Cartilage and bone tissue

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

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

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

Chondroblasts are young, round or ovoid cells capable of mitotic division. 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), Golgi complex, mitochondria, lysosomes, glycogen granules. The chondroblast nucleus, rich in active chromatin, has 1-2 nucleoli.

Chondrocytes are mature large cells of cartilaginous tissue. 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 a lacuna, then such a lacuna is called primary. Most often, cells are located in the form of isogenic groups (2-3 cells) occupying the cavity of the secondary lacuna. The walls of the lacuna consist of two layers: the outer one, formed by collagen fibers, and the inner one, consisting of aggregates of proteoglycans, which come into contact with the glycocalyx of cartilaginous cells.

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

According to the structural features of cartilaginous tissue, there are three types of cartilage: hyaline, fibrous and elastic cartilage.

Hyaline cartilage (from the Greek hyalos - glass) has a bluish color. Its main substance contains thin collagen fibers. Cartilaginous 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 laryngeal cartilages are built from hyaline cartilage.

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

Elastic cartilage is characterized by its elasticity and flexibility. The matrix of elastic cartilage contains, along with collagen fibers, a large number of complexly intertwined elastic fibers. Rounded chondrocytes are located in the lacunae. The epiglottis, 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 of elastic cartilage.

Bone tissue (textus ossei) has special mechanical properties. It consists of bone cells embedded in the bone matrix, which contains collagen fibers and is impregnated with inorganic compounds. There are three types of bone cells: osteoblasts, osteocytes, and osteoclasts.

Osteoblasts are young bone cells with polygonal, cubical dendritic processes. Osteoblasts are rich in elements of the granular endoplasmic reticulum, ribosomes, a well-developed Golgi complex and sharply basophilic cytoplasm. They are located 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 across their entire surface in different directions, which leads to the formation of the walls of the lacunae in which these cells are located. Osteoblasts synthesize components of the intercellular substance (collagen is a component of proteoglycan). In the spaces between the fibers there is an amorphous substance - osteoid tissue, or progenitor bone, which then calcifies. The organic matrix of bone contains hydroxyapatite crystals and amorphous calcium phosphate, the elements of which enter the bone tissue from the blood through tissue fluid.

Osteocytes are mature multi-branched spindle-shaped bone cells with a large round 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 lacunae, but the cell bodies are surrounded by a thin layer of the so-called bone fluid (tissue) and do not directly contact the calcified matrix (lacuna walls). Very long (up to 50 μm) osteocyte processes, rich in actin-like microfilaments, pass through bone canals. 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 (trophism) of osteocytes is carried out. The distance between each osteocyte and the nearest blood capillary does not exceed 100-200 µm.

Osteoclasts are large multinuclear (5-100 nuclei) cells of monocytic origin, up to 190 μm in size. These cells destroy bone and cartilage, resorb bone tissue during its physiological and reparative regeneration. Osteoclast nuclei are rich in chromatin and have clearly visible nucleoli. The cytoplasm contains numerous 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. Such processes are especially numerous on the surface adjacent to the bone being destroyed. This is a corrugated or brush border that increases the area of contact between the osteoclast and the bone. Osteoclast processes also have microvilli, between which there 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, the 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 tendons to bones, in the sutures of the skull after they have healed. Coarse 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, the main 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 adjacent plates are multidirectional and cross almost at a right angle, which provides greater bone strength.

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