Internalization of fibrinogen. (A and C)Overnight incubation of cultured megakaryocytes with medium containing 20% normal plasma. Labeling as indicated on the figures. (A) Type I MVB in close aposition to the Golgi complex (g) containing fibrinogen. (B) Demonstration of fibrinogen in a type I MVB (arrowheads) in bone marrow megakaryocyte. (C) Typical matrix labeling of internalized fibrinogen in type II MVBs. Inset: Detail of a type II MVB showing fibrinogen associated with the matrix, and CD63 typically with the internal vesicles. Bars, 100 nm.