Connective tissues. The structure and functions of connective tissue, the main types of cells Connective tissue with a chaotic arrangement of fibers
Distinguish between collagen and elastic dense formed connective tissues. These include tendons, ligaments, fascia, etc.
Tendons firmly connect the muscles of the skeleton. They are built from different bundles of collagen fibers going in the same direction, i.e.
Orderly (Fig. 111) in the tendons, three orders of collagen fibers are distinguished. I-order bundles are collagen fibers separated from each other by tendon cells. The set of bundles of the first order, united by a thin layer of loose connective tissue, constitutes the bundles of the second order. The set of beams of the second order constitutes the beams of the third order. They are surrounded by a much thicker layer of connective tissue (see Fig. 111) in the layers between the bundles of the II and III orders, blood vessels and nerve fibers pass, feeding and innervating the tendons.
Dense formed elastic connective tissue mainly consists of elastic fibers and layers of loose connective tissue containing collagen fibers and fibroblasts. The elastic tissue is located mainly in the ligaments. Elastic tissue is also represented by extensive membranes, for example, in the walls of large arteries and other organs.
The dermis of the skin is a dense irregular connective tissue. It also mainly consists of a dense network of collagen fibers arranged in different directions. In the cells of the network there are small islands of loose connective tissue with blood vessels that feed the skin, and rare fat cells.
Dense tissues include cartilage and skin tissues.
cartilage tissue. Cartilage tissue is characterized by a dense basic intermediate substance, in which cartilage cells without processes (chondrocytes) are located in groups and singly. Cartilage tissue performs a supporting function and is the basis for laying the skeleton of an animal. In adult animals, cartilage is found on the articular surfaces, the tips of the ribs, in the walls of the trachea and bronchi, the auricle and other places. Cartilage consists of a large amount of intercellular substance and cellular elements. The main intermediate substance is not so dense that vessels and nerves do not grow into it. Therefore, cartilage is fed from the surface through their perichondrium by diffusion of substances. According to the structure of the intermediate substance, three types of cartilage are distinguished: hyaline, elastic and fibrous (Fig. 113). chondroblast cells of the perichondrium multiply by mitosis and, becoming hydrated, turn into chondrocytes, increasing the total mass of the developing cartilage or filling in the places after its damage.
Hyaline (or vitreous) cartilage is characterized by its transparency, has a bluish tint. It is found on the articular surfaces, the tips of the ribs, in the nasal septum, trachea and bronchi. The diameter of chondrocytes is 3-30 microns, their shape is round, oval, angular, discoid. Chondrocytes are often arranged in groups of two to four - these are the so-called isogenic groups. Cartilage cells lying closer to the perichondrium are always located singly. The main intermediate substance of hyaline cartilage consists of amorphous and fibrous (collagen) materials. The older the animal, the more pronounced the content of the main substance, as a result, darker spots are created around groups and individual cells. Lime salts accumulate in the cartilage with age, the cartilage becomes more fragile.
Elastic cartilage in the ground substance, in addition to collagen fibers, contains a network of elastic fibers that give the entire cartilage greater elasticity and flexibility, as well as a yellowish color and less transparency. Chondrocytes and isogenic groups are surrounded by darker capsules. Cells and isogenic groups in the elastic cartilage are arranged in columns (see Fig. 113b). elastic cartilage is present in the auricle, epiglottis, external auditory canal, windpipe of a reindeer. Calcification processes are always absent in elastic cartilage.
Fibrous cartilage is a type of hyaline cartilage that contains ordered bundles of collagen fibers of considerable diameter. A striated structure is created, in which strips of hyaline cartilage alternate with bundles of collagen fibers (see Fig. 113c). Fibrous cartilage occupies an intermediate position between hyaline cartilage, tendons and fascia. It constantly moves from hyaline cartilage to formed connective tissue. Intervertebral discs (menisci) are made up of fibrocartilage, as well as junctions from tendons to bones. Cartilage tissue in addition to the supporting function takes part in the metabolism of carbohydrates.
This type of connective tissue is found in all organs, as it accompanies the blood and lymphatic vessels and forms the stroma of many organs.
Morphofunctional characteristics of cellular elements and intercellular substance.
Structure. It consists of cells and intercellular substance (Fig. 6-1).
There are the followingcells loose fibrous connective tissue:
1. Fibroblasts- the most numerous group of cells, different in degree of differentiation, characterized primarily by the ability to synthesize fibrillar proteins (collagen, elastin) and glycosaminoglycans with their subsequent release into the intercellular substance. In the process of differentiation, a number of cells are formed:
stem cells;
semi-stem progenitor cells;
unspecialized fibroblasts- low-growth cells with a round or oval nucleus and a small nucleolus, basophilic cytoplasm rich in RNA.
