The composition of cartilage. Cartilaginous connective tissue in humans

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cartilage tissue , like bone, refers to skeletal tissues with a musculoskeletal function. According to the classification, three types of cartilage tissue are distinguished - hyaline, elastic and fibrous. Structural features various kinds cartilage depends on its location in the body, mechanical conditions, age of the individual.

Types of cartilage tissue: 1 - hyaline cartilage; 2 - elastic cartilage; 3 - fibrous cartilage

Connective tissues also include cartilage and bone tissue, from which the skeleton of the human body is built. These tissues are called skeletal. Organs built from these tissues perform the functions of support, movement, and protection. They are also involved in mineral metabolism.

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.

Cartilage 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.

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

Hyaline cartilage (from the Greek hyalos - glass) has a bluish color. In its main substance are thin collagen fibers. Cartilage cells have a variety of shapes and structures, depending on the degree of differentiation and their location in the cartilage. Chondrocytes form isogenic groups. Articular, costal cartilages and most of the cartilages of the larynx are built from hyaline cartilage.

Fibrous cartilage, the main substance of which contains a large amount of thick collagen fibers, has increased strength. Cells located between collagen fibers have an elongated shape, they have a long rod-shaped nucleus and a narrow rim of basophilic cytoplasm. Fibrous rings of intervertebral discs, intra-articular discs and menisci are built from fibrous cartilage. This cartilage covers the articular surfaces of the temporomandibular and sternoclavicular joints.

Elastic cartilage is elastic and flexible. In the matrix of elastic cartilage, along with collagen, there are a large number of complexly intertwined elastic fibers. Rounded chondrocytes are located in lacunae. The epiglottis, the sphenoid and corniculate cartilages of the larynx, the vocal process of the arytenoid cartilages, the cartilage of the auricle, and the cartilaginous part of the auditory tube are built from elastic cartilage.

Bone tissue (textus ossei) is distinguished by special mechanical properties. It consists of bone cells immured in the bone ground substance containing collagen fibers and impregnated with inorganic compounds. There are three types of bone cells: osteoblasts, osteocytes and osteoclasts.

Osteoblasts are sprout young bone cells of a polygonal, cubic shape. Osteoblasts are rich in elements of the granular endoplasmic reticulum, ribosomes, a well-developed Golgi complex, and a sharply basophilic cytoplasm. They lie in the superficial layers of the bone. Their round or oval nucleus is rich in chromatin and contains one large nucleolus, usually located on the periphery. Osteoblasts are surrounded by thin collagen microfibrils. Substances synthesized by osteoblasts are secreted through their entire surface in various directions, which leads to the formation of walls of gaps in which these cells lie. Osteoblasts synthesize components of the intercellular substance (collagen is a component of proteoglycan). In the intervals between the fibers there is an amorphous substance - osteoid tissue, or ancestor, which then calcifies. The organic matrix of the bone contains hydroxyapatite crystals and amorphous calcium phosphate, the elements of which enter the bone tissue from the blood through the tissue fluid.

Osteocytes are mature, multi-processed, spindle-shaped bone cells with a large rounded nucleus, in which the nucleolus is clearly visible. The number of organelles is small: mitochondria, elements of the granular endoplasmic reticulum and the Golgi complex. Osteocytes are located in the lacunae, however, the cell bodies are surrounded by a thin layer of the so-called bone fluid (tissue) and do not come into direct contact with the calcified matrix (lacunae walls). Very long (up to 50 μm) processes of osteocytes, rich in actin-like microfilaments, pass through the bone tubules. The processes are also separated from the calcified matrix by a space about 0.1 µm wide, in which tissue (bone) fluid circulates. Due to this fluid, nutrition (trophic) of osteocytes is carried out. The distance between each osteocyte and the nearest blood capillary does not exceed 100-200 microns.

Osteoclasts are large multinucleated (5-100 nuclei) cells of monocytic origin, up to 190 microns in size. These cells destroy bone and cartilage, carry out resorption bone tissue in the process of its physiological and reparative regeneration. Osteoclast nuclei are rich in chromatin and have well-visible nucleoli. The cytoplasm contains many mitochondria, elements of the granular endoplasmic reticulum and the Golgi complex, free ribosomes, and various functional forms of lysosomes. Osteoclasts have numerous villous cytoplasmic processes. There are especially many such processes on the surface adjacent to the destroyed bone. This is a corrugated, or brush, border that increases the area of ​​​​contact of the osteoclast with the bone. Osteoclast processes also have microvilli, between which are hydroxyapatite crystals. These crystals are found in the phagolysosomes of osteoclasts, where they are destroyed. The activity of osteoclasts depends on the level of parathyroid hormone, an increase in the synthesis and secretion of which leads to the activation of osteoclast function and bone destruction.

