Mammalian vitreous humor contains networks of hyaluronan molecules: electron microscopic analysis using the hyaluronan-binding region (G1) of aggrecan and link protein.

Vitreous humor from human, bovine, and chicken eyes was analyzed by rotary shadowing to characterize further the supramolecular organization of the gel-like matrix which forms this tissue. Extensive filamentous networks, distinct from collagen fibrils, were found in both human and bovine vitreous but not in chicken vitreous. The networks consisted of branching structures of various diameters, due to variable numbers of hyaluronan molecules being laterally associated with each other and apparently giving rise to a three-dimensional lattice. These networks could be decorated in a specific and regular manner by the hyaluronan-binding region called G1 purified from bovine nasal septum cartilage. The extent of decoration of hyaluronan was dependent on the relative concentration of G1. In the presence of an excess of G1 the networks were destabilized giving rise to individual unbranched hyaluronan chains of varying length that were saturated with G1. One or more globular proteins, as yet uncharacterized, were seen interacting with the hyaluronan networks, often at branch points. These proteins may serve to stabilize the three-dimensional structure of the matrix although highly ordered networks were also observed without globular proteins. Link protein, which also binds to hyaluronan, bound to the networks in a fashion clearly distinct from G1. Neither G1 nor link protein bound directly to human or bovine vitreous collagen fibrils. However, link protein did bind extensively to the glycosaminoglycan coat of chicken vitreous collagen fibrils described previously (D. W. Wright, and R. Mayne J. Ultrastruct. Mol. Struct. Res. 100, 224-234, 1988), while G1 did not. Digestion of the chicken vitreous collagen fibrils with Streptomyces hyaluronidase did not result in the removal of the glycosaminoglycan coat of the collagen fibrils nor did it affect the binding of G1 or link protein to the fibrils, indicating that hyaluronan is not a component of this structure. These studies demonstrate that proteins with specific binding properties can be used as probes to investigate the structure of the native vitreous humor gel from several species and suggest that this method potentially can be used for structural studies of other connective tissue matrices.