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The structure of the skeleton of any adult includes 206 different bones, all of them are different in structure and role. At first glance, they seem hard, inflexible and lifeless. But this is an erroneous impression, various metabolic processes, destruction and regeneration are constantly taking place in them. They, together with muscles and ligaments, form a special system, which is called "musculoskeletal tissue", the main function of which is musculoskeletal. It is formed from several types of special cells that differ in structure, functional features and significance. Bone cells, their structure and functions will be discussed further.

The structure of bone tissue

Features of lamellar bone tissue

It is formed by bone plates having a thickness of 4-15 microns. They, in turn, consist of three components: osteocytes, ground substance and collagen thin fibers. All the bones of an adult human are formed from this tissue. Collagen fibers of the first type lie parallel to each other and are oriented in a certain direction, while in neighboring bone plates they are directed in the opposite direction and cross almost at a right angle. Between them are the bodies of osteocytes in the gaps. This structure of bone tissue provides it with the greatest strength.

Spongy bone

There is also the name "trabecular substance". If we draw an analogy, then the structure is comparable to an ordinary sponge, built from bone plates with cells between them. They are arranged in an orderly manner, in accordance with the distributed functional load. From the spongy substance, the epiphyses of long bones are mainly built, some are mixed and flat, and all are short. It can be seen that these are mainly light and at the same time strong parts of the human skeleton, which are under load in various directions. The functions of bone tissue are directly related to its structure, which in this case provides a large area for metabolic processes carried out on it, gives high strength in combination with a small mass.

Dense (compact) bone substance: what is it?

The diaphyses of tubular bones consist of a compact substance, in addition, it covers their epiphyses with a thin plate from the outside. It is pierced by narrow channels, through which nerve fibers and blood vessels pass. Some of them are located parallel to the bone surface (central or haversian). Others come to the surface of the bone (feeding holes), through which arteries and nerves penetrate inward, and veins outwardly. The central canal, together with the surrounding bone plates, forms the so-called Haversian system (osteon). This is the main content of a compact substance and they are considered as its morphofunctional unit.

Osteon - structural unit of bone tissue

Its second name is the Haversian system. This is a collection of bone plates that look like cylinders inserted into each other, the space between them is filled with osteocytes. In the center is the Haversian canal, through which the blood vessels that provide metabolism in bone cells pass. Between neighboring structural units there are interstitial (interstitial) plates. In fact, they are the remnants of osteons that existed earlier and collapsed at the moment when the bone tissue was undergoing restructuring. There are also general and surrounding plates, they form the innermost and outermost layer of the compact bone substance, respectively.

Periosteum: structure and meaning

Based on the name, it can be determined that it covers the bones from the outside. It is attached to them with the help of collagen fibers collected in thick bundles that penetrate and intertwine with the outer layer of bone plates. It has two pronounced layers:

• external (it is formed by dense fibrous, unformed connective tissue, it is dominated by fibers located parallel to the surface of the bone);
• the inner layer is well expressed in children and less noticeable in adults (it is formed by loose fibrous connective tissue, in which there are spindle-shaped flat cells - inactive osteoblasts and their precursors).

The periosteum performs several important functions. Firstly, it is trophic, that is, it provides nutrition to the bone, since it contains vessels on the surface that penetrate inside along with the nerves through special nutritional openings. These channels feed the bone marrow. Secondly, regenerative. It is explained by the presence of osteogenic cells, which, when stimulated, transform into active osteoblasts that produce matrix and cause bone tissue to build up, ensuring its regeneration. Thirdly, mechanical or support function. That is, ensuring the mechanical connection of the bone with other structures attached to it (tendons, muscles and ligaments).

Functions of bone tissue

Among the main functions are the following:

1. Motor, support (biomechanical).
2. Protective. Bones protect the brain, blood vessels and nerves, internal organs, etc. from damage.
3. Hematopoietic: in the bone marrow, hemo- and lymphopoiesis occurs.
4. Metabolic function (participation in metabolism).
5. Reparatory and regenerative, consisting in the restoration and regeneration of bone tissue.
6. morphogenesis role.
7. Bone tissue is a kind of depot of minerals and growth factors.

BONE TISSUES

Structure: cells and intercellular substance.

Types of bone tissue: 1) reticulofibrous, 2) lamellar.

Also, bone tissues include tissues specific to teeth: dentin, cementum.

in bone tissue 2 differenton cells: 1) osteocyte and its precursors, 2) osteoclast.

Differenton osteocyte : stem and semi-stem cells, osteogenic cells, osteoblasts, osteocytes.

Cells are formed from poorly differentiated mesenchymal cells; in adults, stem and semi-stem cells are found in the inner layer of the periosteum; during bone formation, they are located on its surface and around the intraosseous vessels.

osteoblasts capable of dividing, arranged in groups, have an uneven surface and short processes connecting them with neighboring cells. The synthetic apparatus is well developed in the cells, because osteoblasts are involved in the formation of intercellular substance: they synthesize matrix proteins (osteonectin, sialoprotein, osteocalcin), collagen fibers, enzymes (alkaline phosphatase, etc.).

The function of osteoblasts: the synthesis of intercellular substance, the provision of mineralization.

The main factors activating osteoblasts are: calcitonin, thyroxine (thyroid hormones); estrogens (ovarian hormones); vitamins C, D; piezo effects that occur in the bone when compressed.

Osteocytes - osteoblasts immured in mineralized intercellular substance. Cells are located in gaps - cavities of the intercellular substance. With their processes, osteocytes are in contact with each other; there is an intercellular fluid around the cells in the lacunae. The synthetic apparatus is less developed than in osteoblasts.

Function of osteocytes: maintenance of homeostasis in bone tissue.

Osteoclast. Differenton osteoclast includes monocyte differon (develops in the red bone marrow), then the monocyte leaves the bloodstream and transforms into a macrophage. Several macrophages fuse to form a multinucleated symplast osteoclast. The osteoclast contains many nuclei and a large volume of cytoplasm. Polarity is characteristic (the presence of functionally unequal surfaces): the cytoplasmic zone adjacent to the bone surface is called the corrugated border, there are many cytoplasmic outgrowths and lysosomes.

Functions of osteoclasts: destruction of fibers and amorphous bone substance.

Bone resorption osteoclast: the first stage is attachment to the bone with the help of proteins (integrins, vitronectins, etc.) to ensure sealing; the second stage is the acidification and dissolution of minerals in the area of ​​destruction by pumping hydrogen ions with the participation of ATPases of the membranes of the corrugated edge; the third stage is the dissolution of the organic substrate of the bone with the help of lysosome enzymes (hydrolases, collagenases, etc.), which the osteoclast removes by exocytosis to the destruction zone.

Factors activating osteoclasts: parathyroid hormone parathyrin; piezo effects that occur in the bone when it is stretched; weightlessness; lack of physical activity (immobilization), etc.

Factors that inhibit osteoclasts: thyroid hormone calciotonin, ovarian hormones estrogen.

intercellular substance of bone consists of collagen fibers (collagen I, V types) and the main (amorphous) substance, consisting of 30% organic and 70% inorganic substances. Organic bone substances: glycosaminoglycans, proteoglycans; inorganic substances: calcium phosphate, mainly in the form of hydroxyapatite crystals.

The largest volume in an adult is lamellar bone tissue, which is compact and spongy. On the surface of the lamellar bones in the area of ​​attachment of the tendons, as well as in the sutures of the skull, there is reticulofibrous bone tissue.

Bone as an organ consists of several tissues: 1) bone tissue, 2) periosteum: 2a) outer layer - PVNST, 2b) inner layer - RVST, with blood vessels and nerves, as well as stem and semi-stem cells.

1. RETICULOFIBROSIS (COARSE FIBER) BONE TISSUE

This tissue is formed in human fetuses as the basis of bones. In adults, it is slightly represented and is located in the sutures of the skull at the points of attachment of the tendons to the bones.

Structure: osteocytes and intercellular substance in which bundles of collagen mineralized fibers are arranged randomly. Osteocytes are found in bone cavities. From the surface, parts of the bone are covered with periosteum, from which reticulofibrous bone tissue receives nutrients by diffusion.

LAMINATE (FINE) BONE TISSUE – the main type of bone tissue in the adult body. Structure: osteocytes and intercellular substance consisting of fibers (collagen or ossein) and amorphous substance. The intercellular substance is represented by plates with a thickness of 3-10 microns. In the plate, the fibers are arranged parallel to each other, the fibers of neighboring plates lie at an angle to each other. Between the plates are the bodies of osteocytes in the gaps, and the bone tubules with processes of osteocytes penetrate the plates at a right angle.

Types of lamellar bone tissue. Made of lamellar bone tissue compact and spongy substance most flat and tubular bones.

in spongy matter bone plates are straight, are part of trabeculae - a complex of 2-3 parallel plates. Trabeculae delimit cavities filled with red bone marrow.

AT compact bone along with straight plates there are concentric plates that form osteons.

Histological structure of the tubular bone as an organ. The tubular bone consists of a diaphysis - a hollow tube consisting of a strong compact bone, and epiphyses - the expanding ends of this tube, built of spongy substance.

Bone as an organ consists of lamellar bone tissue, outside and from the side of the bone marrow cavity, it is covered with connective tissue membranes (periosteum, endosteum). The bone cavity contains red and yellow bone marrow, blood and lymphatic vessels and nerves.

