Types of human bone tissue. Bone tissues: classification, structure, functions. Cytofunctional characteristics of osteoblasts, osteocytes, osteoclasts. The structure of the bone as an organ. General characteristics of human bones

Each human bone is a complex organ: it occupies a certain position in the body, has its own shape and structure, and performs its own function. All types of tissues take part in bone formation, but bone tissue predominates.

General characteristics of human bones

Cartilage covers only the articular surfaces of the bone, the outside of the bone is covered with periosteum, and the bone marrow is located inside. Bone contains adipose tissue, blood and lymphatic vessels, and nerves.

Bone has high mechanical properties, its strength can be compared with the strength of metal. The chemical composition of a living human bone contains: 50% water, 12.5% ​​organic substances of a protein nature (ossein), 21.8% inorganic substances (mainly calcium phosphate) and 15.7% fat.

Types of bones by shape divided into:

• Tubular (long - shoulder, femoral, etc.; short - phalanges of the fingers);
• flat (frontal, parietal, scapula, etc.);
• spongy (ribs, vertebrae);
• mixed (wedge-shaped, zygomatic, lower jaw).

The structure of human bones

The basic structural unit of bone tissue is osteon, which is visible under a microscope at low magnification. Each osteon includes from 5 to 20 concentrically arranged bone plates. They resemble cylinders inserted into each other. Each plate consists of intercellular substance and cells (osteoblasts, osteocytes, osteoclasts). In the center of the osteon there is a channel - the channel of the osteon; blood vessels run through it. Intercalated bone plates are located between adjacent osteons.

Bone is formed by osteoblasts, releasing the intercellular substance and immuring in it, they turn into osteocytes - cells of a process form, incapable of mitosis, with weakly expressed organelles. Accordingly, the formed bone contains mainly osteocytes, and osteoblasts are found only in areas of growth and regeneration of bone tissue.

The largest number of osteoblasts is located in the periosteum - a thin but dense connective tissue plate containing many blood vessels, nerve and lymph endings. The periosteum provides bone growth in thickness and nutrition of the bone.

osteoclasts contain a large number of lysosomes and are able to secrete enzymes, which can explain the dissolution of bone substance by them. These cells take part in the destruction of the bone. In pathological conditions in the bone tissue, their number increases sharply.

Osteoclasts are also important in the process of bone development: in the process of building the final shape of the bone, they destroy calcified cartilage and even newly formed bone, “correcting” its primary shape.

Bone structure: compact and spongy substance

On the cut, sections of the bone, two of its structures are distinguished - compact matter(bone plates are located densely and in an orderly manner), located superficially, and spongy substance(bone elements are located loosely), lying inside the bone.

Such a structure of bones fully corresponds to the basic principle of structural mechanics - to ensure the maximum strength of the structure with the least amount of material and great ease. This is also confirmed by the fact that the location of the tubular systems and the main bone beams corresponds to the direction of action of the forces of compression, tension and twisting.

The structure of bones is a dynamic reactive system that changes throughout a person's life. It is known that in people engaged in heavy physical labor, the compact layer of bone reaches a relatively large development. Depending on the change in the load on individual parts of the body, the location of the bone beams and the structure of the bone as a whole may change.

Connection of human bones

All bone joints can be divided into two groups:

• Continuous connections, earlier in development in phylogenesis, immobile or inactive in function;
• intermittent connections, later in development and more mobile in function.

Between these forms there is a transition - from continuous to discontinuous or vice versa - semi-joint.

The continuous connection of the bones is carried out through connective tissue, cartilage and bone tissue (the bones of the skull itself). A discontinuous connection of bones, or a joint, is a younger formation of a connection between bones. All joints have a common structural plan, including the articular cavity, articular bag and articular surfaces.

Articular cavity it is allocated conditionally, since normally there is no void between the articular bag and the articular ends of the bones, but there is liquid.

