Topic 14. CONNECTIVE TISSUE. SKELETAL CONNECTIVE TISSUES

Skeletal connective tissues include cartilaginous and bone tissues that perform supporting, protective and mechanical functions, as well as taking part in the metabolism of minerals in the body. Each of these types of connective tissue has significant morphological and functional differences, and therefore they are considered separately.

cartilage tissues

Cartilaginous tissue consists of cells - chondrocytes and chondroblasts, as well as dense intercellular substance.

Chondroblasts are located singly along the periphery of the cartilaginous tissue. They are elongated flattened cells with basophilic cytoplasm containing a well-developed granular ER and lamellar complex. These cells synthesize the components of the intercellular substance, release them into the intercellular environment, and gradually differentiate into the definitive cells of the cartilage tissue - chondrocytes. Chondroblasts are capable of mitotic division. The perichondrium surrounding the cartilaginous tissue contains inactive, poorly differentiated forms of chondroblasts, which, under certain conditions, differentiate into chondroblasts that synthesize the intercellular substance, and then into chondrocytes.

An amorphous substance contains a significant amount of mineral substances that do not form crystals, water, or dense fibrous tissue. Vessels in the cartilage tissue are normally absent. Depending on the structure of the intercellular substance, cartilage tissues are divided into hyaline, elastic and fibrous cartilage tissue.

In the human body, hyaline cartilage tissue is widespread and is part of the large cartilages of the larynx (thyroid and cricoid), trachea, and cartilage of the ribs.

Elastic cartilage tissue is characterized by the presence of both collagen and elastic fibers in the cellular substance (cartilaginous tissue of the auricle and cartilaginous part of the external auditory canal, cartilage of the external nose, small cartilages of the larynx and middle bronchi).

Fibrous cartilage tissue is characterized by the content of powerful bundles of parallel collagen fibers in the intercellular substance. In this case, chondrocytes are located between the bundles of fibers in the form of chains. According to physical properties, it is characterized by high strength. It is found in the body only in limited places: it forms part of the intervertebral discs (annulus fibrosus), and is also localized at the points of attachment of ligaments and tendons to hyaline cartilage. In these cases, a gradual transition of connective tissue fibrocytes into cartilage chondrocytes is clearly seen.

When studying cartilage tissues, the concepts of "cartilaginous tissue" and "cartilage" should be clearly understood.

Cartilage tissue is a type of connective tissue, the structure of which is superimposed above. Cartilage is an anatomical organ that consists of cartilage and perichondrium. The perichondrium covers the cartilaginous tissue from the outside (with the exception of the cartilaginous tissue of the articular surfaces) and consists of fibrous connective tissue.

There are two layers in the perichondrium:

1) external - fibrous;

2) internal - cellular (or cambial, germ).

In the inner layer, poorly differentiated cells are localized - prechondroblasts and inactive chondroblasts, which, in the process of embryonic and regenerative histogenesis, first turn into chondroblasts, and then into chondrocytes.

The fibrous layer contains a network of blood vessels. Therefore, the perichondrium, as an integral part of the cartilage, performs the following functions:

1) provides trophic avascular cartilaginous tissue;

2) protects cartilage tissue;

3) provides regeneration of cartilaginous tissue in case of its damage.

The trophism of the hyaline cartilage tissue of the articular surfaces is provided by the synovial fluid of the joints, as well as fluid from the vessels of the bone tissue.

The development of cartilage tissue and cartilage (chondrohystogenesis) is carried out from the mesenchyme.

bone tissues

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 - 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 are 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 EPS 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 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.

The intercellular substance of 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. During the development of bone tissue, 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 as an organ consists of such elements as:

1) bone tissue;

2) periosteum;

3) bone marrow (red, yellow);

4) vessels and nerves.

The periosteum (periosteum) surrounds the bone tissue along the periphery (with the exception of the articular surfaces) and has a structure similar to the perichondrium.

In the periosteum, the outer fibrous and inner cellular (or cambial) layers are isolated. The inner layer contains osteoblasts and osteoclasts. A vascular network is localized in the periosteum, from which small vessels penetrate into the bone tissue through perforating channels.

