[The three-dimensional ultrastructure of the collagen fibers, reticular fibers and elastic fibers: a review].

Fibrous components of the connective tissue are light-microscopically classified into three types: collagen fibers, reticular fibers and elastic fibers. The present paper reviews the three-dimensional ultrastructure of these fibrous components, mainly based on our studies by scanning electron microscopy. The collagen fibers are shaped like tapes or cords about 1 to 20 microns in diameter. Each fiber is a bundle of fibrils running roughly parallel to each other. These collagen fibrils vary in diameter from 30 to 300 nm depending on their locating area of the body, and show a repeating pattern of depressed and protruding segments on the surface. The reticular fibers consist of collagen fibrils about 20-40 nm in diameter, which run singly or in small bundles. They are usually interwoven elaborately to form thin lace-like sheets or sheaths attaching to basal laminae of such cells as epithelial, endothelial and muscular cells. These fibers are considered to play an important role not only in adhering the cells to the collagen fibers, but also in constituting the skeletal framework suitable for individual cells and tissues. The elastic fibers consist of two different components: elastin and fibrillin. Elastin forms unit fibrils of 0.1-0.2 micron thickness which are arranged in bundles or laminae specific to individual organs and tissues. Fibrillin, on the other hand, forms microfibrils about 10 nm in diameter running in or along elastin bundles. These microfibrils also form delicate networks separate from elastin components. For a comprehensive understanding of the fibrous components in the connective tissue, the author proposed categorizing them into two systems: the collagen fibrillar system as a supporting framework of tissues and cells, and the fibrillin-elastin fibrillar system for distributing stressing forces uniformly in tissues.