Articular bag covers the articular surfaces of the bones, forming a hermetic capsule. The articular bag consists of two layers, the outer layer of which passes into the periosteum. The inner layer secretes a fluid into the joint cavity, which plays the role of a lubricant, ensuring the free sliding of the articular surfaces.

Types of joints

The articular surfaces of the articulating bones are covered with articular cartilage. The smooth surface of the articular cartilage promotes movement in the joints. The articular surfaces are very diverse in shape and size, they are usually compared with geometric figures. Hence and names of joints according to shape: spherical (shoulder), elliptical (radio-carpal), cylindrical (radio-ulnar), etc.

Since the movements of the articulating links are made around one, two or many axes, joints are also usually divided by the number of axes of rotation into multiaxial (spherical), biaxial (elliptical, saddle) and uniaxial (cylindrical, block-shaped).

Depending on the number of articulating bones joints are divided into simple, in which two bones are connected, and complex, in which more than two bones are articulated.

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Bone represents a very perfect specialized variety of tissues of the internal environment.

This system harmoniously combines such opposite properties as mechanical strength and functional plasticity, processes of neoformation and destruction.

Bone tissue consists of cells and intercellular substance, which are characterized by a certain histoarchitectonics. The main cells of bone tissue are osteoblasts, osteocytes and osteoclasts.

osteoblasts are oval or cubic in shape. A large light nucleus is not located in the center, it is somewhat shifted to the periphery of the cytoplasm. Often, several nucleoli are found in the nucleus, which indicates a high synthetic activity of the cell.

Electron microscopic studies have shown that a significant part of the osteoblast cytoplasm is filled with numerous ribosomes and polysomes, tubules of the granular endoplasmic reticulum, the Golgi complex, mitochondria, and special matrix vesicles. Osteoblasts have proliferative activity, are producers of intercellular substance and play a major role in the mineralization of the bone matrix. They synthesize and secrete chemical compounds such as alkaline phosphatase, collagens, osteonectin, osteopontin, osteocalcin, bone morphogenetic proteins, etc. The matrix vesicles of osteoblasts contain numerous enzymes that, released outside the cell, initiate bone mineralization processes.

The organic matrix of bone tissue synthesized by osteoblasts consists mainly (90-95%) of type I collagen, collagens III-V and other types, as well as non-collagen proteins (osteocalcin, osteopontin, osteonectin, phosphoproteins, bone morphogenetic proteins) and glycosaminoglycan substances. Proteins of non-collagenous nature have the properties of mineralization regulators, osteoinductive substances, mitogenic factors, regulators of the rate of formation of collagen fibrils. Thrombospondin promotes adhesion of osteoblasts to the subperiosteal osteoid of human bone. Osteocalcin is considered a potential indicator of the function of these cells.

The ultrastructure of osteoblasts indicates that their functional activity is different. Along with functionally active osteoblasts with high synthetic activity, there are inactive cells. Most often they are localized on the periphery of the bone from the side of the medullary canal and are part of the periosteum. The structure of such cells is characterized by a low content of organelles in the cytoplasm.

Osteocytes are more differentiated cells than osteoblasts. They have a process shape.

The processes of osteocytes are located in the tubules penetrating the mineralized bone matrix in various directions. The flattened bodies of osteocytes are located in special cavities - lacunae - and are surrounded on all sides by a mineralized bone matrix. A significant part of the cytoplasm of the osteocyte is occupied by the ovoid nucleus. Synthesis organelles in the cytoplasm are poorly developed: there are a few polysomes, short tubules of the endoplasmic reticulum, and single mitochondria. Due to the fact that the tubules of neighboring lacunae anastomose with each other, the processes of osteocytes are connected to each other using specialized gap junctions. In a small space around the bodies and processes of osteocytes, tissue fluid circulates, containing a certain concentration of Ca 2+ and PO 4 3-, may contain non-mineralized or partially mineralized collagen fibrils.