Red bone marrow is considered as an independent organ and belongs to the organs of hematopoiesis and immunogenesis.

Bone tissue in the formed bones is mainly represented by a lamellar form, however, in different bones, in different parts of the same bone, it has a different structure. In the flat bones and epiphyses of the tubular bones, the bone plates form crossbars (trabeculae) that make up the cancellous substance of the bone. In the diaphysis of tubular bones, the plates are tightly adjacent to each other and form a compact substance.

All types of bone tissue develop mainly from the mesenchyme.

There are two types of osteogenesis:

1) development directly from the mesenchyme (direct osteohistogenesis);

2) development from the mesenchyme through the cartilage stage (indirect osteohistogenesis).

The structure of the diaphysis of a tubular bone. On the transverse section of the diaphysis of the tubular bone, the following layers are distinguished:

1) periosteum (periosteum);

2) the outer layer of common (or general) plates;

3) a layer of osteons;

4) the inner layer of common (or general) plates;

5) internal fibrous plate (endosteum).

External common plates are located under the periosteum in several layers, without forming a single ring. Osteocytes are located between the plates in the gaps. Perforating channels pass through the outer plates, through which perforating fibers and vessels penetrate from the periosteum into the bone tissue. The perforating vessels provide trophism to the bone tissue, and the perforating fibers firmly connect the periosteum with the bone tissue.

The osteon layer consists of two components: osteons and insertion plates between them. The osteon is the structural unit of the compact substance of the tubular bone. Each osteon consists of 5-20 concentrically layered plates and the osteon channel, in which the vessels (arterioles, capillaries, venules) pass. There are anastomoses between the canals of adjacent osteons. Osteons make up the bulk of the bone tissue of the diaphysis of the tubular bone. They are located longitudinally along the tubular bone, respectively, by force (or gravitational) lines and provide a support function. When the direction of the lines of force changes, as a result of a fracture or curvature of the bones, osteons that do not carry a load are destroyed by osteoclasts. However, osteons are not completely destroyed, and part of the bone plates of the osteon along its length are preserved, and such remaining parts of the osteon are called insertion plates.

During postnatal osteogenesis, there is a constant restructuring of the bone tissue, some osteons are resorbed, others are formed, so there are intercalated plates or remnants of previous osteons between the osteons.

The inner layer of the common plates has a structure similar to the outer one, but it is less pronounced, and in the region of the transition of the diaphysis to the epiphyses, the common plates continue into trabeculae.

Endooste - a thin connective tissue plate lining the cavity of the diaphysis canal. The layers in the endosteum are not clearly expressed, but among the cellular elements there are osteoblasts and osteoclasts.

Classification of bone tissue

There are two types of bone tissue:

1) reticulofibrous (coarse-fibered);

2) lamellar (parallel fibrous).

The classification is based on the nature of the location of collagen fibers. In reticulofibrous bone tissue, bundles of collagen fibers are thick, tortuous, and randomly arranged. In the mineralized intercellular substance, osteocytes are randomly located in the lacunae. Lamellar bone tissue consists of bone plates, in which collagen fibers or their bundles are arranged parallel in each plate, but at right angles to the course of the fibers of neighboring plates. Between the plates in the gaps are osteocytes, while their processes pass through the tubules through the plates.

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

Development of bone tissue and bones (osteohistogenesis)

All types of bone tissue develop from one source - from the mesenchyme, but the development of different bones is not the same. There are two types of osteogenesis:

1) development directly from the mesenchyme - direct osteohistogenesis;

2) development from the mesenchyme through the cartilage stage - indirect osteohistogenesis.

With the help of direct osteohistogenesis, a small number of bones develop - the integumentary bones of the skull. At the same time, reticulofibrous bone tissue is first formed, which soon collapses and is replaced by lamellar one.

Direct osteogenesis proceeds in four stages:

1) the stage of formation of skeletal islands in the mesenchyme;

2) the stage of formation of osseoid tissue - an organic matrix;

3) the stage of mineralization (calcification) of osteoid tissue and the formation of reticulofibrous bone tissue;

4) the stage of transformation of reticulofibrous bone tissue into lamellar bone tissue.