There are two types of bone tissue - reticulofibrous (coarse-fibrous) and lamellar. Coarse fibrous bone tissue is present in the embryo. In an adult, it is located in the areas of attachment of the tendons to the bones, in the sutures of the skull after their overgrowth. Rough fibrous bone tissue contains thick disordered bundles of collagen fibers, between which there is an amorphous substance.

Lamellar bone tissue is formed by bone plates with a thickness of 4 to 15 microns, which consist of osteocytes, ground substance, and thin collagen fibers. The fibers (collagen type I) involved in the formation of bone plates lie parallel to each other and are oriented in a certain direction. At the same time, the fibers of neighboring plates are multidirectional and intersect almost at a right angle, which ensures greater bone strength.

Cartilage tissue is a special type of connective tissue and performs a supporting function in the formed organism. IN maxillofacial region cartilage is part of the auricle, auditory tube, nose, articular disc of the temporomandibular joint, and also provides a connection between the small bones of the skull.

Depending on the composition, metabolic activity and ability to regenerate, there are three types of cartilage tissue - hyaline, elastic and fibrous.

hyaline cartilage is formed first at the embryonic stage of development, and under certain conditions, the other two types of cartilage are formed from it. This cartilaginous tissue is found in the costal cartilages, the cartilaginous framework of the nose, and forms the cartilages that cover the surfaces of the joints. It has a higher metabolic activity compared to the elastic and fibrous types and contains a large amount of carbohydrates and lipids. This allows active protein synthesis and differentiation of chondrogenic cells to renew and regenerate hyaline cartilage. With age, hypertrophy and apoptosis of cells occur in hyaline cartilage, followed by calcification of the extracellular matrix.

Elastic cartilage has a similar structure to hyaline cartilage. From such cartilaginous tissue, for example, the auricles, the auditory tube and some cartilages of the larynx are formed. This type of cartilage is characterized by the presence of a network of elastic fibers in the cartilage matrix, a small amount of lipids, carbohydrates and chondroitin sulfates. Due to low metabolic activity, elastic cartilage does not calcify and practically does not regenerate.

fibrocartilage in its structure it occupies an intermediate position between the tendon and hyaline cartilage. characteristic feature fibrocartilage is the presence in the intercellular matrix a large number collagen fibers, mainly type I, which are located parallel to each other, and cells in the form of a chain between them. Fibrous cartilage, due to its special structure, can experience significant mechanical stress both in compression and in tension.

Cartilaginous component of the temporomandibular joint presented in the form of a disk of fibrous cartilage, which is located on the surface of the articular process of the lower jaw and separates it from the articular fossa of the temporal bone. Since fibrocartilage does not have a perichondrium, the cartilage cells are nourished through the synovial fluid. The composition of the synovial fluid depends on the extravasation of metabolites from the blood vessels of the synovial membrane into the joint cavity. The synovial fluid contains low-molecular components - Na + , K + ions, uric acid, urea, glucose, which are close in quantitative ratio to blood plasma. However, the content of proteins in the synovial fluid is 4 times higher than in the blood plasma. In addition to glycoproteins, immunoglobulins, synovial fluid is rich in glycosaminoglycans, among which hyaluronic acid, present in the form of sodium salt, occupies the first place.

2.1. STRUCTURE AND PROPERTIES OF CARTILAGE TISSUE

Cartilage tissue, like any other tissue, contains cells (chondroblasts, chondrocytes) that are embedded in a large intercellular matrix. In the process of morphogenesis, chondrogenic cells differentiate into chondroblasts. Chondroblasts begin to synthesize and secrete proteoglycans into the cartilage matrix, which stimulate the differentiation of chondrocytes.

The intercellular matrix of cartilage tissue provides its complex microarchitectonics and consists of collagens, proteoglycans, and non-collagen proteins - mainly glycoproteins. Collagen fibers are intertwined in a three-dimensional network that connects the rest of the matrix components.