The function of osteocytes is to maintain the integrity of the bone matrix by participating in the regulation of bone mineralization and providing a response to mechanical stimuli. Currently, more and more data is accumulating that these cells are actively involved in the metabolic processes occurring in the intercellular substance of the bone, in maintaining a constant ionic balance in the body. The functional activity of osteocytes largely depends on the stage of their life cycle and the action of hormonal and cytokine factors.

osteoclasts- These are large multinucleated cells with a sharply oxyphilic cytoplasm. They are part of the phagocytic-macrophage system of the body, derivatives of blood monocytes.

On the periphery of the cell, a corrugated brush border is determined. In the cytoplasm, many ribosomes and polysomes, mitochondria, tubules of the endoplasmic reticulum are found, the Golgi complex is well developed. A distinctive feature of the ultrastructure of osteoclasts is the presence of a large number of lysosomes, phagosomes, vacuoles and vesicles.

Osteoclasts have the ability to create an acidic environment locally near their surface as a result of intensive glycolysis processes in these cells. The acidic environment in the area of ​​direct contact between the cytoplasm of osteoclasts and the intercellular substance promotes the dissolution of mineral salts and creates optimal conditions for the action of proteolytic and a number of other enzymes of lysosomes. The cytochemical marker of osteoclasts is the activity of an isoenzyme of acid phosphatase called acid nitrophenyl phosphatase. The functions of osteoclasts are the resorption (destruction) of bone tissue and participation in the process of remodulation of bone structures during embryonic and postnatal development.

The intercellular substance of bone tissues consists of organic and inorganic components. Organic compounds are represented by collagen types I, III, IV, V, IX, XIII (about 95%), non-collagen proteins (bone morphogenetic proteins, osteocalcin, osteopontin, thrombospondin, bone sialoprotein, etc.), glycosaminoglycans and proteoglycans. The inorganic part of the bone matrix is ​​represented by hydroxyapatite crystals containing large amounts of calcium and phosphorus ions; in much smaller quantities, it contains salts of magnesium, potassium, fluorides, bicarbonates.

The intercellular substance of the bone is constantly updated. The destruction of the old intercellular substance is a rather complex and not yet clear in many details process, in which all types of bone tissue cells and a number of humoral factors take part, but osteoclasts play a particularly noticeable and important role.

Bone Types

Depending on the microscopic structure, two main types of bone tissue are distinguished - reticulofibrous (coarse-fibrous) and lamellar.

Reticulofibrous bone tissue It is widely represented in embryogenesis and early postnatal histogenesis of the bones of the skeleton, and in adults it occurs at the sites of attachment of tendons to bones, along the line of overgrowth of cranial sutures, and also in the area of ​​fractures.

Both in embryogenesis and during regeneration, reticulofibrous bone tissue is always replaced by lamellar bone over time. Characteristic in the structure of reticulofibrous bone tissue is a disordered, diffuse arrangement of bone cells in the intercellular substance. Powerful bundles of collagen fibers are weakly mineralized and go in different directions. The density of osteocytes in reticulofibrous bone tissue is higher than in lamellar tissue, and they do not have a specific orientation in relation to collagen (ossein) fibers.

lamellar bone tissue is the main tissue in the composition of almost all human bones. In this type of bone tissue, the mineralized intercellular substance forms special bone plates 5-7 microns thick.

Each bone plate is a collection of closely spaced parallel collagen fibers impregnated with hydroxyapatite crystals. In neighboring plates, the fibers are located at different angles, which gives the bone additional strength. Between the bone plates in the lacunae, bone cells - osteocytes - lie in an orderly manner. The processes of osteocytes through the bone tubules penetrate into the surrounding plates, entering into intercellular contacts with other bone cells. There are three systems of bone plates: surrounding (general, they are external and internal), concentric (included in the structure of the osteon), intercalary (they are the remains of collapsing osteons).

In the composition of the bone, a compact and spongy substance is distinguished. Both of them are formed by lamellar bone tissue. Features of the histoarchitectonics of the lamellar bone will be presented below when describing the bone as an organ.

