Regardless of its histologic type, connective tissue is made up of two components, cells and extracellular matrix. Fibroblasts (which synthesize collagen), fat cells, fixed macrophages, mast cells, plasma cells, and some leukocytes are the principal cellular elements of connective tissue. Extracellular matrix consists of collagenous, elastic, or reticular fibers and a variety of glycoproteins and proteoglycans, which are either sulfated or nonsulfated. These are often present as components of gigantic macromolecular assemblies such as aggrecan proteoglycan aggregates (see Plate 2-25). Among the important sulfated compounds are chondroitin sulfate, keratan sulfate, and heparan sulfate. The principal nonsulfated compound is hyaluronan. In addition, connective tissue also contains blood and lymphatic vessels and nerves that vary in number and size.
The composition and organization of a particular connective tissue largely depends on its function. Loose, or areolar, connective tissue is found throughout the body wherever biologic packing material is needed. It is well vascularized and highly cellular, with a large proportion of matrix. The fibrous component varies in amount and orientation, depending on the mechanical stresses in the region. Adipose tissue is a specialized form of loose connective tissue in which fat cells predominate. Although the fat cell is sometimes thought of as a type of fibroblast, it is increasingly being regarded as a separate cell type that, when it does not contain lipid, resembles a fibroblast.
Dense connective tissue, found in tendons and ligaments, is poorly vascularized. Parallel bundles of densely packed collagen fibers are the predominant elements. Alongside the bundles of collagen fibers are inactive-appearing fibroblasts with densely staining, elongated nuclei. In some species, the fibroblasts of tendons have the tendency to form nodules of ectopic cartilage and bone after injury. Occasionally, nodules of fibrocartilage are seen in tendons.
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