In the bones are distinguished compact (cortical) substance bones and spongy (trabecular) substance, which are formed by lamellar bone tissue. Periosteum, or periosteum, consists of an outer (PVNST or PVOST) and an inner layer (RVST). The inner layer contains osteogenic cambial cells, preosteoblasts, and osteoblasts. The periosteum takes part in bone tissue trophism, development, growth and regeneration. Endost- the membrane covering the bone from the side of the bone marrow is formed by loose fibrous connective tissue, where there are osteoblasts and osteoclasts, as well as other PBST cells. The articular surfaces of the epiphyses do not have periosteum and perichondrium. They are covered with a type of hyaline cartilage called articular cartilage.

The structure of the diaphysis . The diaphysis consists of a compact substance (cortical bone), in which three layers are distinguished: 1) the outer layer of common plates; 2) the middle layer is osteon; 3) the inner layer of common plates.

The outer and inner common plates are straight plates, in which osteocytes will receive nutrition from the periosteum and endosteum. In the outer common plates there are perforating (Volkmann) canals, through which vessels enter the bone from the periosteum into the bone. In the middle layer, most of the bone plates are located in osteons, and between the osteons lie insert plates- remnants of old osteons after bone remodeling.

Osteons are structural units of the compact substance of the tubular bone. They are cylindrical formations, consisting of concentric bone plates, as if inserted into each other. In the bone plates and between them are the bodies of bone cells and their processes, passing in the intercellular substance. Each osteon is delimited from the adjacent osteon by a cleavage line formed by the ground substance. At the center of each osteon is channel (haversian channel), where blood vessels with RVST and osteogenic cells pass. The vessels of the osteon channels communicate with each other and with the vessels of the bone marrow and periosteum. On the inner surface of the diaphysis, bordering the medullary cavity, there are bony crossbars of the cancellous bone.

The structure of the epiphysis. The epiphysis consists of a spongy substance, the bone trabeculae (beams) of which are oriented along the load lines of force, providing strength to the epiphysis. The spaces between the beams contain red bone marrow.

Bone vascularization . Blood vessels form a dense network in the inner layer of the periosteum. From here, thin arterial branches originate, which supply the osteons with blood, penetrate into the bone marrow through the nutrient holes and form a supply network of capillaries passing through the osteons.

bone tissue innervation . In the periosteum, myelinated and unmyelinated nerve fibers form plexuses. Some of the fibers accompany the blood vessels and penetrate with them through the nutrient holes into the osteon channels and then reach the bone marrow.

Bone remodeling and renewal . Throughout a person's life, restructuring and renewal of bone tissue occurs. Primary osteons are destroyed and at the same time new ones appear, both in place of old osteons, and from the side of the periosteum. Under the influence of osteoclasts, the bone plates of the osteon are destroyed, and a cavity forms in this place. This process is called resorption bone tissue. In the cavity around the remaining vessel, osteoblasts appear, which begin to build new plates, concentrically layering on each other. This is how secondary generations of osteons occur. Between the osteons are the remains of destroyed osteons of previous generations - insert plates.

It should be noted that in weightlessness (in the absence of gravity and the forces of gravity of the Earth), osteoclasts destroy bone tissue, which is prevented by physical exercises in astronauts.

Age changes . With age, the total mass of connective tissue formations increases, the ratio of collagen types, glycosaminoglycans changes, and sulfated compounds become more numerous. In the endosteum of aging bone, the population of osteoblasts decreases, but the activity of osteoclasts increases, which leads to thinning of the compact layer and restructuring of the cancellous bone.

In adults, the complete change of bone formations depends on its size and for the hip is 7-12 years, for the rib 1 year. In the elderly, in women in menopause, there is a pronounced decalcification of the bones - osteoporosis.

The development of bone tissue in embryogenesis and in the postnatal period

The human embryo has no bone tissue by the beginning of organogenesis (3-5 weeks). In place of future bones are osteogenic cells or cartilage formations (hyaline cartilage). At the 6th week of embryogenesis, the necessary conditions are created (active development of the chorion - the future placenta, and germination of blood vessels with oxygen supply), and the development of bone tissue begins in embryogenesis, and then after birth (postembryonic development).

The development of bone tissue in the embryo is carried out in two ways: 1) direct osteogenesis- directly from the mesenchyme; and 2) indirect osteogenesis- in place of the cartilaginous bone model previously developed from the mesenchyme. Postembryonic development of bone tissue occurs during physiological regeneration.

direct osteogenesis characteristic in the formation of flat bones (for example, the bones of the skull). It is observed already in the first month of embryogenesis and includes three main stages: 1) formation of osteogenic islets from proliferating mesenchymal cells; 2) differentiation of cells of osteogenic islets into osteoblasts and the formation of an organic bone matrix (osteoid), while some of the osteoblasts turn into osteocytes; the other part of the osteoblasts is not the surface of the intercellular substance, i.e. on the surface of the bone, these osteoblasts will become part of the periosteum; 3) calcification (calcification) of the osteoid - the intercellular substance is impregnated with calcium salts; reticulofibrous bone tissue is formed; 4) restructuring and growth of the bone - old areas of coarse fibrous bone are gradually destroyed and new areas of lamellar bone are formed in their place; due to the periosteum, common bone plates are formed, due to the osteogenic cells located in the adventitia of the vessels of the bone, osteons are formed.

Bone development in place of a previously formed cartilage model (indirect osteogenesis). This type of bone development is characteristic of most bones of the human skeleton (long and short tubular bones, vertebrae, pelvic bones). Initially, a cartilaginous model of the future bone is formed, which serves as the basis for its development, and later the cartilage is destroyed and replaced by bone tissue.

Indirect osteogenesis begins in the second month of embryonic development, ends by the age of 18-25 and includes the following stages:

1) education cartilaginous bone model from the mesenchyme in accordance with the patterns of cartilage histogenesis;

2) education perichondral bone cuff: in the inner layer of the perichondrium, osteoblasts differentiate, which begin to form bone tissue; the perichondrium is replaced by the periosteum;

3) education endochondral bone in the diaphysis: the perichondral bone disrupts the nutrition of the cartilage, as a result, osteogenic islands appear in the diaphysis from the mesenchyme growing here with blood vessels. In parallel, osteoclasts destroy the bone with the formation of a bone marrow cavity;

4) education endochondral bone in the epiphysis;

5) formation epiphyseal plate growth in cartilage (metaepiphyseal cartilage): at the border of the epiphysis and diaphysis, chondrocytes gather in columns, as the growth of unchanged distal cartilage continues. In the column of chondrocytes, there are two oppositely directed processes: on the one hand, the reproduction of chondrocytes and the growth of cartilage ( columnar cells) in its distal section and in the periosseous zone, dystrophic changes ( vesicular chondrocytes).

6) restructuring of reticulofibrous bone tissue into lamellar: the old parts of the bone are gradually destroyed and new ones are formed in their place; due to the periosteum, common bone plates are formed, due to the osteogenic cells located in the adventitia of the vessels of the bone, osteons are formed.

Over time, in the metaepiphyseal plate of cartilage, the processes of cell destruction begin to prevail over the process of neoplasm; the cartilaginous plate becomes thinner and disappears: the bone stops growing in length. The periosteum ensures the growth of tubular bones in thickness by appositional growth. The number of osteons after birth is small, but by the age of 25 their number increases significantly.

Bone regeneration. Physiological regeneration of bone tissues and their renewal occur slowly due to osteogenic cells of the periosteum and osteogenic cells in the osteon canal. Post-traumatic regeneration (reparative) is faster. The sequence of regeneration corresponds to the scheme of osteogenesis. The process of bone mineralization is preceded by the formation of an organic substrate (osteoid), in the thickness of which cartilage beams can form (in case of impaired blood supply). Ossification in this case will follow the type of indirect osteogenesis (see the diagram of indirect osteogenesis).

Bones perform four main functions:

1. They provide strength to limbs and body cavities containing vital organs. In diseases that weaken or disrupt the structure of the skeleton, it is impossible to maintain a straight posture, and disorders of the internal organs occur. An example is cardiopulmonary failure, which develops in patients with severe kyphosis due to compression fractures of the vertebrae.
2. Bones are essential for movement because they form effective levers and attachment points for muscles. Deformation of the bones "spoils" these levers, which leads to severe gait disorders.
3. Bones serve as a large reservoir of ions, from where the body draws the calcium, phosphorus, magnesium and sodium necessary for life when it is impossible to obtain them from the external environment.
4. The bones contain the hematopoietic system. More and more evidence indicates trophic relationships between bone stromal cells and hematopoietic elements.

The structure of the bone

The structure of the bone provides an ideal balance of its hardness and elasticity. Bone is hard enough to withstand external forces, although poorly mineralized bone is brittle and prone to fracture. At the same time, the bone must be light enough to move when the muscles contract. Long bones are built primarily from a compact substance (densely packed layers of mineralized collagen) that gives the tissue its hardness. Trabecular bones appear spongy in cross section, giving them strength and elasticity. Spongy substance makes up the main part of the spine. Diseases accompanied by a violation of the structure or a decrease in the mass of the compact substance of the bone lead to fractures of long bones, and those in which the spongy substance suffers - to fractures of the vertebrae. Fractures of long bones are also possible in cases of defects in the spongy substance.
Two-thirds of bone weight is mineral, and the rest is water and type I collagen. Non-collagenous bone matrix proteins include proteoglycans, γ-carboxyglutamate containing proteins, osteonectin glycoprotein, osteopontin phosphoprotein, and growth factors. There is also a small amount of lipids in the bone tissue.