The cytoplasm of chondroblasts contains a large amount of glycogen and lipids. The breakdown of these macromolecules in oxidative phosphorylation reactions is accompanied by the formation of ATP molecules necessary for protein synthesis. Proteoglycans and glycoproteins synthesized in the granular endoplasmic reticulum and the Golgi complex are packed into vesicles and released into the extracellular matrix.

The elasticity of the cartilage matrix is ​​determined by the amount of water. Proteoglycans are characterized by a high degree of water binding, which determines their size. The cartilage matrix contains up to 75%

water, which is associated with proteoglycans. A high degree of hydration determines the large size of the extracellular matrix and allows the cells to be nourished. Dried agrecan after binding water can increase in volume by 50 times, however, due to the limitations caused by the collagen network, the swelling of the cartilage does not exceed 20% of the maximum possible value.

When cartilage is compressed, water, together with ions, is displaced from the areas around the sulfated and carboxyl groups of the proteoglycan, the groups approach each other, and the repulsive forces between their negative charges prevent further tissue compression. After the load is removed, the electrostatic attraction of cations (Na +, K +, Ca 2+) occurs, followed by the influx of water into the intercellular matrix (Fig. 2.1).

Rice. 2.1.Water binding by proteoglycans in the cartilage matrix. Displacement of water during its compression and restoration of the structure after removal of the load.

Collagen proteins in cartilage

The strength of cartilage tissue is determined by collagen proteins, which are represented by type II, VI, IX, XII, XIV collagens and are immersed in macromolecular aggregates of proteoglycans. Type II collagens account for about 80-90% of all collagen proteins in cartilage. The remaining 15-20% of collagen proteins are the so-called minor collagens of types IX, XII, XIV, which crosslink type II collagen fibrils and covalently bind glycosaminoglycans. A feature of the matrix of hyaline and elastic cartilage is the presence of type VI collagen.

Type IX collagen, found in hyaline cartilage, not only ensures the interaction of type II collagen with proteoglycans, but also regulates the diameter of type II collagen fibrils. Collagen type X is similar in structure to type IX collagen. This type of collagen is synthesized only by hypertrophied growth plate chondrocytes and accumulates around the cells. This unique property of type X collagen suggests the participation of this collagen in bone formation processes.

Proteoglycans. In general, the content of proteoglycans in the cartilage matrix reaches 3%-10%. The main proteoglycan in cartilage is agrecan, which is aggregated with hyaluronic acid. In shape, the agrecan molecule resembles a bottle brush and is represented by one polypeptide chain (core protein) with up to 100 chondroitin sulfate chains and about 30 keratan sulfate chains attached to it (Fig. 2.2).

Rice. 2.2.Proteoglycan aggregate of the cartilage matrix. The proteoglycan aggregate consists of one hyaluronic acid molecule and about 100 agrecan molecules.

Table 2.1

Non-collagenous cartilage proteins

In the structure of the agrecan core protein, an N-terminal domain is isolated, which ensures the binding of agrecan to hyaluronic acid and low molecular weight binding proteins, and a C-terminal domain, which binds agrecan to other molecules of the extracellular matrix. Synthesis of the components of proteoglycan aggregates is carried out by chondrocytes, and the final process of their formation is completed in the extracellular matrix.

Along with large proteoglycans, small proteoglycans are present in the cartilage matrix: decorin, biglycan, and fibromodulin. They make up only 1-2% of the total dry matter mass of cartilage, but their role is very large. Decorin, binding in certain areas with type II collagen fibers, is involved in the processes of fibrillogenesis, and biglycan is involved in the formation of the cartilage protein matrix during embryogenesis. With the growth of the embryo, the amount of biglycan in the cartilage tissue decreases, and after birth, this proteoglycan disappears completely. Regulates the diameter of type II collagen fibromodulin.

In addition to collagens and proteoglycans, the extracellular matrix of cartilage contains inorganic compounds and a small amount of non-collagen proteins, which are characteristic not only for cartilage, but also for other tissues. They are necessary for the binding of proteoglycans to collagen fibers, cells, and individual components of the cartilage matrix into a single network. These are adhesive proteins - fibronectin, laminin and integrins. Most of the specific non-collagen proteins in the cartilage matrix are present only during the period of morphogenesis, calcification of the cartilage matrix, or appear during pathological conditions(Table 2.1). Most often, these are calcium-binding proteins containing 7-carboxyglutamic acid residues, as well as glycoproteins rich in leucine.