Function: have a very low level of protein synthesis and secretion.
differentiated fibroblasts(mature) - large-sized cells (40-50 microns or more). Their nuclei are light, contain 1-2 large nucleoli. Cell borders are indistinct, blurred. The cytoplasm contains a well-developed granular endoplasmic reticulum.
Function: Intensive biosynthesis of RNA, collagen and elastic proteins, as well as glycosminoglycans and proteoglycans, necessary for the formation of the ground substance and fibers.
fibrocytes— definitive forms of fibroblast development. They have a spindle shape and pterygoid processes. They contain a small number of organelles, vacuoles, lipids and glycogen.
Function: the synthesis of collagen and other substances in these cells is sharply reduced.
- myofibroblasts- functionally similar to smooth muscle cells, but unlike the latter, they have a well-developed endoplasmic reticulum.
Function: these cells are observed in the granulation tissue of the wound process and in the uterus during the development of pregnancy.
- fibroclasts.- cells with high phagocytic and hydrolytic activity, they contain a large number of lysosomes.
Function: take part in the resorption of the intercellular substance.
Rice. 6-1. Loose connective tissue. 1. Collagen fibers. 2. Elastic fibers. 3. Fibroblast. 4. Fibrocyte. 5. Macrophage. 6. Plasma cell. 7. Fat cell. 8. Tissue basophil (mast cell). 9. Pericyte. 10. Pigment cell. 11. Adventitial cage. 12. Basic substance. 13. Blood cells (leukocytes). 14. Reticular cell.
2. Macrophages wandering, actively phagocytic cells. The shape of macrophages is different: there are flattened, rounded, elongated and irregularly shaped cells. Their borders are always clearly defined, and the edges are uneven. . The cytolemma of macrophages forms deep folds and long microprotrusions, with the help of which these cells capture foreign particles. As a rule, they have one core. The cytoplasm is basophilic, rich in lysosomes, phagosomes and pinocytic vesicles, contains a moderate amount of mitochondria, granular endoplasmic reticulum, Golgi complex, inclusions of glycogen, lipids, etc.
Function: phagocytosis, biologically active factors and enzymes (interferon, lysozyme, pyrogens, proteases, acid hydrolases, etc.) are secreted into the intercellular substance, which ensures their various protective functions; produce monokine mediators, interleukin I, which activates DNA synthesis in lymphocytes; factors that activate the production of immunoglobulins, stimulating the differentiation of T- and B-lymphocytes, as well as cytolytic factors; provide processing and presentation of antigens.
3. Plasma cells (plasmocytes). Their size ranges from 7 to 10 microns. Cell shape is round or oval. The nuclei are relatively small, round or oval in shape, located eccentrically. The cytoplasm is sharply basophilic, contains a well-developed granular endoplasmic reticulum in which proteins (antibodies) are synthesized. Only a small light zone near the nucleus, forming the so-called sphere, or courtyard, is deprived of basophilia. Centrioles and the Golgi complex are found here.
Functions: These cells provide humoral immunity. They synthesize antibodies - gamma globulins (proteins) that are produced when an antigen appears in the body and neutralize it.
4. Tissue basophils (mast cells). Their cells have a varied shape, sometimes with short, wide processes, which is due to their ability to amoeboid movements. In the cytoplasm there is a specific granularity (blue), resembling granules of basophilic leukocytes. It contains heparin, hyaluronic acid, histamine and serotonin. Mast cell organelles are poorly developed.
Function: tissue basophils are regulators of local connective tissue homeostasis. In particular, heparin reduces the permeability of the intercellular substance, blood clotting, and has an anti-inflammatory effect. Histamine acts as its antagonist.
5. Adipocytes (fat cells) - located in groups, less often - one by one. Accumulating in large quantities, these cells form adipose tissue. The form of solitary fat cells is spherical, they contain one large drop of neutral fat (triglycerides), which occupies the entire central part of the cell and is surrounded by a thin cytoplasmic rim, in the thickened part of which lies the nucleus. In this regard, adipocytes have a cricoid shape. In addition, in the cytoplasm of adipocytes there is a small amount of cholesterol, phospholipids, free fatty acids, etc.
Function: have the ability to accumulate large amounts of reserve fat, which is involved in trophism, energy production and water metabolism.
6. Pigment cells- have short, irregularly shaped processes. These cells contain the melanin pigment in their cytoplasm, which is capable of absorbing UV radiation.