Bone Minerals
Bone contains minerals in two forms. The main form is hydroxyapatite crystals of various maturity. The rest are amorphous calcium phosphate salts with a lower ratio of calcium to phosphate than in pure hydroxyapatite. These salts are localized in areas of active bone tissue formation and are present in greater amounts in young bone.

bone cells
Bone is made up of three types of cells: osteoblasts, osteocytes, and osteoclasts.

osteoblasts
Osteoblasts are the main bone-forming cells. Their precursors are bone marrow mesenchymal cells, which in the process of differentiation begin to express PTH and vitamin D receptors, alkaline phosphatase (released into the extracellular environment), as well as bone matrix proteins (type I collagen, osteocalcin, osteopontin, etc.). Mature osteoblasts move to the surface of the bone, where they line the areas of bone tissue neoplasm, located under the bone matrix (osteoid) and causing its mineralization - the deposition of hydroxyapatite crystals on collagen layers. As a result, lamellar bone tissue is formed. Mineralization requires the presence of sufficient calcium and phosphate in the extracellular fluid, as well as alkaline phosphatase, which is secreted by active osteoblasts. Some "aging" osteoblasts flatten out, turning into inactive cells lining the surface of the trabeculae, others sink into the compact bone substance, turning into osteocytes, and still others undergo apoptosis.

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Osteocytes
Osteoblasts remaining in the compact bone during its renewal turn into osteocytes. Their ability to synthesize protein drops sharply, but many processes (tubules) appear in the cells, stretching beyond the resorption cavity (lacunae) and connecting with capillaries, processes of other osteocytes of this bone unit (osteon) and processes of superficial osteoblasts. It is believed that osteocytes form syncytium, which ensures the movement of minerals from the bone surface, and, in addition, play the role of mechanical load sensors that generate the main signal for the formation and renewal of bone tissue.

osteoclasts
Osteoclasts are giant multinucleated cells that specialize in bone resorption. They come from hematopoietic cells and no longer divide. Osteoclast formation is stimulated by osteoblasts, which interact with their surface molecule RANKL with the nuclear factor-kappa-B activating receptor (RANK) on the surface of precursors and mature osteoclasts. Osteoblasts also secrete macrophage colony-stimulating factor-1 (M-CSF-1), which enhances the effect of RANKL on osteoclastogenesis. In addition, osteoblasts and other cells produce a decoy osteoprotegerin (OPG) receptor that binds to RANKL and blocks its action. PTH and 1,25(OH) 2 D (as well as cytokines IL-1, IL-6 and IL-11) stimulate RANKL synthesis in osteoblasts. TNF potentiates the stimulating effect of RANKL on osteoclastogenesis, while IFNγ blocks this process by acting directly on osteoclasts.
Mobile osteoclasts surround the area of ​​the bone surface with a dense ring, and their membrane adjacent to the bone folds into a special structure called a corrugated border. The corrugated border is a separate organelle but acts like a giant lysosome that dissolves and breaks down the bone matrix by secreting acid and proteases (primarily cathepsin K). Collagen peptides formed as a result of bone resorption contain pyridinoline structures, the level of which in urine can be used to judge the intensity of bone resorption. Thus, bone resorption depends on the rate of maturation of osteoclasts and the activity of their mature forms. Mature osteoclasts have receptors for calcitonin, but not for PTH or vitamin D.

Bone update

Bone renewal is a continuous process of destruction and formation of bone tissue that continues throughout life. In childhood and adolescence, bone renewal proceeds at a high rate, but the process of bone formation and an increase in bone mass quantitatively predominates. After the bone mass reaches its maximum, the processes that determine the dynamics of bone mass throughout the rest of life begin to dominate. Renewal occurs in separate areas of the bone surface throughout the skeleton. Normally, about 90% of the bone surface is at rest, being covered with a thin layer of cells. In response to physical or biochemical signals, bone marrow progenitor cells migrate to specific locations on the bone surface, where they fuse to form multinucleated osteoclasts that “eat away” the cavity in the bone.
The renewal of the compact bone substance begins from inside the conical cavity, which continues into the tunnel. Osteoblasts crawl into this tunnel, forming a cylinder of new bone and gradually narrowing the tunnel until a narrow Haversian canal remains, through which the cells remaining in the form of osteocytes feed. A bone formed in one conical cavity is called an osteon.
During resorption of the spongy substance, a jagged area of ​​the bone surface is formed, called the gauship lacuna. After 2-3 months, the resorption phase ends, leaving behind a cavity about 60 µm deep, into the base of which osteoblast precursors grow from the bone marrow stroma. These cells acquire the osteoblast phenotype, that is, they begin to secrete bone proteins such as alkaline phosphatase, osteopontin, and osteocalcin, and gradually replace the resorbed bone with new bone matrix. When the newly formed osteoid reaches a thickness of about 20 µm, mineralization begins. The entire cycle of bone renewal normally lasts about 6 months.
This process does not need hormonal influences, with the only exception that 1,25(OH) 2 D supports the absorption of minerals in the intestines and thus provides the renewing bone with calcium and phosphorus. For example, with hypoparathyroidism, nothing happens to the bone tissue, except for a slowdown in its metabolism. However, systemic hormones use bones as a source of minerals to maintain a constant extracellular level of calcium. At the same time, bone mass is replenished. For example, when PTH activates bone resorption (to correct hypocalcemia), the processes of new bone tissue formation are also enhanced, aimed at replenishing its mass. The role of osteoblasts in the regulation of osteoclast activity has been studied in some detail, but the mechanism of “attraction” of osteoblasts to bone resorption foci remains unclear. One possibility is that during bone resorption, IGF-1 is released from the bone matrix, which stimulates the proliferation and differentiation of osteoblasts.
Resorbed bone is not completely replaced, and at the end of each renewal cycle, some bone mass deficit remains. Over the course of life, the deficit increases, which determines the well-known phenomenon of age-related decrease in bone mass. This process begins shortly after the cessation of body growth. Various influences (malnutrition, hormones and medicinal substances) affect bone metabolism in a common way - through a change in the rate of bone tissue renewal, but by different mechanisms. Changes in the hormonal environment (hyperthyroidism, hyperparathyroidism, hypervitaminosis D) usually increase the number of renewal foci. Other factors (high doses of glucocorticoids or ethanol) impair osteoblast activity. Estrogens or androgen deficiency increase osteoclast activity. At any given time, there is a transient shortage of bone mass called "renewal space", ie. still unfilled area of ​​bone resorption. In response to any stimulus that changes the initial number of renewal sites (“renewal units”), the renewal space either increases or decreases until a new equilibrium is established. This is manifested by an increase or decrease in bone mass.

Bone tissue forms the basis of the skeleton. It is responsible for the protection of internal organs, movement, and is involved in metabolism. Bone tissue also includes dental tissue. Bone is a hard and flexible organ. Its features continue to be studied. There are more than 270 bones in the human body, each of which performs its own function.

Bone tissue is a type of connective tissue. One is both ductile and resistant to deformation, durable.

There are 2 main types of bone tissue depending on its structure:

1. Coarse fiber. This is a denser, but less elastic bone tissue. In the body of an adult, it is very small. It is mainly found at the junction of bone with cartilage, at the junction of cranial sutures, as well as at the fusion of fractures. Coarse-fibrous bone tissue is found in large quantities during the period of human embryonic development. It acts as the rudiment of the skeleton, and then gradually degenerates into a lamellar one. The peculiarity of this type of tissue is that its cells are arranged randomly, which makes it denser.
2. Lamellar. Lamellar bone tissue is the main one in the human skeleton. It is part of all the bones of the human body. A feature of this tissue is the arrangement of cells. They form fibers, which in turn form plates. The fibers that make up the plates can be located at different angles, which makes the fabric strong and elastic at the same time, but the plates themselves are parallel to each other.

In turn, lamellar bone tissue is divided into 2 types - spongy and compact. Spongy tissue has the appearance of cells and is looser. However, despite the reduced strength, spongy tissue is more voluminous, lighter, and less dense.

It is the spongy tissue that contains the bone marrow involved in the hematopoietic process.

Compact bone tissue performs a protective function, so it is denser, stronger and heavier. Most often, this tissue is located outside the bone, covering and protecting it from damage, cracks, and fractures. Compact bone tissue makes up the majority of the skeleton (about 80%).

The structure and functions of lamellar bone tissue

Lamellar bone tissue is the most common type of bone tissue in the human body.

The functions of lamellar bone tissue are very important for the body. It protects the internal organs from damage (the lungs in the chest, the brain inside, the pelvic organs, etc.), and also allows a person to move, bearing the weight of other tissues.

Bone tissue is resistant to deformation, can withstand a lot of weight, and is also able to regenerate and grow together in case of fractures.

