Morphology of platelets on fibrinogen or collagen matrices. Platelets in suspension were allowed to adhere to a coverslip coated with fibrinogen (A through D) or collagen (E and F ) in the presence of 2 mmol/L CaCl₂ and 0.2 U/mL apyrase. Phase-contrast images were continuously recorded with a bright-field camera. Video prints are shown from various microscopic fields after 20 minutes of adhesion to immobilized fibrinogen in the absence (A and B) or presence (C and D) of 10 μmol/L H-Arg-Gly-Asp-Ser-OH. Video prints are also shown from two fields after 20 minutes of adhesion to collagen (E and F ). Note the rounded, balloon-like platelets with small dark spots, attached to the collagen fibers (E and F, arrows). Bar indicates 5 μm.