Function: protection of cells from UV radiation.
7. Adventitial cells - unspecialized cells accompanying blood vessels. They have a flattened or fusiform shape with weakly basophilic cytoplasm, an oval nucleus, and underdeveloped organelles.
Function: acts as a cambium.
8. Pericytes have a process shape and surround the blood capillaries in the form of a basket, located in the crevices of their basement membrane.
Function: regulate changes in the lumen of blood capillaries.
9. Leukocytes migrate to the connective tissue from the blood.
Function: see blood cells.
intercellular substance consists of the main substance and the fibers located in them - collagen, elastic and reticular.
TO 
collagen fibers in loose, unformed fibrous connective tissue, they are located in different directions in the form of twisted rounded or flattened strands 1-3 microns thick or more. Their length is indefinite. The internal structure of the collagen fiber is determined by the fibrillar protein - collagen, which is synthesized in the ribosomes of the granular endoplasmic reticulum of fibroblasts. In the structure of these fibers, several levels of organization are distinguished (Fig. 6-2):
— The first one is the molecular level — represented by collagen protein molecules, having a length of about 280 nm and a width of 1.4 nm. They are built from triplets - three polypeptide chains of the collagen precursor - procollagen, twisted into a single helix. Each procollagen chain contains sets of three different amino acids, repeatedly and regularly repeated throughout its length. The first amino acid in such a set can be any, the second is proline or lysine, the third is glycine.
Rice. 6-2. Levels of structural organization of collagen fibers (scheme).
A. I. Polypeptide chain.
II. Molecules of collagen (tropocollagen).
III. Protofibrils (microfibrils).
IV. Fibril of minimum thickness, in which transverse striation becomes visible.
V. Collagen fiber.
B. Spiral structure of a collagen macromolecule (according to Rich); small light circles - glycine, large light circles - proline, shaded circles - hydroxyproline. (According to Yu. I. Afanasiev, N. A. Yurina).
- The second - supramolecular, extracellular level - represents collagen molecules connected in length and cross-linked by means of hydrogen bonds. First formed protoftsbrills, and 5-b protofibrils, fastened together by side bonds, make up microfibrils, about 10 nm thick. They are distinguishable in an electron microscope in the form of slightly sinuous threads.
— The third, fibrillar level. With the participation of glycosaminoglycans and glycoproteins, microfibrils form fibril bundles. They are transversely striated structures with an average thickness of 50–100 nm. The repetition period of dark and light areas is 64 nm.
— Fourth, fiber level. Depending on the topography, the composition of the collagen fiber (thickness 1-10 microns) includes from several fibrils to several tens .
Function: determine the strength of connective tissues.
Elastic fibers - their shape is rounded or flattened, widely anastomose with each other. The thickness of elastic fibers is usually less than collagen. The main chemical component of elastic fibers is a globular protein elastin, synthesized by fibroblasts. Electron microscopy made it possible to establish that the elastic fibers in the center contain amorphous component, and on the periphery microfibrillar. In terms of strength, elastic fibers are inferior to collagen ones.
Function: determines the elasticity and extensibility of the connective tissue.
Reticular fibers belong to the type of collagen fibers, but differ in smaller thickness, branching and anastomoses. Contain increased amount carbohydrates, which are synthesized by reticular cells and lipids. Resistant to acids and alkalis. They form a three-dimensional network (reticulum), from which they take their name.
Base substance is a gelatinous hydrophilic medium, in the formation of which fibroblasts play an important role. It consists of sulfated (chondroitinsulfuric acid, keratin sulfate, etc.) and non-sulfated ( hyaluronic acid) glycosaminoglycans, which determine the consistency and functional features of the main substance. In addition to these components, the composition of the main substance includes lipids, albumins and blood globulins, minerals (salts of sodium, potassium, calcium, etc.).
Function: transport of metabolites between cells and blood; mechanical (binding of cells and fibers, cell adhesion, etc.); support; protective; water metabolism; regulation of ionic composition.
Classification. The connective tissue itself is divided into:
1) fibrous connective tissues:
loose fibrous connective tissue;
dense fibrous connective tissue:
a) dense unformed connective tissue;
b) densely formed connective tissue;
2) connective tissues with special properties.
This classification is based on the principle of the ratio of cells and intercellular structures, as well as the degree of orderliness of the arrangement of connective tissue fibers.
Fibrous connective tissues
Loose fibrous connective tissue
This type of connective tissue is found in all organs, as it accompanies the blood and lymphatic vessels and forms the stroma of many organs.
Structure. It consists of cells and intercellular substance (Fig. 6-1).