Bone tissue consists of intercellular substance, as well as 3 types of bone cells:

1. osteoblasts. These are the youngest, often oval cells of bone tissue with a diameter of no more than 20 microns. It is these cells that synthesize the substance that fills the intercellular space of the bone tissue. This is the main function of cells. When a sufficient amount of this substance is formed, osteoblasts become overgrown with it and become osteocytes. Osteoblasts are able to divide, and also have an uneven surface with small processes, with which they are attached to neighboring cells. There are also inactive osteoblasts, they are often localized in the densest parts of the bone and have a small number of organelles.
2. Osteocytes. These are stem cells that can often be found inside the tissues of the periosteum (the upper, strong layer of the bone that protects it and allows it to quickly heal when damaged). When osteoblasts are overgrown with intercellular substance, they turn into osteocytes and are localized in the intercellular space. Their ability to synthesize is somewhat lower than that of osteoblasts.
3. Osteoclasts. The largest multinucleated bone tissue cells that are found only in vertebrates. Their main function is the regulation and destruction of old bone tissue. Osteoblasts create new bone cells, while osteoclasts break down old ones. Each such cell contains up to 20 nuclei.

You can find out the state of the bone tissue with the help of. Lamellar bone tissue plays an important role in the body, but it can be destroyed, worn out with a lack of calcium, and also due to infections.

Diseases of lamellar bone tissue:

• Tumors. There is the concept of "bone cancer", but most often the tumor grows into the bone from other tissues, and does not originate in it. The tumor can originate from the cells of the bone marrow, but not the bone itself. Sarcoma (primary bone cancer) is quite rare. This disease is accompanied by severe pain in the bones, swelling of the soft tissues, limited mobility, swelling and deformity of the joints.
• Osteoporosis. This is the most common bone disease, accompanied by a decrease in the amount of bone tissue, thinning of the bones. This is a complex disease that is asymptomatic for a long time. Spongy tissue begins to suffer first. The plates in it begin to empty, and the tissue itself is damaged from daily stress.
• Osteonecrosis. Part of the bone dies due to impaired blood circulation. Osteocytes begin to die, which leads to necrosis. The hip bones are most commonly affected by osteonecrosis. Thrombosis and bacterial infections lead to this disease.
• Paget's disease. This disease is more common in the elderly. Paget's disease is characterized by bone deformity and severe pain. The normal process of bone tissue repair is disrupted. The causes of this disease are unknown. In the affected areas, the bone thickens, deforms and becomes very brittle.

You can learn more about osteoporosis from the video.

Bone tissue is a type of connective tissue and consists of cells and intercellular substance, which contains a large amount of mineral salts, mainly calcium phosphate. Minerals make up 70% of bone tissue, organic - 30%.

Functions of bone tissue

mechanical;

protective;

participation in the mineral metabolism of the body - the depot of calcium and phosphorus.

bone cells: osteoblasts, osteocytes, osteoclasts.

The main cells in the formed bone tissue are osteocytes.

osteoblasts

osteoblasts found only in developing bone tissue. They are absent in the formed bone tissue, but are usually contained in an inactive form in the periosteum. In developing bone tissue, they cover each bone plate along the periphery, tightly adhering to each other, forming a kind of epithelial layer. The shape of such actively functioning cells can be cubic, prismatic, angular.

Oteoclasts

There are no bone-destroying cells in the formed bone tissue. But they are contained in the periosteum and in places of destruction and restructuring of bone tissue. Since local processes of bone tissue restructuring are continuously carried out in ontogenesis, osteoclasts are necessarily present in these places. In the process of embryonic osteogenesis, these cells play an important role and are found in large numbers.

intercellular substance bone tissue

consists of the main substance and fibers, which contain calcium salts. The fibers consist of type I collagen and are folded into bundles that can be arranged in parallel (ordered) or disordered, on the basis of which the histological classification of bone tissues is built. The main substance of bone tissue, like other types of connective tissues, consists of glycosaminoglycans and proteoglycans, but the chemical composition of these substances is different. In particular, bone tissue contains less chondroitin sulfuric acids, but more citric and other acids that form complexes with calcium salts. In the process of development of bone tissue, an organic matrix, the main substance and collagen (ossein, type II collagen) fibers are first formed, and then calcium salts (mainly phosphate) are deposited in them. Calcium salts form hydroxyapatite crystals, which are deposited both in the amorphous substance and in the fibers, but a small part of the salts is deposited amorphously. Providing bone strength, calcium phosphate salts are simultaneously a depot of calcium and phosphorus in the body. Therefore, bone tissue takes part in mineral metabolism.

Classification of bone tissue

There are two types of bone tissue:

reticulofibrous (coarse-fibrous);

lamellar (parallel fibrous).

AT reticulofibrous bone tissue bundles of collagen fibers are thick, tortuous and randomly arranged. In the mineralized intercellular substance, osteocytes are randomly located in the lacunae. lamellar bone tissue consists of bone plates in which collagen fibers or their bundles are parallel in each plate, but at right angles to the course of the fibers in adjacent plates. Between the plates in the gaps are osteocytes, while their processes pass through the tubules through the plates.

In the human body, bone tissue is represented almost exclusively by a lamellar form. Reticulofibrous bone tissue occurs only as a stage in the development of some bones (parietal, frontal). In adults, they are located in the area of ​​attachment of the tendons to the bones, as well as in place of the ossified sutures of the skull (sagittal suture of the scales of the frontal bone).

When studying bone tissue, it is necessary to differentiate the concepts of bone tissue and bone.

Bone

Bone is an anatomical organ, the main structural component of which is bone. Bone as an organ is made up of the following items:

bone;

periosteum;

bone marrow (red, yellow);

vessels and nerves.

Periosteum

(periosteum) surrounds the bone tissue along the periphery (with the exception of the articular surfaces) and has a structure similar to the perichondrium. In the periosteum, the outer fibrous and inner cellular or cambial layers are isolated. The inner layer contains osteoblasts and osteoclasts. A pronounced vascular network is localized in the periosteum, from which small vessels penetrate into the bone tissue through perforating channels. Red bone marrow is considered as an independent organ and belongs to the organs of hematopoiesis and immunogenesis.

The skeleton represents the framework that helps the body keep its shape, protect organs, move in space, and much more. In general, the structure of bone cells, like any tissue, is very specialized, due to which there is strength to mechanical stress, and with it plasticity, in parallel with this, regeneration processes occur. In addition, the cells are in a strictly defined mutual arrangement, due to which the bone, and not other tissue, is much stronger than the connective tissue. The main components of bone tissue are osteoblasts, osteoclasts, and osteocytes.

It is these cells that maintain the properties of the tissue, providing its histological structure. What is the secret of these three cells, which the bone has in its composition, determining many functions. After all, only the teeth, which contain the alveoli of the jaw, are stronger than the bones. Vessels and nerves pass through the bones, as in the skull, they contain the brain, which is the source of hematopoiesis, and protect the internal organs. Covered with a layer of cartilage on top, they provide normal movement.

What is an osteoblast

The structure of this cell is specific, it is an oval or cubic formation visible under a microscope. Laboratory equipment showed that inside the cytoplasm the nucleus of the osteoblast is large, light in color, located not centrally, but somewhat towards the periphery. There are a couple of nucleoli nearby, which indicates that the cell is capable of synthesizing many substances. It also has many ribosomes, organelles, due to which the synthesis of substances occurs. Also involved in this process is the granular endoplasmic reticulum, the Golgi complex, which brings the synthesis products out.

Numerous mitochondria are responsible for what energy supply will be. They have a lot of work to do, a lot of them are contained in muscle tissue. But in the cartilaginous, coarse fibrous connective tissue, in contrast to muscle, there are much fewer mitochondria.

Cell functions

The main job of the cell is to produce intercellular substance. They also provide mineralization of bone tissue, due to this it has a special strength. In addition, cells are involved in the synthesis of many important bone tissue enzymes, the main of which is alkaline phosphatase, collagen fibers of special strength, and much more. Enzymes, leaving the cell, provide bone mineralization.

Varieties of osteoblasts

In addition to the fact that the structure of cells is specific, they are functionally active to varying degrees. Active ones have a high synthetic ability, but inactive ones are located in the peripheral part of the bone. The latter are located near the bone canal, are part of the periosteum, the membrane that covers the bone. Their structure is reduced to a small number of organelles.

Osteocyte, its structure

This bone tissue cell is more differentiated than the previous one. The osteocyte has processes that are located in the tubules passing through the mineralized matrix of the bone, their direction is different. A flat body is located in a recess - lacunae, surrounded on all sides by a mineralized component. In the cytoplasm there is an oval-shaped nucleusoccupying almost its entire volume.

Organelles are poorly developed, a small number of ribosomes, the channels of the endoplasmic reticulum are short, mitochondria, in contrast to muscle, cartilage tissue, are few. Through channels with gaps, cells can interact with each other. The microscopic space around the cell has a meager amount of tissue fluid. It contains calcium ions, residue, phosphorus, collagen fibers (mineralized or not).

Function

The task of the cell is to regulate the integrity of bone tissue, to participate in mineralization. Also, the function of the cell is to respond to the emerging load.

Recently, the fact that cells are involved in the processes of bone tissue metabolism, including the jaw, has become increasingly popular. There is an assumption that the work of the cell is additionally to regulate the ionic balance of the body.

In many ways, the functions of osteocytes depend on the stage of the life cycle, like cartilage, muscle tissue, as well as the effects of hormones on them.

Osteoclast, its secret

These cells are of considerable size, contain many nuclei, and, in essence, are derivatives of blood monocytes. On the periphery, the cell has a corrugated brush border. In the cytoplasm of the cell there are many ribosomes, mitochondria, tubules of the endoplasmic reticulum, as well as the Golgi complex. Also, the cell contains a large number of lysosomes, phagocytic organelles, all kinds of vacuoles, vesicles.