2.2. FORMATION OF CARTILAGE TISSUE

At an early stage embryonic development cartilage tissue consists of undifferentiated cells contained in the form of an amorphous mass. In the process of morphogenesis, the cells begin to differentiate, the amorphous mass increases and takes the form of the future cartilage (Fig. 2.3).

In the extracellular matrix of the developing cartilage tissue, the composition of proteoglycans, hyaluronic acid, fibronectin and collagen proteins changes quantitatively and qualitatively. Transfer from

Rice. 2.3.Stages of formation of cartilaginous tissue.

prechondrogenic mesenchymal cells to chondroblasts is characterized by sulfation of glycosaminoglycans, an increase in the amount of hyaluronic acid and precedes the onset of the synthesis of a cartilage-specific large proteoglycan (agrecan). At primary

stages of morphogenesis, high-molecular binding proteins are synthesized, which later undergo limited proteolysis with the formation of low-molecular proteins. Molecules of agrecan bind to hyaluronic acid with the help of low molecular weight binding proteins and proteoglycan aggregates are formed. Subsequently, the amount of hyaluronic acid decreases, which is associated with both a decrease in the synthesis of hyaluronic acid and an increase in the activity of hyaluronidase. Despite the decrease in the amount of hyaluronic acid, the length of its individual molecules, necessary for the formation of proteoglycan aggregates during chondrogenesis, increases. The synthesis of type II collagen by chondroblasts occurs later than the synthesis of proteoglycans. Initially, prechondrogenic cells synthesize type I and III collagens; therefore, type I collagen is found in the cytoplasm of mature chondrocytes. Further, in the process of chondrogenesis, there is a change in the components of the extracellular matrix that control the morphogenesis and differentiation of chondrogenic cells.

Cartilage as a precursor to bone

All bookmarks of the bone skeleton go through three stages: mesenchymal, cartilaginous and bone.

The mechanism of cartilage calcification is very complex process and has not yet been fully explored. Ossification points, longitudinal septa in the lower hypertrophic zone of cartilage rudiments, as well as the layer of articular cartilage adjacent to the bone are subject to physiological calcification. The likely reason for this development of events is the presence of alkaline phosphatase on the surface of hypertrophic chondrocytes. In the matrix subject to calcification, so-called matrix vesicles containing phosphatase are formed. It is believed that these vesicles are, apparently, the primary area of ​​cartilage mineralization. Around chondrocytes, the local concentration of phosphate ions increases, which contributes to tissue mineralization. Hypertrophic chondrocytes synthesize and release into the cartilage matrix a protein - chondrocalcin, which has the ability to bind calcium. Mineralized areas are characterized by high concentrations of phospholipids. Their presence stimulates the formation of hydroxyapatite crystals in these places. In the zone of cartilage calcification, partial degradation of proteoglycans occurs. Those of them that have not been affected by degradation slow down calcification.

Violation of inductive relationships, as well as a change (delay or acceleration) in the timing of the appearance and synostesis of ossification centers in the composition of individual bone anlages, cause the formation of structural defects of the skull in the human embryo.

Cartilage regeneration

Cartilage transplantation within the same species (so-called allogeneic transplants) is usually not accompanied by symptoms of a rejection reaction in the recipient. This effect cannot be achieved with respect to other tissues, since the grafts of these tissues are attacked and destroyed by cells of the immune system. The difficult contact of the donor's chondrocytes with the cells of the recipient's immune system is primarily due to the presence of a large amount of intercellular substance in the cartilage.

The hyaline cartilage has the highest regenerative capacity, which is associated with the high metabolic activity of chondrocytes, as well as the presence of the perichondrium, a dense fibrous unformed connective tissue surrounding the cartilage and containing a large number of blood vessels. Type I collagen is present in the outer layer of the perichondrium, while the inner layer is formed by chondrogenic cells.

Due to these features, cartilage tissue transplantation is practiced in plastic surgery, for example, for the reconstruction of a disfigured nose contour. In this case, allogeneic transplantation of chondrocytes alone, without the surrounding tissue, is accompanied by graft rejection.