There are the followingcells loose fibrous connective tissue:
1. Fibroblasts- the most numerous group of cells, different in degree of differentiation, characterized primarily by the ability to synthesize fibrillar proteins (collagen, elastin) and glycosaminoglycans with their subsequent release into the intercellular substance. In the process of differentiation, a number of cells are formed:
stem cells;
semi-stem progenitor cells;
unspecialized fibroblasts- low-growth cells with a round or oval nucleus and a small nucleolus, basophilic cytoplasm rich in RNA.
Function: have a very low level of protein synthesis and secretion.
differentiated fibroblasts(mature) - large-sized cells (40-50 microns and more). Their nuclei are light, contain 1-2 large nucleoli. Cell borders are indistinct, blurred. The cytoplasm contains a well-developed granular endoplasmic reticulum.
Function: Intensive biosynthesis of RNA, collagen and elastic proteins, as well as glycosminoglycans and proteoglycans, necessary for the formation of the ground substance and fibers.
fibrocytes- definitive forms of fibroblast development. They have a spindle shape and pterygoid processes. They contain a small number of organelles, vacuoles, lipids and glycogen.
Function: the synthesis of collagen and other substances in these cells is sharply reduced.
- myofibroblasts- functionally similar to smooth muscle cells, but unlike the latter, they have a well-developed endoplasmic reticulum.
Function: these cells are observed in the granulation tissue of the wound process and in the uterus during the development of pregnancy.
- fibroclasts.- cells with high phagocytic and hydrolytic activity, they contain a large number of lysosomes.
Function: take part in the resorption of the intercellular substance.
Rice. 6-1. Loose connective tissue. 1. Collagen fibers. 2. Elastic fibers. 3. Fibroblast. 4. Fibrocyte. 5. Macrophage. 6. Plasma cell. 7. Fat cell. 8. Tissue basophil (mast cell). 9. Pericyte. 10. Pigment cell. 11. Adventitial cage. 12. Basic substance. 13. Blood cells (leukocytes). 14. Reticular cell.
2. Macrophages wandering, actively phagocytic cells. The shape of macrophages is different: there are flattened, rounded, elongated and irregularly shaped cells. Their borders are always clearly defined, and the edges are uneven. . The cytolemma of macrophages forms deep folds and long microprotrusions, with the help of which these cells capture foreign particles. As a rule, they have one core. The cytoplasm is basophilic, rich in lysosomes, phagosomes and pinocytic vesicles, contains a moderate amount of mitochondria, granular endoplasmic reticulum, Golgi complex, inclusions of glycogen, lipids, etc.
Function: phagocytosis, biologically active factors and enzymes (interferon, lysozyme, pyrogens, proteases, acid hydrolases, etc.) are secreted into the intercellular substance, which ensures their various protective functions; produce monokine mediators, interleukin I, which activates DNA synthesis in lymphocytes; factors that activate the production of immunoglobulins, stimulating the differentiation of T- and B-lymphocytes, as well as cytolytic factors; provide processing and presentation of antigens.
3. Plasma cells (plasmocytes). Their size ranges from 7 to 10 microns. Cell shape is round or oval. The nuclei are relatively small, round or oval in shape, located eccentrically. The cytoplasm is sharply basophilic, contains a well-developed granular endoplasmic reticulum in which proteins (antibodies) are synthesized. Only a small light zone near the nucleus, forming the so-called sphere, or courtyard, is deprived of basophilia. Centrioles and the Golgi complex are found here.
Functions: These cells provide humoral immunity. They synthesize antibodies - gamma globulins (proteins) that are produced when an antigen appears in the body and neutralize it.
4. Tissue basophils (mast cells). Their cells have a varied shape, sometimes with short, wide processes, which is due to their ability to amoeboid movements. In the cytoplasm there is a specific granularity (blue), resembling granules of basophilic leukocytes. It contains heparin, hyaluronic acid, histamine and serotonin. Mast cell organelles are poorly developed.
Function: tissue basophils are regulators of local connective tissue homeostasis. In particular, heparin reduces the permeability of the intercellular substance, blood clotting, and has an anti-inflammatory effect. Histamine acts as its antagonist.
5. Adipocytes (fat cells) - located in groups, less often - one by one. Accumulating in large quantities, these cells form adipose tissue. The form of solitary fat cells is spherical, they contain one large drop of neutral fat (triglycerides), which occupies the entire central part of the cell and is surrounded by a thin cytoplasmic rim, in the thickened part of which lies the nucleus. In this regard, adipocytes have a cricoid shape. In addition, in the cytoplasm of adipocytes there is a small amount of cholesterol, phospholipids, free fatty acids, etc.