Tasks

This cell has its own tasks, it can create an acidic environment around itself as a result of biochemical reactions in bone tissue. As a result, mineral salts dissolve, after which old or dead cells are dissolved and digested by enzymes and lysosomes.

Thus, the job of the cell is to gradually destroy the outdated tissue, but at the same time the structure of the bone tissue is updated. As a result, a new one appears in its place, due to which the bone structure is updated.

Other components

Despite its strength (as in the thigh or lower jaw), organic substances are present in the bone, which are complemented by inorganic ones. The organic component is represented by 95% collagen proteins, the rest is occupied by non-collagen, as well as glycosminoglycans, proteoglycans.

The inorganic component of bone tissue is crystals of a substance called hydroxyapatite, which contains large amounts of calcium and phosphorus ions. Less in the lamellar structure of the bone contains salts of magnesium, potassium, fluorides, bicarbonates. There is a constant renewal of the lamellar structure, the intercellular substance around the cell.

Varieties

In total, bone tissue has two types, it all depends on its microscopic structure. The first is called reticulofibrous or coarse-fibered, the second is lamellar. Let's consider each separately.

In an embryo, a newborn

Reticulofibrous is widely represented in the embryo, the child after birth. An adult has a lot of connective tissue, and this variety is found only in the place where the tendon is attached to the bone, at the junction of the sutures on the skull, at the fracture line. Gradually, the reticulofibrous tissue is replaced by a lamellar one.

This bone tissue has a special structure, its cells are randomly located in the intercellular substance. Collagen fibers, which are a type of connective tissue, are powerful, poorly mineralized, and have a different direction. Reticulofibrous bone has a high density, but the cells are not oriented along the connective tissue of collagen fibers.

In an adult

As an infant grows, its bone contains mostly lamellar bone. This variety is interesting in that bone plates are formed by mineralized intercellular substance, having a thickness of 5 to 7 microns. Any plate consists of collagen fibers of the connective tissue, located in parallel, as close as possible, as well as impregnated with crystals of a special mineral - hydroxyapatite.

In neighboring plates, connective tissue fibers run at different angles, which provides strength, for example, in the thigh or jaw. Lacunas or alveoli between the plates in an orderly manner contain bone cells - osteocytes. Their processes through the tubules penetrate into adjacent plates, due to which intercellular contacts of neighboring cells are formed.

There are some record systems:

• surrounding (external or located from the inside);
• concentric (included in the structure of the osteon);
• intercalary (residue of a collapsing osteon).

The structure of the cortical, spongy layer

At the heart of this layer are mineral salts, in the jaw it is here that implants are implanted through the alveoli. The basal layer is located the deepest, is the most durable, there are many partitions in the jaw, penetrated by capillaries, but there are few of them.

In the central section there is a spongy substance, there are some subtleties in its structure. It is built from partitions, capillaries. Due to the partitions, the bone has a density, and through the capillaries it receives blood. Their functions in the jaw are to nourish the teeth, oxygenate.

In the bones of the body, including the jaw, which contains alveoli, there is a compact, and then a spongy substance following it. Both of these components have a slightly different structure, but are formed by a lamellar-type tissue. The compact substance is located outside, muscle, cartilage or connective tissue is attached to it. Its function is to give bone density, as, for example, in the jaw, the alveoli of which bear the load from chewing food.

The spongy substance is located inside any bone, including the jaw, in the lower part it contains alveoli. Its functions are reduced to additional strengthening of the bone, in giving it plasticity, this part is the receptacle of the bone marrow, which produces blood cells.

Some facts

In total, a person contains from 208 to 214 bones, which consist of half of the inorganic component, a quarter is organic matter, and another quarter is water. All this is interconnected by connective tissue, collagen fibers and proteoglycans.

The composition of the bone has an organic component, as in muscle, connective or cartilage tissue, in total from 20 to 40%. The share of inorganic minerals is from 50 to 70%, cellular elements contain from 5 to 10%, and fats - 3%.

The weight of the human skeleton is on average 5 kg, much depends on age, gender, amount of connective tissue, body structure and growth rates. The amount of cortical bone is on average 4 kg, which is 80%. The spongy substance of tubular bones, jaws and others weighs about a kilogram, which is 20%. The volume of the skeleton is 1.4 liters.

The bone in the human skeleton is a separate organ that can have its own specific problems. It is in the bones that injuries often occur, which, depending on the type, have different healing times. If you look at the bone with the naked eye, it becomes clear that each of them differs in its shape. This is due to what functions it performs, what load it affects, how many muscles are attached.

Bones allow a person to move in space, they are protection for internal organs. And the more important the organ, the more it is surrounded by bones. With age, the ability to recover decreases and the fracture heals more slowly, cells lose the ability to rapidly divide. This is proved by microscopic studies, as well as the properties of bone tissue. The degree of mineralization of collagen fibers decreases, so injuries last longer.

It is the main supporting tissue and structural material for bones, i.e. for the skeleton. Fully differentiated bone is the strongest material in the body, with the exception of tooth enamel. It is highly resistant to compression and stretching and is exceptionally resistant to deformation. The surface of the bone (with the exception of the articulating surfaces) is covered with a membrane (periosteum) that provides bone healing after fractures.

Bone cells and intercellular substance

Bone cells (osteocytes) are interconnected by long processes and are surrounded on all sides by the main substance of the bone (extracellular matrix). The composition and structure of the basic substance of the bone is peculiar. The extracellular matrix is ​​filled with collagen fibers located in the ground substance rich in inorganic salts (calcium salts, primarily phosphate and carbonate).

It contains 20-25% water, 25-30% organic matter and 50% various inorganic compounds. The minerals of the bone are in crystalline form, thus providing its high mechanical strength.

Due to a good blood supply, which favors increased metabolism, the bone has biological plasticity. Rigid and extremely durable bone material is a living tissue that can easily adapt to changes in static loads, including when they change direction. There are no distinct boundaries between the organic and mineral components of the bone, and therefore their presence can only be established by microscopic examination. When burned, the bone retains only its mineral base and becomes brittle. If the bone is placed in acid, then only organic substances remain, and it becomes flexible, like rubber.

The structure of the tubular bone

The structure of the bone especially clearly seen in the longitudinal cut of a long bone. Distinguish thick outer layer (substantia compacta, compacts, compact substance) and inner (spongy) layer (substancia spongiosa, spongiosa). While a dense outer layer is characteristic of long bones and is especially noticeable on the body of the bone (diaphysis), the spongy layer is mainly found inside its ends (epiphyses).

This "lightweight design" provides bone strength with minimal material consumption. The bone adapts to the resulting loads through the orientation of the bony crossbars (trabeculae). Trabeculae are located along the lines of compression and tension that occur during loading. The space between the trabeculae in spongy bones is filled with red bone marrow, which provides hematopoiesis. White bone marrow (fatty marrow) is mainly located in the cavity of the diaphysis.

In long bones, the outer layer has a lamellar (lamellar) structure. Therefore, the bones are also called lamellar. The architecture of the lamellar network (osteon, or Haversian system) is clearly visible on the cuts. At the center of each osteon is a blood vessel, through which nutrients are supplied from the blood to the bone.

Osteocytes and extracellular matrix are grouped around it. Osteocytes are always located between the plates, which contain spiralized collagen fibrils. Cells are connected to each other by means of processes passing through the smallest bone tubules (canalicules). Nutrients flow from the internal blood vessels through these tubules. As the osteon develops, the bone-forming cells (osteoblasts) begin to come in large numbers from the inside of the bone, forming the outer plate of the osteon. Collagen fibrils are superimposed on this plate, which spiralize. Crystals of inorganic salts are ordered between the fibrils.

Then the next plate is formed from the inside, in which the collagen fibrils are located perpendicular to the fibrils of the first plate. The process continues until there is only room in the center for the so-called Haversian canal, through which the blood vessel passes. Also in the channel is a small amount of connective tissue. Mature osteon reaches about 1 cm in length and consists of 10-20 cylindrical plates inserted one into the other. Bone cells are, as it were, walled up between the plates and are connected to neighboring cells through long thin processes. The osteons are connected to each other by canals (Volkmann canals), through which the branches of the vessels pass into the Haversian canals.

Spongy bones also have a lamellar structure, but in this case the plates are arranged in layers, as in a sheet of plywood. Since cancellous bone cells also have high metabolic activity and require nutrients, the lamellae are thin (about 0.5 mm) in this case. This is due to the fact that the exchange of nutrients between cells and bone marrow occurs solely due to diffusion.

Throughout the life of the organism, the osteons of the dense layer and the plates of spongy bones can adapt well to changes in static loads (for example, to fractures). At the same time, in a dense and spongy substance, the old lamellar structures are destroyed, and new ones arise. The plates are destroyed by special cells called osteoclasts, and the osteons that are in the process of renewal are called interstitial plates.

Bone development

At the first stage of human bone differentiation, lamellar tissue is not formed. Instead, reticulofibrous (coarse-fibrous) bone develops. This occurs in the embryonic period, as well as during the healing of fractures. In the coarse fibrous bone, the vessels and collagen fibers are arranged randomly, which makes it reminiscent of a strong, fiber-rich connective tissue. Rough fibrous bone can be formed in two ways.

1. Membrane bone develops directly from the mesenchyme. This type of ossification is called intramembrane ossification or desmal ossification(straight way).

2. First, a cartilaginous rudiment is formed in the mesenchyme, which then turns into bone (endochondral bone). The process is called endochondral or indirect ossification.

