Human tissues are composed not only of cells. Most mammalian cells are located in tissues, where they are surrounded by a complex extracellular matrix. The matrix includes different polysaccharides and proteins spontaneously organized to form ordered structures. The tissue in which extracellular matrix occupies a considerably larger volume than cells is often referred to as "connective tissue" (Fig. 1.1).
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Fig. 1.1. Organization: extracellular matrix-basal membrane-epithelium
The connective tissue contacts with the layer of epithelial cells. The basal membrane (very thin and stiff film) resides between epithelium and extracellular matrix.
Connective tissue provides:
► active exchange of metabolites and ions between the blood and tissues;
► formation of the structure of organs and tissues during embryogenesis and the postnatal period;
► flexibility of interacting surfaces ofjoints because it forms cartilage;
► protection against external influence by regulation of the functional activity of phagocytes and immune system cells;
► regeneration and replacement of imperfection by stimulating functional activity and proliferation of tissue cells.
The share of connective tissue varies in different organs: in bones and skin it is the main component. Connective tissue may be mineralized forming hard bone or tooth structures; and also form transparent matter of eye cornea, or a cord-like structure making tendons resistant to rupture.