Function: have the ability to accumulate large amounts of reserve fat, which is involved in trophism, energy production and water metabolism.
6. Pigment cells- have short, irregularly shaped processes. These cells contain the melanin pigment in their cytoplasm, which is capable of absorbing UV radiation.
Function: protection of cells from UV radiation.
7. Adventitial cells - unspecialized cells accompanying blood vessels. They have a flattened or fusiform shape with weakly basophilic cytoplasm, an oval nucleus, and underdeveloped organelles.
Function: acts as a cambium.
8. Pericytes have a process shape and surround the blood capillaries in the form of a basket, located in the crevices of their basement membrane.
Function: regulate changes in the lumen of blood capillaries.
9. Leukocytes migrate to the connective tissue from the blood.
Function: see blood cells.
intercellular substance consists of the main substance and the fibers located in them - collagen, elastic and reticular.
TO collagen fibers in loose, unformed fibrous connective tissue, they are located in different directions in the form of twisted rounded or flattened strands 1-3 microns thick or more. Their length is indefinite. The internal structure of the collagen fiber is determined by the fibrillar protein - collagen, which is synthesized in the ribosomes of the granular endoplasmic reticulum of fibroblasts. In the structure of these fibers, several levels of organization are distinguished (Fig. 6-2):
- The first is the molecular level - represented by collagen protein molecules, having a length of about 280 nm and a width of 1.4 nm. They are built from triplets - three polypeptide chains of the collagen precursor - procollagen, twisted into a single helix. Each procollagen chain contains sets of three different amino acids, repeatedly and regularly repeated throughout its length. The first amino acid in such a set can be any, the second is proline or lysine, the third is glycine.
Rice. 6-2. Levels of structural organization of collagen fibers (scheme).
A. I. Polypeptide chain.
II. Molecules of collagen (tropocollagen).
III. Protofibrils (microfibrils).
IV. Fibril of minimum thickness, in which transverse striation becomes visible.
V. Collagen fiber.
B. Spiral structure of a collagen macromolecule (according to Rich); small light circles - glycine, large light circles - proline, shaded circles - hydroxyproline. (According to Yu. I. Afanasiev, N. A. Yurina).
- The second - supramolecular, extracellular level - represents collagen molecules connected in length and cross-linked by means of hydrogen bonds. First formed protoftsbrills, and 5-b protofibrils, fastened together by side bonds, make up microfibrils, about 10 nm thick. They are distinguishable in an electron microscope in the form of slightly sinuous threads.
- The third, fibrillar level. With the participation of glycosaminoglycans and glycoproteins, microfibrils form fibril bundles. They are transversely striated structures with an average thickness of 50–100 nm. The repetition period of dark and light areas is 64 nm.
- Fourth, fiber level. Depending on the topography, the composition of the collagen fiber (thickness 1-10 microns) includes from several fibrils to several tens .
Function: determine the strength of connective tissues.
Elastic fibers - their shape is rounded or flattened, widely anastomose with each other. The thickness of elastic fibers is usually less than collagen. The main chemical component of elastic fibers is a globular protein elastin, synthesized by fibroblasts. Electron microscopy made it possible to establish that the elastic fibers in the center contain amorphous component, and on the periphery microfibrillar. In terms of strength, elastic fibers are inferior to collagen ones.
Function: determines the elasticity and extensibility of the connective tissue.
Reticular fibers belong to the type of collagen fibers, but differ in smaller thickness, branching and anastomoses. They contain an increased amount of carbohydrates, which are synthesized by reticular cells and lipids. Resistant to acids and alkalis. They form a three-dimensional network (reticulum), from which they take their name.
Base substance is a gelatinous hydrophilic medium, in the formation of which fibroblasts play an important role. It consists of sulfated (chondroitinsulfuric acid, keratin sulfate, etc.) and non-sulfated (hyaluronic acid) glycosaminoglycans, which determine the consistency and functional features of the main substance. In addition to these components, the composition of the main substance includes lipids, albumins and blood globulins, minerals (salts of sodium, potassium, calcium, etc.).
Function: transport of metabolites between cells and blood; mechanical (binding of cells and fibers, cell adhesion, etc.); support; protective; water metabolism; regulation of ionic composition.
Dense fibrous connective tissue
It is characterized by a relatively large number of densely arranged fibers (collagen), a small amount of cellular elements (fibrocytes, fibroblasts) and the main substance between them.
Depending on the nature of the location of the fibrous structures, this tissue is divided into:
Dense irregular connective tissue.