Adapting to the needs of a growing organism, developing bones are constantly changing shape. Lamellar bones also change according to functional load, for example, as body weight increases.

Development of long bones

Most bones develop from the cartilaginous primordium along an indirect path. Only some bones (skulls and clavicles) are formed by intramembrane ossification. However, parts of long bones can form in a straight path even if the cartilage is already laid down, for example, in the form of a perichondral bony cuff, due to which the bone thickens (perichondral ossification).

Inside the bone, tissue is laid down along an indirect path, with cartilage cells being first removed by chondroclasts and then replaced by chondral ossification. On the border of the diaphysis and the epiphysis, the epiphyseal plate (cartilage) develops. In this place, the bone begins to grow in length due to the division of cartilage cells. Division continues until growth stops. Because the epiphyseal cartilage plate does not contain calcium, it is not visible on an x-ray. Bone growth within the epiphyses (ossification centers) begins only from the moment of birth. Many centers of ossification develop only in the first years of life. In places where muscles attach to bones (apophyses), special centers of ossification are formed.

Differences between bone and cartilage

Avascular bone cells form a dense substance that performs transport functions. Such a bone regenerates well and constantly adapts to changing static conditions. In avascular cartilage, cells are isolated from each other and from nutrient sources. Compared to bone, cartilage is less able to regenerate and has little adaptive capacity.

Bone tissue is a specialized type of connective tissue, the organic intercellular substance of which contains up to 70% of inorganic compounds - calcium and phosphorus salts and more than 30 trace element compounds. The composition of the organic matrix includes collagen-type proteins (ossein), lipids chondroitin sulfates. In addition, it includes citric acid and other acids that form complex compounds with calcium that impregnate the intercellular substance.

There are 2 types of bone tissue: coarse fibrous (reticulofibrous) and lamellar.

The intercellular substance of bone tissue contains Cellular elements : osteogenic cells, osteoblasts and osteocytes, which are formed from the mesenchyme and represent the bone differon. Another population of cells are osteoclasts.

osteogenic cells are stem cells of bone tissue that separate from the mesenchyme at an early stage of osteogenesis. They are able to produce growth factors that induce hematopoiesis. In the process of differentiation, they turn into osteoblasts.

osteoblasts localized in the inner layer of the periosteum, during the formation of the bone are on its surface and around the intraosseous vessels; cells are cubic, pyramidal, angular in shape, with well-developed HES and other synthesis organelles. They produce collagen proteins and components of the amorphous matrix, actively divide.

Osteocytes - are formed from osteoblasts, located inside the bone in a kind of bone lacunae, have a process shape. They lose the ability to divide. The secretion of the intercellular substance of the bone in them is weakly expressed.

osteoclasts - polynuclear macrophages of bone tissue, are formed from blood monocytes. Can contain up to 40 or more cores. The volume of the cytoplasm is large; the cytoplasmic zone adjacent to the bone surface forms a corrugated border formed by cytoplasmic outgrowths, which contains many lysosomes.

Functions - destruction of fibers and amorphous bone substance.

intercellular substance represented by collagen fibers (collagen types I, V) and an amorphous component, which contains calcium phosphate (mainly in the form of hydroxyapatite crystals and a little in the amorphous state), a small amount of magnesium phosphate and very few glycosaminoglycans and proteoglycans.

Coarse-fibered (reticulofibrous) bone tissue is characterized by a disordered arrangement of ossein fibers. In the lamellar (mature) bone tissue, the ossein fibers in the bone plates have a strictly ordered arrangement. Moreover, in each bone plate, the fibers have the same parallel arrangement, and in the adjacent bone plate they are at right angles to the previous one. The cells between the bone plates are localized in special lacunae, they can be walled up in the intercellular substance or located on the surface of the bone and around the vessels penetrating the bone.

Bone as an organ histologically it consists of three layers: periosteum, compact substance and endosteum.

Periosteum It has a structure similar to the perichondrium, that is, it consists of 2 similar layers, the inner of which, osteogenic, is formed by loose connective tissue, where there are many osteoblasts, osteoclasts and many vessels.

Endost lines the medullary canal. It is formed by loose fibrous connective tissue, where there are osteoblasts and osteoclasts, as well as other cells of loose connective tissue.

Functions of the periosteum and endosteum: bone trophism, bone growth in thickness, bone regeneration.

Compact matter The bone is made up of 3 layers. The outer and inner are the general (common) bone plates, and between them is the osteon layer.

The structural and functional unit of bone as an organ is Osteon , which is a cavity formation, consisting of concentrically layered bone plates in the form of several cylinders inserted one into the other. Between the bone plates there are gaps in which osteocytes lie. A blood vessel passes through the cavity of the osteon. The bony canal in which the blood vessel is located is called the osteon canal or Haversian canal. Intercalated bone plates (remnants of collapsing osteons) are located between the osteons.

Histogenesis of bone tissue. The source of the development of bone tissue is mesenchymal cells that migrate from sclerotomes. At the same time, its histogenesis is carried out in two ways: directly from the mesenchyme (direct osteohistogenesis) or from the mesenchyme at the site of the previously formed hyaline cartilage (indirect osteohistogenesis).

Direct osteogenesis. Coarse fibrous (reticulofibrous) bone tissue is formed directly from the mesenchyme, which is subsequently replaced by lamellar bone tissue. There are 4 stages in direct osteogenesis:

1. isolation of the osteogenic island - in the area of ​​bone tissue formation, mesenchymal cells actively divide and turn into osteogenic cells and osteoblasts, blood vessels form here;

2. osteoid stage - osteoblasts begin to form the intercellular substance of the bone tissue, while some of the osteoblasts are inside the intercellular substance, these osteoblasts turn into osteocytes; the other part of the osteoblasts is on the surface of the intercellular substance, i.e. on the surface of the formed bone tissue, these osteoblasts will become part of the periosteum;

3. mineralization of the intercellular substance (impregnation with calcium salts). Mineralization is carried out due to the intake of calcium glycerophosphate from the blood, which, under the influence of alkaline phosphatase, is split into glycerol and a phosphoric acid residue, which reacts with calcium chloride, resulting in the formation of calcium phosphate; the latter turns into hydroapatite;

4. restructuring and growth of the bone - old areas of coarse fibrous bone are gradually destroyed and new areas of lamellar bone are formed in their place; due to the periosteum, common bone plates are formed, due to the osteogenic cells located in the adventitia of the vessels of the bone, osteons are formed.

indirect osteohistogenesis carried out in place of the cartilage. In this case, lamellar bone tissue is immediately formed. In this case, 4 stages can also be distinguished:

1. formation of a cartilaginous model of the future bone;

2. in the region of the diaphysis of this model, perichondral ossification occurs, while the perichondrium turns into a periosteum, in which stem (osteogenic) cells differentiate into osteoblasts; osteoblasts begin the formation of bone tissue in the form of common plates that form the bone cuff;

3. in parallel with this, endochondral ossification is also observed, which occurs both in the area of ​​the diaphysis and in the area of ​​the epiphysis; ossification of the epiphysis is carried out only by endochondral ossification; blood vessels grow into the cartilage, in the adventitia of which there are osteogenic cells that turn into osteoblasts. Osteoblasts, producing intercellular substance, form bone plates around the vessels in the form of osteons; simultaneously with the formation of bone, the destruction of cartilage by chondroclasts occurs;

4. restructuring and growth of the bone - the old parts of the bone are gradually destroyed and new ones are formed in their place; due to the periosteum, common bone plates are formed, due to the osteogenic cells located in the adventitia of the vessels of the bone, osteons are formed.

In the bone tissue throughout life, both the processes of creation and destruction are constantly taking place. Normally, they balance each other. The destruction of bone tissue (resorption) is carried out by osteoclasts, and the destroyed areas are replaced by newly built bone tissue, in the formation of which osteoblasts take part. The regulation of these processes is carried out with the participation of hormones produced by the thyroid, parathyroid and other endocrine glands. The structure of bone tissue is affected by vitamins A, D, C. Inadequate intake of vitamin D in the early postnatal period leads to the development of the disease Rickets.

• mechanical - bones, cartilage and muscles form the musculoskeletal system. Bone strength is a prerequisite for this function.
• protective - bones form a frame for vital internal organs. In addition, the bone itself is a container for the bone marrow, which performs hematopoietic and immune functions.
• metabolic - bone tissue is a depot of calcium and phosphorus in the body and plays an important role in maintaining a constant concentration of these elements in the blood

1. flat bones(skull bones, scapula, lower jaw, ilium)
2. tubular bones(long and short) (femur, humerus, lower leg and forearm bones)

   In long bones, two wide ends (epiphyses), a more or less cylindrical middle part (diaphysis) and a part of the bone where the diaphysis passes into the epiphysis (metaphysis) are distinguished. The metaphysis and epiphysis of long bones are separated by a layer of cartilage - the epiphyseal cartilage (the so-called growth pads).

3. voluminous bones(long, short, sesamoid)
4. mixed bones

The structural unit of the bone is the osteon or the Haversian system, i.e. a system of 20 or more concentrically arranged bone plates around the central canal, in which the vessels of the microvasculature, unmyelinated nerve fibers, lymphatic capillaries pass, accompanied by elements of loose fibrous connective tissue containing osteogenic cells, perivascular cells, osteoblasts and macrophages. Osteons do not adhere tightly to each other, between them there is an intercellular substance, together with which osteons form the main middle layer of bone substance, covered from the inside by the endosteum. The endosteum is a dynamic structure formed by a thin layer of connective tissue that includes bone-lining cells, osteogenic cells, and osteoclasts. In places of active osteogenesis under the layer of osteoblasts there is a thin layer of non-mineralized matrix - osteoid. The endosteum is surrounded by a cavity containing the bone marrow.

