Cells of connective tissue

Fibroblasts are the main cells of connective tissue. They are spindle-shaped, from the surface of the fibroblasts thin short and long processes depart. The amount of fibroblasts in different types of connective tissue is different, especially many in loose fibrous connective tissue. Fibroblasts have an oval core filled with small chromatin blocks, a distinct nucleolus and a basophilic cytoplasm containing many free and attached ribosomes. Fibroblasts have a well-developed granular endoplasmic reticulum. The Golgi complex is also well developed. On the cellular surface of fibroblasts is fibronectin - an adhesive protein, to which collagen and elastic fibers are attached. On the inner surface of the fibroblast cytolemma, there are micropinocytosis vesicles. Their presence testifies to intensive endocytosis. The cytoplasm of fibroblasts is filled by a three-dimensional microtrabecular network formed by thin protein filaments 5-7 nm thick, which connect actinic, myosin and intermediate filaments among themselves. Movement of fibroblasts is possible due to the connection of their actin and myosin filaments located under the cell's cytolemma.

Fibroblasts synthesize and secrete the main components of the intercellular substance, namely the amorphous substance and the fibers. Amorphous (basic) matter is a gelatinous hydrophilic medium, consists of proteoglycans, glycoproteins (adhesive proteins), and water. Proteoglycans, in turn, consist of glycosaminoglycans (sulphated: keratin sulfate, dermatan sulfate, chondroitin sulfate, heparin sulfate, etc.) associated with proteins. Proteoglycans, together with specific proteins, are combined into complexes connected with hyaluronic acid (nonsulfated glycosaminoglycans). Glycosaminoglycans have a negative charge, and water is a dipole (±), so it binds to glycosaminoglycans. This water is called bound. The amount of bound water depends on the number and length of glycosaminoglycan molecules. For example, in a loose connective tissue there are many glycosaminoglycans, so there is a lot of water in it. In the bone tissue of the molecule glycosaminoglycans short, it has little water.

Collagen fibers begin to form in the Golgi complex of fibroblasts, where prokollagen aggregates are formed, passing into "secretory" granules. During prokollagen secretion from cells, this collagen on the surface turns into a tropocollagen. Molecules of tropocollagen in the extracellular space are joined together by "self-assembling", forming protofibrils. Five or six protofibrils, joining together with the help of lateral bonds, form microfibrils with a thickness of about 10 nm. Microfibrils, in turn, combine into long transversely striated fibrils up to 300 nm thick, which form collagen fibers with a thickness of 1 to 20 μm. Finally, a lot of fibers, gathering, make collagen beams up to 150 microns thick.

An important role in fibrillogenesis belongs to the fibroblast itself, which not only secrets the components of the intercellular substance, but also creates the direction (orientation) of connective tissue fibers. This direction corresponds to the length of the axis of fibroblasts that regulate the assembly and the three-dimensional arrangement of fibers and their bundles in the intercellular substance.

Elastic fibers with a thickness of 1 to 10 μm consist of an elastin protein. Proelastin molecules are synthesized by fibroblasts on the ribosomes of the granular endoplasmic reticulum and secreted into the extracellular space, where microfibrils are formed. Elastic microfibrils about 13 nm thick near the cell surface in the extracellular space form a loopy network. Elastic fibers anastomose and intertwine, forming networks, fenestrated plates and membranes. Unlike collagen, elastic fibers can stretch 1.5 times, after which they return to their original state.

Reticular fibers thin (thickness from 100 nm to 1.5 μm), branched, form small-looped networks, in cells of which cells are located. Together with the reticular cells, the reticular fibers form the framework (stroma) of the lymph nodes, spleen, red bone marrow, and together with collagen elastic fibers participate in the formation of the stroma of many other organs. Reticular fibers are derived from fibroblasts and reticular cells. Each reticular fiber contains a variety of fibrils 30 nm in diameter with transverse striation similar to that of collagen fibers. Reticular fibers contain type III collagen, covered with carbohydrates, which allows them to be identified using the Schick reaction. They are painted black when impregnated with silver.

Fibrocytes are also cells of connective tissue. Fibroblasts grow into fibroblasts as they age. The fibrocyte is a spindle-shaped cell with a large ellipsoid nucleus, a small nucleolus, and a small amount of cytoplasm poor in organelles. The granular endoplasmic reticulum and the Golgi complex are poorly developed. Each cell contains both lysosomes, and autophagosomes, and other organelles.

Along with the cells that synthesize the components of the intercellular substance, there are cells in the loose fibrous connective tissue that destroy it. These cells - fibroblasts - in their structure are very similar to fibroblasts (in shape, development of the granular endoplasmic reticulum and the Golgi complex). At the same time, they are rich in lysosomes, which makes them look like macrophages. Fibroclasts have a large phagocytic and hydrolytic activity.

In a loose fibrous tissue, macrophages, lymphocytes, tissue basophils (fat cells), fatty, pigmentary, adventitial, plasma and other cells also function and perform certain functions.

Macrophages, or macrophages (from the Greek makros - large, devouring), are mobile cells. They capture and devour foreign substances, interact with cells of lymphoid tissue - lymphocytes. Macrophages have different shapes, their sizes are from 10 to 20 microns, the cytolemma forms numerous processes. The nucleus of macrophages is rounded, ovoid or bean-shaped. In the cytoplasm there are many lysosomes. Macrophages secrete (secrete) a large number of different substances in the intercellular substance: enzymes (lysosomal, collagenase, protease, elastase) and other biologically active substances, including stimulating the production of B-lymphocytes and immunoglobulins, which increase the activity of T-lymphocytes.

Tissue basophils (mast cells) are usually located in the loose fibrous connective tissue of the internal organs, as well as near the blood vessels. They are rounded or ovoid. In their cytoplasm there are many different sizes of granules containing heparin, hyaluronic acid, chondroitin sulfates. With degranulation (separation of granules), heparin reduces coagulability of blood, increases the permeability of blood vessels, thereby causing edema. Heparin is an anticoagulant. Eosinophils containing histamine block the effect of histamine and the slow factor of anaphylaxin. It should be noted that pellet ejection (degranulation) is the result of allergy, immediate-type hypersensitivity reaction and anaphylaxis.

Fat cells, or adipocytes, are large (up to 100-200 microns in diameter), globose, almost completely filled with a drop of fat, which accumulates as a backup material. The fat cells are usually arranged in groups, forming a fatty tissue. Loss of fat from adipocytes occurs under the influence of lipolytic action hormones (epinephrine, insulin) and lipase (a lipotic enzyme). In this case triglycerides of fat cells are split up to glycerin and fatty acids, which enter the blood and are transferred to other tissues. Human adipocytes do not divide. New adipocytes can be formed from adventitial cells, which are located near the blood capillaries.

Adventitial cells are poorly differentiated cells of the fibroblastic series. They adhere to the blood capillaries, fusiform or flattened. The nucleus is ovoid, organelles are poorly developed.

Pericytes (pericapillary cells, or Rugee cells) are located outside the endothelium, inside the basal layer of the blood capillaries. These are process cells that touch the appendages with each neighboring endotheliocyte.

Pigment cells, or pigment cells, process, contain in their cytoplasm a pigment melanin. These cells are abundant in the iris and vascular membranes of the eye, the skin of the nipple and the sucking mug of the breast and in other parts of the body.

Plasma cells (plasmocytes) and lymphocytes are the "working" cells of the immune system, they actively move in tissues, including in the connective, participate in the reactions of humoral and cellular immunity.

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