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Connective tissue cells
Last reviewed: 06.07.2025

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Fibroblasts are the main cells of connective tissue. They are spindle-shaped, with thin short and long processes extending from the surface of fibroblasts. The number of fibroblasts in different types of connective tissue varies, and they are especially numerous in loose fibrous connective tissue. Fibroblasts have an oval nucleus filled with small chromatin lumps, a clearly distinguishable nucleolus, and basophilic cytoplasm containing many free and attached ribosomes. Fibroblasts have a well-developed granular endoplasmic reticulum. The Golgi complex is also well developed. Fibronectin, an adhesive protein to which collagen and elastic fibers are attached, is located on the cell surface of fibroblasts. Micropinocytic vesicles are present on the inner surface of the fibroblast cytolemma. Their presence indicates intensive endocytosis. The cytoplasm of fibroblasts is filled with a three-dimensional microtrabecular network formed by thin protein filaments 5-7 nm thick, which connect actin, myosin and intermediate filaments. Fibroblast movements are possible due to the connection of their actin and myosin filaments located under the cell cytolemma.
Fibroblasts synthesize and secrete the main components of the intercellular substance, namely the amorphous substance and fibers. The amorphous (basic) substance is a gelatinous hydrophilic medium, consists of proteoglycans, glycoproteins (adhesive proteins) and water. Proteoglycans, in turn, consist of glycosaminoglycans (sulfated: keratin sulfate, dermatan sulfate, chondroitin sulfate, heparin sulfate, etc.) associated with proteins. Proteoglycans together with specific proteins combine into complexes connected with hyaluronic acid (non-sulfated 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, loose connective tissue contains a lot of glycosaminoglycans, so it contains a lot of water. In bone tissue, the glycosaminoglycan molecules are short, and it contains little water.
Collagen fibers begin to form in the Golgi complex of fibroblasts, where procollagen aggregates are formed, turning into “secretory” granules. During the secretion of procollagen from cells, this procollagen on the surface is transformed into tropocollagen. Tropocollagen molecules in the extracellular space combine with each other by “self-assembly”, forming protofibrils. Five to six protofibrils, joining together with the help of lateral bonds, form microfibrils about 10 nm thick. Microfibrils, in turn, combine into long transversely striated fibrils up to 300 nm thick, which form collagen fibers from 1 to 20 μm thick. Finally, many fibers, gathering, form collagen bundles up to 150 μm thick.
An important role in fibrillogenesis belongs to the fibroblast itself, which not only secretes components of the intercellular substance, but also creates the direction (orientation) of connective tissue fibers. This direction corresponds to the length of the fibroblast axis, which regulates the assembly and three-dimensional arrangement of fibers and their bundles in the intercellular substance.
Elastic fibers with a thickness of 1 to 10 μm consist of the protein elastin. 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 with a thickness of about 13 nm near the cell surface in the extracellular space form a looped network. Elastic fibers anastomose and intertwine with each other, forming networks, fenestrated plates and membranes. Unlike collagen fibers, elastic fibers can stretch 1.5 times, after which they return to their original state.
Reticular fibers are thin (from 100 nm to 1.5 μm thick), branched, and form fine-meshed networks in the cells of which cells are located. Together with reticular cells, 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 derivatives of fibroblasts and reticular cells. Each reticular fiber contains many fibrils 30 nm in diameter with transverse striation similar to that of collagen fibers. Reticular fibers contain type III collagen and are covered with carbohydrates, which allows them to be detected using the Schick reaction. They are stained black when impregnated with silver.
Fibrocytes are also connective tissue cells. Fibroblasts turn into fibrocytes as they age. A fibrocyte is a spindle-shaped cell with a large ellipsoid nucleus, a small nucleolus, and a small amount of organelle-poor cytoplasm. The granular endoplasmic reticulum and Golgi complex are poorly developed. Each cell contains lysosomes, autophagosomes, and other organelles.
Along with the cells synthesizing the components of the intercellular substance, there are cells in the loose fibrous connective tissue that destroy it. These cells - fibroclasts - are very similar in structure 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 similar to macrophages. Fibroclasts have great phagocytic and hydrolytic activity.
Loose fibrous tissue also contains and performs certain functions macrophages, lymphocytes, tissue basophils (mast cells), fat, pigment, adventitial, plasma and other cells.
Macrophages, or macrophocytes (from the Greek makros - large, devouring), are mobile cells. They capture and devour foreign substances, interact with lymphoid tissue cells - lymphocytes. Macrophages have different shapes, their sizes range from 10 to 20 µm, the cytolemma forms numerous processes. The nucleus of macrophages is round, ovoid or bean-shaped. There are many lysosomes in the cytoplasm. Macrophages secrete a large number of different substances into the intercellular substance: enzymes (lysosomal, collagenase, protease, elastase) and other biologically active substances, including those stimulating the production of B-lymphocytes and immunoglobulins, increasing the activity of T-lymphocytes.
Tissue basophils (mast cells) are usually located in the loose fibrous connective tissue of internal organs, as well as near blood vessels. They are round or ovoid. Their cytoplasm contains many granules of various sizes containing heparin, hyaluronic acid, chondroitin sulfates. During degranulation (release of granules), heparin reduces blood clotting, increases the permeability of blood vessels, thereby causing edema. Heparin is an anticoagulant. Eosinophils containing histaminase block the effect of histamine and the slow factor of anaphylaxin. It should be noted that the release of granules (degranulation) is the result of allergy, immediate hypersensitivity reaction and anaphylaxis.
Fat cells, or adipocytes, are large (up to 100-200 µm in diameter), spherical, and almost completely filled with a drop of fat, which accumulates as a reserve material. Fat cells are usually located in groups, forming adipose tissue. Fat loss from adipocytes occurs under the influence of lipolytic hormones (adrenaline, insulin) and lipase (a lipolytic enzyme). In this case, triglycerides of fat cells are broken down into glycerol and fatty acids, which enter the blood and are transported to other tissues. Human adipocytes do not divide. New adipocytes can form from adventitial cells, which are located near blood capillaries.
Adventitial cells are poorly differentiated cells of the fibroblastic series. They are adjacent to blood capillaries, spindle-shaped or flattened. Their nucleus is ovoid, organelles are poorly developed.
Pericytes (pericapillary cells, or Rouget cells) are located outside the endothelium, inside the basal layer of blood capillaries. These are process cells that contact each neighboring endothelial cell with their processes.
Pigment cells, or pigmentocytes, dendritic, contain the pigment melanin in their cytoplasm. These cells are abundant in the iris and vascular membranes of the eye, the skin of the nipple and areola of the mammary gland, and in other areas of the body.
Plasma cells (plasmocytes) and lymphocytes are the “working” cells of the immune system; they actively move in tissues, including connective tissue, and participate in humoral and cellular immunity reactions.