Cartilage and bone tissue

The connective tissues are also cartilaginous and bone tissue, of which the skeleton of the human body is built. These tissues are called skeletal. The organs constructed from these tissues perform the functions of support, movement, 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. It consists of cartilaginous tissue from cartilaginous cells (chondroblasts and chondrocytes) and dense, elastic intercellular substance.

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

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

Chondrocytes are mature large cells of cartilaginous tissue. They are rounded, oval or polygonal, with processes, developed organelles. Chondrocytes are located in cavities - lacunae, surrounded by intercellular substance. If there is one cell in the lacuna, then this lacuna is called the primary one. Most often, the cells are arranged 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 layer, formed by collagen fibers, and the internal, consisting of aggregates of proteoglycans, which come into contact with the glycocalysis of the cartilaginous cells.

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

In accordance with the peculiarities of the structure of the cartilaginous tissue, three types of cartilage are distinguished: hyaline, fibrous and elastic cartilage.

Hyaline cartilage (from the Greek hyalos - glass) has a bluish color. Thin collagen fibers are located in its main substance. Cartilage cells have a variety of shape and structure, depending on the degree of differentiation and location of their cartilage. Chondrocytes form isogenic groups. From hyaline cartilage articular, costal cartilages and most cartilages of the larynx are constructed.

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

Elastic cartilage is characterized by elasticity, flexibility. In the matrix of the elastic cartilage along with collagen contains a large number of difficultly intertwining elastic fibers. Rounded chondrocytes are located in lacunae. From the elastic cartilage, an epiglottis, wedge-shaped and carob-shaped cartilages of the larynx, a voice process of the arytenoid cartilages, a cartilage of the auricle, a cartilaginous part of the auditory tube are constructed.

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

Osteoblasts are process young bone cells of a polygonal, cubic shape. Osteoblasts are rich in elements of a granular endoplasmic reticulum, ribosomes, a well-developed Golgi complex, and a sharply basophilic cytoplasm. They lie in the surface layers of the bone. The rounded 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 excreted through their entire surface in different directions, which leads to the formation of lacunae walls in which these cells lie. Osteoblasts synthesize the components of the intercellular substance (collagen is a component of proteoglycan). In between the fibers there is an amorphous substance - an osteoid tissue, or an ancestry that is then calcified. The organic matrix of bone contains crystals of hydroxyapatite and amorphous calcium phosphate, the elements of which enter the bone tissue from the blood through the tissue fluid.

Osteocytes are mature, multiflex 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 lacunae, but the body cells are surrounded by a thin layer of so-called bone fluid (tissue) and do not touch directly with the calcified matrix (lacunar walls). Very long (up to 50 μm) processes of osteocytes, rich in actin-like microfilaments, pass through the bony tubules. The processes are also separated from the calcified matrix by a space of about 0.1 μm in width, in which the 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 μm.

Osteoclasts are large multinucleated (5-100 nuclei) cells of monocytic origin, up to 190 μm in size. These cells destroy bone and cartilage, carry out the resorption of bone tissue during 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, 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. It is corrugated, or brush, a border that increases the area of contact of the osteoclast with the bone. Osteoclasts processes also have microvilli, between which are crystals of hydroxyapatite. These crystals are found in phagolysosomes of osteoclasts, where they are destroyed. The activity of osteoclasts depends on the level of parathyroid hormone, an increase in synthesis and secretion of which leads to activation of osteoclast function and destruction of bone.

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 seams of the skull after their overgrowth. Coarse-fibrous bone tissue contains thick, disordered bundles of collagen fibers, between which is an amorphous substance.

The lamellar bone tissue is formed by bone plates 4 to 15 μm in thickness, which consist of osteocytes, a basic substance, thin collagen fibers. Fibers (type I collagen) involved in the formation of bone plates lie parallel to each other and are oriented in a certain direction. In this case, the fibers of adjacent plates are multidirectional and cross at almost a right angle, which provides greater strength of the bone.

[1], [2], [3], [4]