Regulation of cartilage metabolism

The formation and growth of cartilage tissue is regulated by hormones, growth factors and cytokines. Chondroblasts are target cells for thyroxine, testosterone and somatotropin, which stimulate the growth of cartilage tissue. Glucocorticoids (cortisol) inhibit cell proliferation and differentiation. A certain role in the regulation of the functional state of the cartilage tissue is played by sex hormones that inhibit the release of proteolytic enzymes that destroy the cartilage matrix. In addition, the cartilage itself synthesizes proteinase inhibitors that suppress the activity of proteinases.

A number of growth factors - TGF-(3, fibroblast growth factor, insulin-like growth factor-1 stimulate growth and development

cartilage tissue. By binding to chondrocyte membrane receptors, they activate the synthesis of collagens and proteoglycans and thereby help maintain the cartilage matrix constancy.

Violation of hormonal regulation is accompanied by excessive or insufficient synthesis of growth factors, which leads to a variety of defects in the formation of cells and extracellular matrix. So, rheumatoid arthritis, osteoarthritis and other diseases are associated with increased formation of skeletal cells, and cartilage begins to be replaced by bone. Under the influence of platelet growth factor, chondrocytes themselves begin to synthesize IL-1α and IL-1(3), the accumulation of which inhibits the synthesis of proteoglycans and collagen types II and IX. This contributes to chondrocyte hypertrophy and, ultimately, calcification of the intercellular matrix of cartilage tissue. Destructive changes are also associated activation of matrix metalloproteinases involved in the degradation of the cartilage matrix.

Age-related changes in cartilage

With aging, degenerative changes occur in the cartilage, the quality and quantitative composition glycosaminoglycans. Thus, the chains of chondroitin sulfate in the proteoglycan molecule synthesized by young chondrocytes are almost 2 times longer than the chains produced by more mature cells. The longer the chondroitin sulfate molecules in the proteoglycan, the more water structures the proteoglycan. In this regard, the proteoglycan of old chondrocytes binds less water, so the cartilage matrix of the elderly becomes less elastic. Changes in the microarchitectonics of the intercellular matrix in some cases are the cause of the development of osteoarthritis. Also, the composition of proteoglycans synthesized by young chondrocytes contains a large amount of chondroitin-6-sulfate, while in older people, on the contrary, chondroitin-4-sulfates predominate in the cartilaginous matrix. The state of the cartilage matrix is ​​also determined by the length of glycosaminoglycan chains. In young people, chondrocytes synthesize short-chain keratan sulfate, and with age, these chains lengthen. A decrease in the size of proteoglycan aggregates is also observed due to the shortening of not only glycosaminoglycan chains, but also the length of the core protein in one proteoglycan molecule. With aging, the content of hyaluronic acid in cartilage increases from 0.05 to 6%.

A characteristic manifestation of degenerative changes in cartilage tissue is its non-physiological calcification. It usually occurs in the elderly and is characterized by primary degeneration of the articular cartilage followed by damage to the articulating components of the joint. The structure of collagen proteins changes and the system of bonds between collagen fibers is destroyed. These changes are associated with both chondrocytes and matrix components. The resulting hypertrophy of chondrocytes leads to an increase in cartilage mass in the area of ​​cartilage cavities. Type II collagen gradually disappears, which is replaced by type X collagen, which takes part in the processes of bone formation.

Diseases associated with malformations of cartilage tissue

In dental practice, manipulations are most often performed on the upper and lower jaws. There are a number of features of their embryonic development, which are associated with different paths of evolution of these structures. In the human embryo at the early stages of embryogenesis, cartilage is found in the composition of the upper and lower jaws.

On the 6-7th week of intrauterine development, the formation of bone tissue begins in the mesenchyme of the mandibular processes. The upper jaw develops along with the bones of the facial skeleton and undergoes ossification much earlier than the mandible. By the age of 3 months, there are no fusion sites on the anterior surface of the bone upper jaw with skull bones.

At the 10th week of embryogenesis, secondary cartilage is formed in the future branches of the lower jaw. One of them corresponds to the condylar process, which in the middle of fetal development is replaced by bone tissue according to the principle of endochondral ossification. Secondary cartilage also forms along the anterior margin of the coronoid process, which disappears just before birth. In the place of fusion of the two halves of the lower jaw, there are one or two islands of cartilaginous tissue, which ossify in the last months of intrauterine development. At the 12th week of embryogenesis, the condylar cartilage appears. At the 16th week, the condyle of the mandibular branch comes into contact with the anlage of the temporal bone. It should be noted that fetal hypoxia, the absence or weak movement of the embryo contributes to the disruption of the formation of joint spaces or the complete fusion of the epiphyses of the opposite bone anlages. This leads to deformation of the mandibular processes and their fusion with the temporal bone (ankylosis).