Outside, the bone substance is covered with a periosteum (periosteum), consisting of two layers: outer - fibrous and inner, adjacent to the surface of the bone - osteogenic or cambial, which is a source of cells during physiological and reparative regeneration of bone tissue. The periosteum is permeated with blood vessels that go from it to the bone substance in special channels called Volkmann's. The beginning of these channels is visible on the macerated bone in the form of numerous vascular openings. The vessels of the Haversian and Volkmann canals provide metabolism in the bone.

Bone tissue can be mature - lamellar and immature - reticulofibrous. Reticulofibrous bone tissue is represented mainly in the fetal skeleton; in adults - in places of attachment of tendons to bones, in overgrown sutures of the skull bones, as well as in bone regenerate during fracture consolidation.

Lamellar tissue forms a compact or spongy (trabecular) bone substance. From a compact substance, for example, the diaphyses of tubular bones are built. The trabecular substance forms the epiphyses of tubular bones, fills flat, mixed and voluminous bones. The spaces surrounding these trabeculae are filled with bone marrow, as are the cavities of the diaphysis.

Both compact and spongy substance have an osteon structure. The difference lies in the osteon organization.

Morphologically, the composition of bone tissue includes cellular elements and intercellular substance (bone matrix). Cellular elements occupy a small volume.

osteoblasts are large cells with basophilic cytoplasm. Active synthesizing osteoblasts are cuboidal or cylindrical cells with thin processes. The main enzyme of osteoblasts is alkaline phosphatase (AP). Active osteoblasts cover 2-8% of the bone surface, inactive (resting cells) - 80-92%, forming a continuous cell layer near the sinus of the medullary canal. The main function of osteoblasts is protein synthesis. They form osteoid plates by deposition of collagen fibers and proteoglycans. 1-2 microns of osteoid (newly formed non-calcified bone tissue) is deposited daily. After 8-9 days, the final thickness of this layer reaches 12 microns. After ten days of maturation, mineralization begins from the side opposite to the osteoblast, the mineralization front moves in the direction of the osteoblast. At the end of the cycle, every tenth osteoblast is immured as an osteocyte. The remaining osteoblasts remain on the surface as inactive. They are involved in bone metabolism.

osteoclasts- giant multinucleated cells (4-20 nuclei). They are usually in contact with calcified bony surfaces and within gauspinal lacunae resulting from their own resorptive activity. The main enzyme is acid phosphatase. Osteoclasts are mobile cells. They surround the part of the bone that needs to be resorbed. Their life expectancy is from 2 to 20 days. The main function of osteoclasts is the resorption of bone tissue due to lysosomal enzymes in the brush border area.

Osteocytes- metabolic inactive bone cells. They are found in small osteocytic lacunae deeply embedded in the bone. Osteocytes originate from osteoblasts immured in their own bone matrix, which later calcifies. These cells have numerous long processes in order to contact the cell processes of other osteocytes. They form a network of thin tubules extending to the entire bone matrix. The main role of osteocytes is the intracellular and extracellular transport of nutrients and minerals.

19 types of collagen proteins have been identified (Kadurina T.I., 2000). Collagen isoforms differ in amino acid composition, immunological, chromatographic properties, macromolecular organization and distribution in tissues. In morphofunctional terms, all isoforms are divided into interstitial collagens (types I, II, III, V), which form large fibrils; non-fibrillar (minor) collagens (IV, VI-XIX types), forming small fibrils and lining the basement membranes. Collagen types I and V are called pericellular. They are deposited around cells, forming support structures. For bone tissue, type I collagen is most characteristic.

The collagen molecule consists of three alpha chains wrapped one around the other and forming a dextrorotatory helix. Alpha chains are built from frequently repeated fragments with a characteristic triplet sequence -Gly-X-Y. The X position is often occupied by proline (Pro) or 4-hydroxyproline (4Hyp), Y by hydroxylysine, and the third position is always occupied by glycine, which ensures dense packing of three polypeptide chains into a fibril.

The terminal sections of the alpha chains at the N- and C-termini of the molecules are telopeptides (PINP and PICP, respectively). The arrangement of glycine here is disordered, as a result of which there is no tightly packed triple helix in this part of the molecule.

Telopeptides are involved in the mechanism of polymerization of molecules into fibrils, the formation of intermolecular cross-links, which are trivalent pyridinolines, which are released during bone resorption, and in the manifestation of the antigenic properties of collagen.

The level of released PINP and PICP can indirectly judge the ability of osteoblasts to synthesize type I collagen, since one molecule of collagen and one N- and C-terminal telopeptide are formed from one procollagen molecule. For the quantitative determination of PINP and PICP, radioimmunoassay and enzyme immunoassay methods have been developed (Taubman M.B., Goldberg B., Sherr C., 1974; Pedersen B.J., Bonde M., 1994). The clinical significance of these indicators is discussed (Linkhart S.G., et al., 1993; Mellko J., et al., 1990; Mellko J., et al., 1996).

Collagen formation includes two stages.

1. At the first stage, intracellular synthesis by osteoblasts of the collagen precursor, procollagen, takes place. The synthesized procollagen chain undergoes intracellular post-translational modification with hydroxylation of proline and lysine, and glycosylation of hydroxylysine residues in the collagen structure. Three procollagen chains form a procollagen molecule. Assembly of procollagen occurs with the formation of disulfide bonds in the C-terminal regions, after which a three-chain structure is formed, twisted together into a spiral. Such a molecule is secreted by osteoblasts into the extracellular space.
2. After secretion, tropocollagen, the monomer of collagen, is assembled in the extracellular space. At the same time, under the influence of extracellular lysine oxidase, interfibrillar cross-links characteristic of mature collagen are formed - pyridinoline bridges, resulting in the formation of collagen fibrils.

The rest of the organic part of the bone matrix can be classified into:

• non-collagen proteins that carry out cell adhesion (fibronectin, thrombospondin, osteopontin, bone sialoprotein). The same proteins are able to intensively bind with calcium and participate in the mineralization of bone tissue;
• glycoproteins (alkaline phosphatase, osteonectin);
• proteoglycans (acid polysaccharides and glycosaminoglycans - chondroitin sulfate and heparan sulfate);
• non-collagen gamma carboxylated (Gla) proteins (osteocalcin, Gla matrix protein (MGP));
• growth factors (fibroblast growth factor, transforming growth factors, bone morphogenetic proteins) - cytokines secreted by bone tissue and blood cells, which carry out local regulation of osteogenesis.

Alkaline phosphatase (AP). The synthesis of this protein is considered one of the most characteristic properties of osteoblastic cells. However, it should be borne in mind that this enzyme has several isoforms (bone, liver, intestinal, placental). The exact mechanism of action of alkaline phosphatase has not been established. It is assumed that this enzyme cleaves phosphate groups from other proteins, thereby increasing the local concentration of phosphorus; he is also credited with destroying a mineralization inhibitor, pyrophosphate. The half-life in the blood is 1-2 days, excreted by the kidneys (Coleman J.E., 1992). The determination of the activity of the bone fraction of ALP is more specific than the determination of the activity of total ALP in the blood, since an increase in the latter may be associated with an increase in the amount of other isoenzymes. A significant increase in the amount of bone ALP in serum / plasma is observed with bone growth, Paget's disease, hyperparathyroidism, osteomalacia and is associated with a high intensity of osteogenesis (Defton L.J., Wolfert R.L., Hill C.S., 1990; Moss D.W., 1992). The most adequate methods for determining the activity of bone ALP are enzyme immunoassay and chromatography (Hill C.S., Grafstein E., Rao S., Wolfert R.L., 1991; Gomez B.Jr., et al., 1995; Hata K., et al., 1996 ).

Osteonectin- a glycoprotein of bone and dentin, has a high affinity for type I collagen and hydroxyapatite, contains Ca-binding domains. It maintains the concentration of Ca and P in the presence of collagen. It is assumed that the protein is involved in the interaction of the cell and the matrix.

osteopontin- phosphorylated sialoprotein. Its determination by IHC methods can be used to characterize the protein composition of the matrix, in particular interfaces, where it is the main component and accumulates in the form of a dense cover called cementation lines (lamina limitans). Due to its physicochemical properties, it regulates the calcification of the matrix, specifically participates in the adhesion of cells to the matrix or matrix to the matrix. The production of osteopontin is one of the earliest manifestations of osteoblast activity.

