Gingival Reticulin: Identification and Role in Histogenesis of Collagen Fibers

¹ Department of Dental Science, Royal College of Surgeons of England, London, England

Argyrophilic fibers are distinct from nonargyrophilic mature collagen, and occur as two different morphologic types, reticulin and argyrophilic young collagen. In light microscopic examination of developing connective tissue, including oral connective tissue, a reticulin network can be seen early in development. Argyrophilic young collagen fibers soon appear in increasing numbers. These gradually obscure the reticulin and are transformed into nonargyrophilic mature collagen. Argyrophilic young collagen is absent from mature connective tissue, but reticulin persists in adult parenchymatous organs and in mature gingiva and skin, where it is largely hidden by nonargyrophilic collagen. The electron microscope shows reticulin to have a matrix that contains fine unbanded filaments, and typically banded collagen fibrils distributed either sparsely and apparently haphazardly or in bundles. A hypothesis describing the histogenesis of collagen fibers has been proposed. In developing connective tissue, the formative cells move apart and deposit intercellular substance in the form of reticulin, which comprises collagen building blocks of unknown size, lipid, and glycoprotein. The collagen aggregates to form filaments and banded fibrils arranged sparsely or in bundles, some sufficiently large to be fibers. The reticulin then has an appearance similar to that in mature tissues. Further development is inhibited in collagen-poor connective tissues. In collagen-rich tissue, further aggregation of collagen leads to thickening of fibers that at first are argyrophilic in the light microscope. The developing fibers replace the reticulin, but new reticulin is concurrently deposited. Reticulin appears to provide a milieu in which collagen building blocks may be aggregated to form fibers or "stored" for varying periods, extending to the life span of the organism. Ultrastructural changes observed in connective tissue of developing mouse skin appear to be consistent with those described in the hypothesis.