Collagen fibril organisation in mammalian vitreous by freeze etch/rotary shadowing electron microscopy

Mammalian vitreous gel contains two major network-forming polymeric systems : long, thin fibrils comprising predominantly type II collagen and a meshwo rk of hyaluronan. The gel structure is maintained primarily by the collagen component, but little is known about the mechanisms of spacing of the coll agen fibrils and of interactions between fibrils to form a stable network. In this study we have applied the technique of freeze etching/rotary shadow ing electron microscopy in order to reveal the fibrillar network in central , cortical and basal vitreous and to understand the structural relationship between the collagen fibrils. The fibrils were arranged side by side in na rrow bundles that frequently branched to link one bundle to another. Only a minor part of the fibrillar network consisted of segments that had a diame ter of a single fibril (16.4 nm mean diameter). In addition, three morpholo gically distinct filamentous structures were observed that appeared to form links within the collagen fibrillar network: short, single interlinking fi laments of 7.0 nm mean diameter, network-forming filaments of 6.7 nm mean d iameter, and longer filaments of 8.2 nm mean diameter. All three types of f ilamentous structure were removed by digestion of the vitreous gels with St reptomyces hyaluronan lyase prior to freeze etching, indicating that these structures contain or are stabilised by hyaluronan. These filamentous struc tures may contribute to the structural stability of the vitreous gel. (C) 2 000 Elsevier Science Ltd. All rights reserved.