It is located in the dermis of the skin and is characterized by a disordered arrangement of fibers.
Densely formed connective tissue.
It occurs in tendons, ligaments, fibrous membranes and is characterized by a strictly ordered arrangement of fibers.
Tendon consists of thick, tightly packed, parallel bundles of collagen fibers separated by fibrocytes, a small number of fibroblasts, and ground substance. Each bundle of collagen fibers is called beam of the first order. Several bundles of the first order, surrounded by thin layers of loose fibrous connective tissue (endothenonium), make up second-order beams. Second-order beams are composed of bundles of the third order, separated by thicker layers of loose connective tissue (peritenonium). In large tendons, there may be bundles of the fourth order. Blood vessels and nerves pass through the perithenonium and endotenonium.
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This type of connective tissue is characterized by a quantitative predominance of fibers over the main substance and cells. Depending on the relative position of the fibers and the bundles and networks formed from them, two main types of dense connective tissue are distinguished: unformed and formed.
In dense irregular connective tissue fibers form a complex system of intersecting bundles and networks. This arrangement of them reflects the versatility of mechanical effects on a given area of tissue, according to which these fibers are located, ensuring the strength of the entire tissue system. Dense unformed tissue is found in large quantities in the composition of the skin of animals, where it performs a supporting function. Along with intertwining collagen fibers, it has a network of elastic fibers, which determines the ability of the tissue system to stretch and return to its original state after the termination of the external mechanical factor. Varieties of dense unformed tissue are part of the perichondrium and periosteum, membranes and capsules of many organs.
Rice. 112. Dense formed connective tissue of the tendon in the longitudinal section:
1 - collagen fibers - bundles of the first order; 2 - tendon bundle II order; 3 - nuclei of fibrocytes; 4 - layers of loose connective tissue.
Densely formed connective tissue characterized by ordered fibers, which corresponds to the action of the mechanical tension of the fabric in one direction. In accordance with the type of predominant fibers, collagen and elastic densely shaped tissues are distinguished. Dense, well-formed collagen tissue is most typically found in tendons. It consists of tightly lying collagen fibers oriented parallel along the tendon and bundles formed from them (Fig. 112). Each collagen fiber, consisting of numerous fibrils, is designated as a bundle of the first order. Between the fibers (beams of the first order), clamped by them, there are also longitudinally oriented fibrocytes. A set of bundles of the first order form bundles of the second order, surrounded by a thin layer of loose connective tissue - endotenonium. Several bundles of the II order form a bundle of the III order, surrounded by a thicker layer of loose connective tissue - perithenonium. In large tendons, there may also be bundles of the IV order. The perithenonium and endotenonium contain blood vessels that feed the tendon, nerve endings and fibers that send to the central nervous system signals about the state of tissue tension.
Dense formed elastic tissue in animals is found in ligaments (for example, in the vulva). It is formed by a network of thick longitudinally elongated elastic fibers. Fibrocytes and thin intertwining collagen fibrils are located in narrow slit-like spaces between elastic fibers. In some places there are wider layers of loose connective tissue through which blood vessels pass. This tissue, represented by a system of circularly located membranes and elastic networks, is present in large arterial vessels.
Dense connective tissues contain less of the main substance, and fibrous structures predominate in the intercellular substance. They have few cells and less diverse cellular composition. The fibers are predominantly collagenous, densely arranged to each other. In dense unformed connective tissue, collagen fibers form bundles, there are fibroblasts between the fibers, but fibrocytes predominate. The bundles of collagen fibers are intertwined with each other, and between the bundles there are thin layers of loose connective tissue with capillaries. This tissue forms the reticular layer of the skin. The ability of regeneration is lower than in loose.
Dense formed connective tissue.
Dense formed connective tissue forms fibrous membranes, ligaments and tendons, with all fibers running parallel and tight. Tendons contain collagen fibers. Each individual fiber is a bundle of the first order, between them are fibrocytes. These fibers form a bundle of the second order. Between the bundles of the second order there are layers of connective tissue with blood capillaries that form endotenonium. The bundles of the second order are combined into bundles of the third order, which are separated from each other by a large layer of connective tissue - perithenonium. The ability to regenerate is low.
Connective tissue with special properties.
1. Reticular tissue. Contains reticular cells, which connect with their processes and form a network. Along the processes, deepening into the cytolemma, there are reticular fibers. The reticular tissue forms the stroma of the circulatory organs and regenerates very well.