Cartilage tissue is functionally inherent in the supporting role. It does not work in tension, like a dense connective tissue, but due to internal tension, it resists compression well and serves as a shock absorber for the bone apparatus.

This special tissue serves for the fixed connection of bones, forming synchondrosis. Covering the articular surfaces of the bones, softens the movement and friction in the joints.

Cartilage tissue is very dense and at the same time quite elastic. Its biochemical composition is rich in dense amorphous matter. Cartilage develops from intermediate mesenchyme.

At the site of the future cartilage, mesenchymal cells multiply rapidly, their processes are shortened and the cells are in close contact with each other.

Then an intermediate substance appears, due to which mononuclear sections are clearly visible in the rudiment, which are the primary cartilaginous cells - chondroblasts. They multiply and give more and more masses of the intermediate substance.

The rate of reproduction of cartilage cells by this period is greatly slowed down, and due to the large amount of intermediate substance, they are far removed from each other. Soon, cells lose the ability to divide by mitosis, but still retain the ability to divide amitotically.

However, now the daughter cells do not diverge far, as the intermediate substance surrounding them has condensed.

Therefore, cartilage cells are located in the mass of the main substance in groups of 2-5 or more cells. All of them come from one initial cell.

Such a group of cells is called isogenic (isos - equal, identical, genesis - occurrence).

Rice. 1.

A - hyaline cartilage of the trachea;

B - elastic cartilage of the auricle of the calf;

B - fibrocartilage of the intervertebral disc of the calf;

a - perichondrium; b ~ cartilage; in - an older section of cartilage;

• 1 - chondroblast; 2 - chondrocyte;
• 3 - isogenic group of chondrocytes; 4 - elastic fibers;
• 5 - bundles of collagen fibers; 6 - the main substance;
• 7 - chondrocyte capsule; 8 - basophilic and 9 - oxyphilic zone of the main substance around the isogenic group.

Cells of the isogenic group do not divide by mitosis, they give little intermediate substance of a slightly different chemical composition, which forms cartilaginous capsules around individual cells, and fields around the isogenic group.

The cartilage capsule, as revealed by electron microscopy, is formed by thin fibrils concentrically located around the cell.

Consequently, at the beginning of the development of the cartilage tissue of animals, its growth occurs by increasing the mass of the cartilage from the inside.

Then the oldest part of the cartilage, where cells do not multiply and no intermediate substance is formed, ceases to increase in size, and cartilage cells even degenerate.

However, the growth of cartilage as a whole does not stop. Around the obsolete cartilage, a layer of cells separates from the surrounding mesenchyme, which become chondroblasts. They secrete around themselves the intermediate substance of the cartilage and gradually thicken with it.

At the same time, as they develop, chondroblasts lose the ability to divide by mitosis, form less intermediate substance and become chondrocytes. On the layer of cartilage formed in this way, due to the surrounding mesenchyme, more and more layers of it are superimposed. Consequently, cartilage grows not only from the inside, but also from the outside.

In mammals, there are: hyaline (vitreous), elastic and fibrous cartilage.

Hyaline cartilage (Fig. 1--A) is the most common, milky white and somewhat translucent, so it is often called vitreous.

It covers the articular surfaces of all bones; costal cartilages, cartilages of the trachea and some cartilages of the larynx are formed from it. Hyaline cartilage consists, like all tissues of the internal environment, of cells and an intermediate substance.

Cartilage cells are represented by chondroblasts and chondrocytes. It differs from hyaline cartilage in the strong development of collagen fibers, which form bundles that lie almost parallel to each other, as in tendons!

There is less amorphous substance in fibrous cartilage than in hyaline. Rounded light cells of fibrocartilage lie between the fibers in parallel rows.

In places where fibrocartilage is located between hyaline cartilage and formed dense connective tissue, a gradual transition from one type of tissue to another is observed in its structure. Thus, closer to the connective tissue, collagen fibers in cartilage form coarse parallel bundles, and cartilage cells lie in rows between them, like fibrocytes of dense connective tissue. Closer to the hyaline cartilage, the bundles are divided into individual collagen fibers that form a delicate network, and the cells lose their correct location.