The role of vitreous collagen in platelet aggregation in vitro and in vivo.

Abstract The reaction between platelets and the macromolecular constituents present in connective tissue was studied using the vitreous as a model system. Direct injection of platelets into the rabbit vitreous resulted in the prompt formation of a membrane. In vitro aggregation experiments were performed using washed human platelets and fractions obtained from the bovine, rabbit, and owl monkey vitreous. In all three species only those fractions containing hydroxylysine and hydroxyproline supported platelet aggregation. Amino acid and carbohydrate analysis, as well as electron microscopic examination indicated that the guanidine-insoluble fraction from the bovine vitreous was composed only of collagen and that this appeared to be the constituent which caused platelet aggregation in vivo. The mechanism of platelet aggregation thus appears to be similar in vitro and in vivo. Support for this conclusion was obtained by analysis of the vitreous membrane formed in vivo which showed the presence of amino acids characteristic of collagen as well as an unidentified component also present in platelets. In experiments performed with platelets whose function had been modified by treatment with aspirin, membrane formation was inhibited in vivo. This finding indicated that adhesion to collagen is an essential prerequisite for aggregation. In vivo membrane formation occurred with less than 10 μg of collagen per milliliter and was not impaired by either its association with other structural proteins or the unusual organization of the fibers.