2. Adipose tissue. Adults have white fat. It is represented by an accumulation of fat cells that form lobules. They are separated by a layer of connective tissue that contains blood capillaries. They are filled with neutral fat. It is easily digested, but difficult to give. Adipose tissue forms subcutaneous adipose tissue, fatty capsules around organs. This fabric is a source of water, energy, plastic material. Brown fat is found in embryogenesis and in newborns. It is more energy efficient.
3. Pigment tissue - accumulation of pigment cells.
4. Mucous tissue. Normal - only in embryogenesis and in the umbilical cord. There are few cells in it, few collagen fibers, a semi-liquid ground substance is well expressed.
5. Skeletal tissue is divided into:
a) Cartilaginous
b) Skeletal
Skeletal connective tissue.
cartilage tissue mainly performs a trophic function. It has reduced water content to 70-80%, increased content of mineral salts up to 4-7% and organic matter up to 10-15%. These tissues are more dense and elastic, all contain cells and intercellular substance. Cartilage tissue cells are the same and are called chondroblasts. They have a spindle-shaped or oval shape with a basophilic cytoplasm, a developed protein-synthesizing apparatus, some of them are stem cells and are able to proliferate. Chondroblasts produce extracellular substance and differentiate into young chondrocytes. These are small oval-shaped cells with a developed protein-synthesizing apparatus, retain the ability to proliferate and produce intercellular substance, and eventually turn into mature chondrocytes. They are larger and lose their ability to proliferate over time. All these cells are located in cavities that correspond in volume to their size. The cavity is limited by a capsule consisting of collagen fibers. Several chondrocytes can accumulate in it, then isogenic groups of cells are formed.
Cartilaginous tissues differ from each other in the structure of the intercellular substance, primarily in the structure of intercellular fibers capable of calcification. There are hyaline, elastic and fibrous cartilage tissue.
Hyaline cartilage is the most common (the articulation of the ribs with the sternum, in the wall of the airways, in the formation of articular surfaces). Outside covered with perichondrium (perichondria). The outer layer is formed by a denser fibrous connective tissue, the inner layer is looser. The inner shell contains fibroblasts and chondroblasts. In the shell are blood vessels. Chondroblasts proliferate and produce intercellular substance, secrete it around themselves and wall up. Due to this, cartilage grows from the outside - appositional. Deeper is the own substance of the cartilage. In its peripheral part are young chondrocytes. They also divide, produce and secrete intercellular substance and determine the growth of cartilage from the inside - interstitial growth. Mature chondrocytes are located in the middle part of the cartilage substance, and isogenic groups of chondrocytes are located in the center. Between the cells is an intercellular substance containing collagen fibers and the ground substance. They have the same refractive index, so they are difficult to distinguish. In a growing organism, the intercellular substance is oxyphilic; with age, as glycosaminoglycones accumulate, it becomes basophilic. There are no blood vessels in the cartilage, nutrition occurs diffusely. With aging, calcium salts are deposited, calcification occurs, cartilage becomes brittle and brittle.
Elastic cartilage is part of the wall of the airways, forms the basis of the auricle. It has a similar structure, but has a number of features. Elastic fibers are located in the intercellular substance, the intercellular substance is oxyphilic all the time, it is not normally calcified.
Fibrous cartilage is found in the junction of the tendon and bone, in the intervertebral discs. On the one hand, the cartilage is formed by dense, formed connective tissue, and on the other, by hyaline cartilage. With age, fibrocartilage calcifies. Cartilaginous tissues regenerate all the time.
bone tissue possess a high degree mineralization (calcium phosphate content - 70%), hard, durable and form bone. In very low water content, organic matter is dominated by proteins. Distinguish:
1. Coarse fibrous (reticulofibrous) skeletal tissue. It is present in embryogenesis, and in adults it forms sutures and joints of the bones of the skull.
2. Lamellar bone tissue.
Bone tissues contain cells that produce an intercellular substance, in which collagen fibers sharply predominate. A small volume is occupied by the main (gluing) substance. Its cellular composition is the same, represented by osteoblasts - cells that form bone tissue. These are large, round-shaped cells with a round nucleus, with a well-developed protein-synthesizing apparatus, they produce an intercellular substance (collagen fibers). The number of these cells is large in a growing organism, during regeneration. Osteocytes are also referred to as bone cells. They have a thin body and long thin processes that lie in the bone tubules, anastomose with the processes of other cells and transport tissue fluid through the bone tubules. There are also osteoclasts - cells that destroy bone tissue. They flutter from blood monocytes and belong to the macrophage system. These are large, multinucleated cells with a well-developed lysosomal apparatus. On one surface of the cell there are microvilli. Lysosomal enzymes are secreted into the microvillus area and break down the protein matrix, which leads to the release of calcium and leaching it from the bone.