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Cartilaginous tissue (textus cartilaginus) forms articular cartilages, intervertebral discs, cartilages of the larynx, trachea, bronchi, external nose. Cartilage tissue consists of cartilage cells (chondroblasts and chondrocytes) and a dense, elastic intercellular substance.

Cartilaginous tissue contains about 70-80% water, 10-15% organic matter, 4-7% salts. About 50-70% of the dry matter of cartilage tissue is collagen. The intercellular substance (matrix) produced by cartilage cells consists of complex compounds, which include proteoglycans. hyaluronic acid, glycosaminoglycan molecules. There are two types of cells in the cartilaginous tissue: chondroblasts (from the Greek chondros - cartilage) and chondrocytes.

Chondroblasts are young, capable of mitotic division, rounded or ovoid cells. They produce components of the intercellular substance of cartilage: proteoglycans, glycoproteins, collagen, elastin. The cytolemma of chondroblasts forms many microvilli. The cytoplasm is rich in RNA, a well-developed endoplasmic reticulum (granular and non-granular), the Golgi complex, mitochondria, lysosomes, and glycogen granules. The chondroblast nucleus, rich in active chromatin, has 1-2 nucleoli.

Chondrocytes are mature large cartilage cells. They are round, oval or polygonal, with processes, developed organelles. Chondrocytes are located in cavities - lacunae, surrounded by intercellular substance. If there is one cell in the gap, then such a gap is called primary. Most often, the cells are located in the form of isogenic groups (2-3 cells) occupying the cavity of the secondary lacuna. The walls of the lacunae consist of two layers: the outer one, formed by collagen fibers, and the inner one, consisting of aggregates of proteoglycans that come into contact with the glycocalyx of cartilage cells.

The structural and functional unit of cartilage is the chondron, formed by a cell or an isogenic group of cells, a pericellular matrix, and a lacuna capsule.

Cartilage tissue is nourished by diffusion of substances from the blood vessels of the perichondrium. Nutrients enter the articular cartilage tissue from the synovial fluid or from the vessels of the adjacent bone. Nerve fibers are also localized in the perichondrium, from where individual branches of amyopiatic nerve fibers can penetrate into the cartilaginous tissue.

In accordance with the structural features of the cartilage tissue, there are three types of cartilage: hyaline, fibrous and elastic cartilage.

hyaline cartilage, from which the cartilages of the respiratory tract, the thoracic ends of the ribs and the articular surfaces of the bones are formed in humans. In a light microscope, its main substance appears to be homogeneous. Cartilage cells or their isogenic groups are surrounded by an oxyphilic capsule. In differentiated areas of cartilage, a basophilic zone adjacent to the capsule and an oxyphilic zone located outside of it are distinguished; Together, these zones form a cellular territory, or chondrin ball. A complex of chondrocytes with a chondrin ball is usually taken as a functional unit of cartilage tissue - a chondron. The ground substance between chondrons is called interterritorial spaces.
Elastic cartilage(synonym: mesh, elastic) differs from hyaline by the presence of branching networks of elastic fibers in the main substance. The cartilage of the auricle, epiglottis, vrisberg and santorin cartilages of the larynx are built from it.
fibrocartilage(a synonym for connective tissue) is located at the transition points of dense fibrous connective tissue into hyaline cartilage and differs from the latter by the presence of real collagen fibers in the ground substance.

7. Bone tissue - location, structure, functions

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:

1) support;

2) mechanical;

3) protective (mechanical protection);

4) participation in the mineral metabolism of the body (depot of calcium and phosphorus).

Bone cells - osteoblasts, osteocytes, osteoclasts. The main cells in the formed bone tissue are osteocytes. These are process-shaped cells with a large nucleus and weakly expressed cytoplasm (nuclear-type cells). The cell bodies are localized in the bone cavities (lacunae), and the processes are located in the bone tubules. Numerous bone tubules, anastomosing with each other, penetrate the bone tissue, communicating with the perivascular space, form the drainage system of the bone tissue. This drainage system contains tissue fluid, through which the exchange of substances is ensured not only between cells and tissue fluid, but also in the intercellular substance.

