According to the ultrastructural features, 6 types of fibroblasts are distinguished of which two are immature forms (little-differentiated and young fibroblasts) and the rest (collagenoblasts, myofibroblasts, fibroclasts, fibrocytes) are mature cells with the functional specificity participating in the collagen production, contraction, remodeling, involution and stabilization of the connective tissue, respectively. In their turn, collagenoblasts differ from each other in the intensity of biosynthesis and the mode of collagen secretion from the cell. Fibroblasts also produce proteoglycans which are important in fibrillogenesis. Proteoglycans form cuffing around each collagen fibril as well as are components of microfibrils and floccular material which form the ultrastructural basis of the cementating substance. A system of collagen-lytic enzymes and ultrastructural signs of collagen resorption have been found in fibroblasts attesting to their role in the maintenance of equilibrium between biosynthesis and catabolism of collagen under normal and pathological conditions.