Bone tissues differ in the structure of the intercellular substance. In coarse-fibered bone tissue, collagen fibers form bundles that intertwine with each other. Osteocytes are located between the fibers, but in an adult there are few thin bones. In lamellar bone tissue, collagen fibers run parallel to each other, are tightly glued together and form bone plates. The strength of bone tissue is ensured by the fact that the plates go at different angles. Between the plates are osteocytes. Their processes penetrate the bone plates in all areas.
Lamellar bone tissue forms a compact bone. It contains osteons and spongy part where osteons are absent.
The diaphysis of a tubular bone is built from compact bone tissue. Outside, the diaphysis is covered with a periosteum (periosteum), its outer layer consists of a denser fibrous tissue, and the inner layer of a looser one, contains fibroblasts, osteoblasts. Part of the collagen fibers goes into the substance of the bone, so the periosteum is tightly connected to the bone. It contains a large number of receptors and blood vessels are also located here.
The diaphysis is built from lamellar bone tissue. Outside, there is a layer of large bone plates that run concentrically along the diameter of the entire bone. Next, the inner layer of the common plates is isolated, and from the inside lies the endosteum, consisting of loose connective tissue containing blood vessels. Between them is a wide middle osteogenic layer. It contains osteons - structural and functional units of bone. Osteons are located along the axis of the diaphysis and consist of concentric bone plates of different diameters. Within each osteon is the osteon canal, which contains a blood vessel. Between the osteons are the remains of bone plates - these are the remains of osteons. Normally, in humans, osteons are gradually destroyed, and new osteons are formed. Osteocytes are located between the bone plates of all layers, and their processes penetrate the bone plates and an extensive network of tubules is created. The blood vessels of the periosteum through perforating channels enter the osteons, go through their channels, anastomose with each other and deliver nutrients into the osteon canal. From there, along the bone tubules, calcium phosphates spread very quickly to all parts of the bone. There are two mechanisms of bone formation: direct osteogenesis - the process of formation of flat bones directly from the mesenchyme. Mesenchymal cells proliferate and group together to form skeletal islets. They turn into osteoblasts, produce intercellular substance, immure themselves and turn into osteocytes. In this way, bone beams are formed. On their surface, otseoblasts are produced, calcification of the intercellular substance occurs. Bone beams are built from coarse-fibered bone tissue. Bone beams grow into blood vessels. With the help of osteoblasts, coarse fibrous bone tissue is destroyed and, as blood vessels grow in, it is replaced by lamellar bone tissue with the help of osteoblasts. This is how lamellar bones develop.
The tubular bone develops in place of the hyaline cartilage. This is indirect osteogenesis. In the second month of embryogenesis, the germ of hyaline cartilage is laid. This is the future bone small size. Outside, it is covered with the perichondrium, then in the region of the diaphysis between the perichondrium and the substance of the cartilage, a bone cuff is formed from coarse-fibrous bone tissue. It completely surrounds the diaphysis and disrupts the nutrition of the cartilaginous tissue of the diaphysis. Part of the cartilage in the diaphysis is destroyed, the remaining parts of the cartilage are calcified. The perichondrium turns into a periosteum and blood vessels grow inside it. They penetrate the bone cuff, while its coarse-fibered bone tissue is replaced by lamellar one, the vessels grow deeper into the cartilage zone, while osteoclasts destroy the cartilage, and osteoblasts around the remnants calcify the cartilage, forming endochondral bone from lamellar bone tissue. The calcified cartilage is completely destroyed, the endochondral bone grows, connects with the perichondral bone, osteoclasts destroy the bone tissue in the middle of the diaphysis and form the medullary cavity. Red bone marrow is formed in it from mesenchymal cells. The epiphysis is represented by hyaline cartilage. It later undergoes ossification. And between the epiphysis and the diaphysis there is a metoepiphyseal plate - a growth zone (due to it, the bones grow in length). Here, a layer of vesicular cells, a columnar layer and a boundary layer are distinguished
(similar in structure to hyaline cartilage). This plate ossifies at the age of 18-20. Bone tissue regenerates well. At the beginning, loose connective tissue is formed in the damage zone due to fibrocytes, then, due to osteoblasts, it is replaced by coarse fibrous connective tissue, which fills the defect and forms a callus. By the end of the second week, the coarse fibrous connective tissue begins to fill with lamellar connective tissue. The growth and regeneration of bones is influenced by exercise stress, the content of protein, calcium salts, vitamins D, C, A in food, hormones.