Indirect osteogenesis begins from the 2nd month of intrauterine development. First, in the mesenchyme, due to the activity of chondroblasts, a cartilaginous model of the future bone from hyaline cartilage tissue, covered with perichondrium, is laid. Then there is a replacement, first in the diaphysis, and then in the epiphyses of the bone cartilage tissue. Ossification in the diaphysis is carried out in two ways:

1) perichondral;

2) endochondral.

First, in the area of ​​the diaphysis of the cartilaginous anlage of the bone, osteoblasts are evicted from the perichondrium and form reticulofibrous bone tissue, which, in the form of a cuff, covers the cartilaginous tissue along the periphery. As a result, the perichondrium turns into a periosteum. This method of bone formation is called perichondral. After the formation of the bone cuff, the trophism of the deep sections of the hyaline cartilage in the area of ​​the diaphysis is disturbed, as a result of which calcium salts are deposited here - cartilage shoaling. Then, under the inductive influence of calcified cartilage, blood vessels grow into this zone from the periosteum through the holes in the bone cuff, the adventitia of which contains osteoclasts and osteoblasts. Osteoclasts destroy the stagnant cartilage, and around the vessels, due to the activity of osteoblasts, lamellar bone tissue is formed in the form of primary osteons, which are characterized by a wide lumen (channel) in the center and fuzzy boundaries between the plates. This method of bone tissue formation in the depth of cartilage tissue is called endochondral. Simultaneously with endochondral ossification, the coarse-fibered bone cuff is restructured into lamellar bone tissue, which makes up the outer layer of the general plates. As a result of perichondral and endochondral ossification, the cartilaginous tissue in the area of ​​the diaphysis is replaced by bone. In this case, a cavity of the diaphysis is formed, which is first filled with red bone marrow, which is then replaced by white bone marrow.

The epiphyses of tubular bones and spongy bones develop only endochondral. Initially, in the deep parts of the cartilaginous tissue of the epiphysis, shallowing is noted. Then, vessels with osteoclasts and osteoblasts penetrate there, and due to their activity, the cartilage tissue is replaced by lamellar tissue in the form of trabeculae. The peripheral part of the cartilage tissue is preserved in the form of articular cartilage. Between the diaphysis and the epiphysis, cartilage tissue is preserved for a long time - the metaepiphyseal plate, due to the constant reproduction of the cells of which the bone grows in length.

In the metaepiphyseal plate, the following cell zones are distinguished:

1) border zone;

2) zone of columnar cells;

3) zone of vesicular cells.

Approximately by the age of 20, the metaepiphyseal plate is reduced, synostosis of the epiphyses and diaphysis occurs, after which the growth of the bone in length stops. In the process of bone development due to the activity of osteoblasts of the periosteum, bones grow in thickness. Regeneration of bones after their damage and fractures is carried out due to the activity of periosteal osteoblasts. Reorganization of bone tissue is carried out constantly throughout osteogenesis: some osteons or their parts are destroyed, others are formed.

Factors affecting the process of osteohistogenesis and the state of bone tissue

The following factors influence the process of osteohistogenesis on the state of bone tissue.

1. The content of vitamins A, C, D. The lack of these vitamins in food leads to a violation of the synthesis of collagen fibers and to the disintegration of existing ones, which is manifested by fragility and increased fragility of bones. Insufficient formation of vitamin D in the skin leads to a violation of bone tissue calcification and is accompanied by insufficient bone strength and flexibility (for example, with rickets). An excess of vitamin A activates the activity of osteoclasts, which is accompanied by bone resorption.

2. The optimal content of thyroid and parathyroid hormones - calcitonin and parathyroid hormone, which regulate the calcium content in the blood serum. The state of bone tissue is also influenced by the level of sex hormones.

3. Bone curvature leads to the development of a piezoelectric effect - stimulation of osteoclasts and bone resorption.

4. Social factors - food, etc.

5. Environmental factors.

Age-related changes in bone tissue

With increasing age, the ratio of organic and inorganic substances in the bone tissue changes towards an increase in inorganic and a decrease in organic, which is accompanied by an increase in bone fragility. This may explain the significant increase in the incidence of fractures in the elderly.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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