Osteocalcin- a small protein is most widely represented in the bone matrix. Participates in the process of calcification, serves as a marker for assessing the activity of bone tissue metabolism, accounting for 15% of extractable non-collagen proteins. Consists of 49 amino acid residues, three of which are calcium-binding. Synthesized and secreted osteocalcin on osteoblasts. Its synthesis at the level of transcription controls calcitriol (1,25 - dihydroxycholecalciferol), in addition, in the process of "maturation" in osteoblasts, vitamin K-dependent carboxylation of three glutamic acid residues undergoes. A protein similar to osteocalcin, bone gla protein (BGP), contains 5 glutamic acid residues. In the extracellular matrix, carboxylated carboxyglutamic acid residues are able to bind ionized Ca 2+ and, thus, osteocalcin is strongly associated with hydroxyapatite (Price P.A., Williamson M.K., Lothringer J.W., 1981). 90% of the protein is bound. 10% of the newly synthesized osteocalcin immediately diffuses into the blood, where it can be detected. Osteocalcin circulating in the peripheral blood is a sensitive marker of bone metabolism, and its determination is of diagnostic value in osteoporosis, hyperparathyroidism and osteodystrophy (Charhon S.A., et al., 1986; Edelson G.W., Kleevehoper M., 1998). During osteoclastic resorption, osteocalcin from the bone matrix is ​​released into the blood in the form of polypeptide fragments. As a result, metabolites of α-carboxyglutamic acid appear in the urine. Thus, an increase in total serum osteocalcin reflects the activation of osteogenesis.

Bone morphogenetic proteins (BMPs)- cytokines belonging to the main subclass of transforming growth factors. It is known that they are able to induce bone tissue growth, namely, to influence the proliferation and differentiation of four types of cells - osteoblasts, osteoclasts, chondroblasts and chondrocytes. In addition, morphogenetic proteins block myogenesis and adipogenesis. It has been shown that osteoblasts and bone marrow stromal cells express BMP type I and II receptors. Treatment of their BMP for 4 weeks causes matrix mineralization, an increase in alkaline phosphatase activity and mRNA concentration. It has been shown that BMP is distributed along the collagen fibers of the bone tissue, in the cells of the osteogenic layer of the periosteum; in moderate amounts it is present in the cells of the lamellar bone and is present in excess in the tissues of the tooth.

Proteoglycans- this is a class of macromolecules with a molecular weight of 70-80 kDa, consisting of a core protein, with which chains of glycosaminoglycans (GAGs) are covalently linked, the latter consist of repeating disaccharide subunits: chondroitin, dermatan, keratan, heparan (Fig. 9). GAGs are divided into two groups - non-sulfated (hyaluronic acid, chondroitin) and sulfated (heparan sulfate, dermatan sulfate, keratan sulfate).

The bone matrix is ​​formed by collagen fibrils oriented in one direction. They make up 90% of all bone proteins. Fusiform and lamellar hydroxyapatite crystals are found on collagen fibers, within them and in the surrounding space. As a rule, they are oriented in the same direction as the collagen fibers. The ground substance consists of glycoproteins and proteoglycans. These highly ionized complexes have a pronounced ion-binding ability and therefore play an important role in the calcification and fixation of hydroxyapatite crystals to collagen fibers. Bone collagen is represented by type 1 collagen, and type II, V, XI collagens are found only in trace amounts. Numerous non-collagen proteins are also present in the bone matrix. Most of them are synthesized by bone-forming cells. Their function is not clear enough, but it has been established that the level of these proteins decreases as the matrix matures.

Calcium. Calcium enters the body with food. Its consumption is 0.9 (in women) - 1.1 (in men) g / day, and absorption is from 0.12 to 0.67 g / day. More than 90% of the calcium in the body is found in bone tissue. Plasma calcium concentration is about 10 mg/100 ml. Daily fluctuations do not exceed 3%. About 40% are associated with the protein, and only half are in the ionized form. Calcium ions are a key regulator of cellular metabolism, therefore the level of ionized calcium is strictly controlled and considered as a physiological constant (Brikman A., 1999). Every day, 10 mmol (0.4 g) of calcium enters the bones and the same amount leaves the skeleton, thus maintaining a stable level of calcium in the blood. The regulation of this process is carried out by three organs - the intestines, kidneys, bones and three main hormones - parathyroid, calcitriol, calcitonin.

Dietary calcium is absorbed in the small intestine through two independent processes. The first is a saturable (transcellular) pathway regulated by vitamin D and occurs mainly in the initial section of the small intestine (Heath D., Marx S.J., 1982). The second process - unsaturated - is a passive diffusion of calcium from the intestinal lumen into the blood and lymph. The amount absorbed in this way depends linearly on the amount of dissolved calcium in the gut. This process is not subject to direct endocrine regulation. The combined action of the two mechanisms provides an increase in the effective absorption of calcium during periods of high physiological need, with a low content of calcium in products. In addition, calcium absorption depends on age (Brazier M., 1995). In the first days after birth, almost all the calcium received is absorbed, and calcium absorption remains high during the growth period. A marked decrease in calcium absorption occurs after 60 years of age. The amount of available calcium also depends on the diet, since phosphates, oxalates, and fats bind calcium. Insoluble salts with calcium are formed by phytic acid, a large amount of which is found in wheat flour. Calcium absorption is increased by a high-calorie protein diet and growth hormone. With thyrotoxicosis, a negative calcium balance can be observed. Poor absorption of calcium contribute to acute and chronic kidney disease, gastrectomy, resection of large segments of the small intestine, bowel disease.

The kidneys play the most important role in the metabolism of this cation. 97-99% of the filtered calcium is reabsorbed and no more than 5 mmol / day (0.2 g / day) is excreted in the urine. Sodium balance also influences calcium excretion by the kidneys. Sodium chloride infusion or increased dietary sodium intake increases urinary calcium excretion (Nordin B.E.C., 1984).

Phosphorus. About 80% of phosphorus in the human body is associated with calcium and forms the inorganic basis of bones and serves as a reservoir of phosphorus (Dolgov V.V., Ermakova I.P., 1998). Intracellular phosphorus is represented by high-energy compounds, it is acid-soluble phosphorus. Phosphorus is also an integral part of phospholipids - the main structural components of membranes.

The daily intake of phosphorus is 0.6-2.8 g (Moskalev Yu.I., 1985). Usually about 70% of dietary phosphorus is absorbed, and this process depends on the calcium content of foods and the formation of insoluble salts. Phosphorus and calcium form poorly soluble compounds, therefore their total concentration does not exceed a certain level and an increase in one of them is usually accompanied by a decrease in the other (Pak C.Y.C., 1992). The high content of magnesium, iron and aluminum in food also reduces the absorption of phosphorus. Vitamin D and lipids, on the contrary, contribute to the absorption of phosphorus.

In plasma, inorganic phosphorus is contained in the form of HPO4-2 and H2PO4- anions, their total amount is 1-2 mM. About 95% are free anions, 5% are bound to the protein.

In renal failure, a decrease in glomerular filtration by 20% relative to the norm causes hyperphosphatemia. As a result, the synthesis of calcitriol and the absorption of calcium in the intestine are reduced (Rowe P.S., 1994). Tissue catabolism is a common cause of hyperphosphatemia in patients with diabetic ketoacidosis. The causes of hypophosphatemia are vitamin D deficiency, malabsorption syndrome, primary and secondary hyperparathyroidism, diabetic ketoacidosis (recovery phase), renal tubular insufficiency, renal tubular insufficiency, renal tubular insufficiency, alcoholic delirium, alkalosis, hypomagnesemia. Normal tubular reabsorption is 83-95%. The decrease in tubular reabsorption of phosphate is due to an increase in the level of PTH or a primary defect in the reabsorption of phosphate in the renal tubules.

Magnesium. About half of the body's magnesium is found in the bones. It has been shown that the Mg-ATP complex is necessary for the functioning of the Ca-pump, which determines the level of impulsation of cells with the property of automation (Moskalev Yu.I., 1985; Ryan M.F., 1991). In plasma, magnesium is distributed in three fractions: free (ionized) - approximately 70-80%; associated (with albumin and other proteins) - 20-30%; fully connected (complex) - 1-2%. Physiologically active is ionized magnesium. An increase in the concentration of magnesium suppresses the secretion of PTH (Brown E.M., Chen C.J., 1989).

Hypomagnesemia is the most common cause of hypocalcemia (Mundy G.R., 1990). When magnesium is replenished, calcium levels quickly normalize. Magnesium deficiency can develop with hereditary absorption deficiencies, with alcoholism with malnutrition, impaired renal function, treatment with gentamicin, tobramycin, amikacin, cyclosporine, malnutrition. With magnesium deficiency, hypocalcemia develops due to a decrease in PTH secretion and the development of resistance of bone tissue and kidneys to PTH (Ryan M.F., 1991). Excretion of magnesium in the urine increases with an excess volume of extracellular fluid, hypercalcemia, hypermagnesemia and decreases in opposite situations.

Total magnesium is measured photometrically, ionized - using ion-selective electrodes. The values ​​of ionized magnesium depend on pH (Ryan M.F., 1991).

The growth of bones in length occurs due to the presence of a metaepiphyseal cartilaginous growth plate in the transitional section between the diaphysis and the epiphysis. It has four zones. The surface, facing the epiphysis, is called the reserve zone. Following it, the formed cells form a zone of proliferation, the chondroblasts and chondrocytes located here are continuously dividing. Due to hypoxic conditions in the deep layers of this area, cells experience oxygen starvation and hypertrophy. The totality of such chondrocytes forms the third zone - the zone of hypertrophied chondrocytes. Finally, metabolic disturbances lead to cell death. Dead chondrocytes with a mineralized matrix are observed in the area of ​​calcified cartilage. From the side of the diaphysis, a large number of vessels grow here. Under conditions of good oxygenation, osteogenic cells located near blood vessels differentiate into osteoblasts and form bone trabeculae. Since such a process occurs at both ends of the organ, the bone lengthens proportionally.