Fibronectin in the vitreous body--distribution and possible functional role

The presence of fibronectin in the bovine vitreous was demonstrated by immunohistochemical procedures which showed a uniform coating of the vitreous collagen network. A fractional extraction of bovine vitreous was carried out in order to determine the distribution of fibronectin and glycosaminoglycans as related to collagen fibers. About half of total fibronectin could be extracted with aqueous buffers with increasing concentrations of KCl, part of fibronectin remained however strongly associated with the insoluble collagen network even after a final extraction with 4 M urea and 0.05 M DTT. Total extractable fibronectin was of the order of 76 micrograms per vitreous, corresponding to approximately 0.17 nM fibronectin. Total quantity of GAG-s determined as uronic acid were of the order of 2200 micrograms/vitreous corresponding approximately to 4400 micrograms disaccharide units that is to about 11 nM disaccharide units of GAG per vitreous. The persistence of fibronectin, strongly associated with the collagen fibers even after repeated KCl and urea-DTT extractions was confirmed using immuno-gold labelling of vitreous collagen fibers. Gold particle density on the collagen fibers increased with the molarity of KCl used for the extractions. These findings suggest that KCl mainly removed fiber associated components probably GAG-s, which hindered the immune recognition of fiber-bound fibronectin. The strong association of fibronectin with vitreous collagen suggested a modified model for vitreous structure taking in account the binding of fibronectin both by collagen and GAG-s.