Osteocytes are definitive forms of cells and do not divide. They are formed from osteoblasts.

osteoblasts found only in developing bone tissue. In the formed bone tissue, they are usually contained in an inactive form in the periosteum. In developing bone tissue, osteoblasts surround each bone plate along the periphery, tightly adhering to each other.

The shape of these cells can be cubic, prismatic and angular. The cytoplasm of osteoblasts contains a well-developed endoplasmic reticulum, the Golgi lamellar complex, many mitochondria, which indicates a high synthetic activity of these cells. Osteoblasts synthesize collagen and glycosaminoglycans, which are then released into the extracellular space. Due to these components, an organic matrix of bone tissue is formed.

These cells provide mineralization of the intercellular substance through the release of calcium salts. Gradually releasing the intercellular substance, they seem to be walled up and turn into osteocytes. At the same time, intracellular organelles are significantly reduced, synthetic and secretory activity is reduced, and the functional activity characteristic of osteocytes is preserved. Osteoblasts localized in the cambial layer of the periosteum are in an inactive state; synthetic and transport organelles are poorly developed in them. When these cells are irritated (in case of injuries, bone fractures, etc.), a granular ER and a lamellar complex rapidly develop in the cytoplasm, active synthesis and release of collagen and glycosaminoglycans, the formation of an organic matrix (bone callus), and then the formation of a definitive bone fabrics. In this way, due to the activity of osteoblasts of the periosteum, bones regenerate when they are damaged.

osteoclasts- bone-destroying cells are absent in the formed bone tissue, but 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 ontogeny, osteoclasts are also necessarily present in these places. In the process of embryonic osteohistogenesis, these cells play a very important role and are present in large numbers. Osteoclasts have a characteristic morphology: these cells are multinucleated (3-5 or more nuclei), have a rather large size (about 90 microns) and a characteristic shape - oval, but the part of the cell adjacent to the bone tissue has a flat shape. In the flat part, two zones can be distinguished: the central (corrugated part, containing numerous folds and processes), and the peripheral part (transparent) in close contact with the bone tissue. In the cytoplasm of the cell, under the nuclei, there are numerous lysosomes and vacuoles of various sizes.

The functional activity of the osteoclast is manifested as follows: in the central (corrugated) zone of the cell base, carbonic acid and proteolytic enzymes are released from the cytoplasm. The released carbonic acid causes demineralization of bone tissue, and proteolytic enzymes destroy the organic matrix of the intercellular substance. Fragments of collagen fibers are phagocytosed by osteoclasts and destroyed intracellularly. Through these mechanisms, resorption (destruction) of bone tissue occurs, and therefore osteoclasts are usually localized in the depressions of bone tissue. After the destruction of bone tissue due to the activity of osteoblasts, which are evicted from the connective tissue of the vessels, a new bone tissue is built.

intercellular substance bone tissue consists of the main (amorphous) substance and fibers, which contain calcium salts. The fibers consist of collagen and are folded into bundles, which can be arranged in parallel (orderly) or randomly, on the basis of which the histological classification of bone tissues is built. The main substance of bone tissue, as well as other types of connective tissues, consists of glycosamino- and proteoglycans.

The bone tissue contains less chondroitin sulfuric acids, but more citric and others, which form complexes with calcium salts. In the process of bone tissue development, an organic matrix is ​​first formed - the main substance and collagen fibers, and then calcium salts are deposited in them. They form crystals - hydroxyapatites, which are deposited both in an amorphous substance and in fibers. Providing bone strength, calcium phosphate salts are also both a depot of calcium and phosphorus in the body. Thus, bone tissue takes part in the mineral metabolism of the body.

When studying bone tissue, one should also clearly separate the concepts of “bone tissue” and “bone”.

Bone is an organ whose main structural component is bone tissue.

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 attraction of the Earth) the destruction of bone tissue by osteoclasts occurs, which is prevented in astronauts by physical exercises.

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).

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, are transformed 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.