α₂β₁ integrin-independent collagen-induced pulmonary embolism. (A) The absolute decrement in the platelet count from baseline of α₂^+/+ or α₂^-/- mice one minute after injection of the indicated amount of collagen (or saline) plus epinephrine (1 μg) was determined. The values represent the mean ± SEM for each concentration of collagen injected (25 μg [n = 21, α₂^+/+; n = 21, α₂^-/-]), 12 μg [n = 6, α₂^+/+; n = 6, α₂^-/-], 6 μg [n = 7, α₂^+/+; n = 8, α₂^-/-], or 3 μg [n = 7, α₂^+/+; n = 7, α₂^-/-]). (B) The number of thrombi in the lungs at the time of killing 3 minutes after injection of the indicated amount of collagen was determined. Lung sections stained with hematoxylin and eosin were digitally imaged using an Olympus Camedia C3040 Zoom camera. The number of thrombi in × 20 fields was quantitated and expressed as thrombi per millimeter squared ± SEM for each concentration of collagen injected (25 μg [n = 27, α₂^+/+; n = 23, α₂^-/-], 12 μg [n = 6, α₂^+/+; n = 7, α₂^-/-], 6 μg [n = 7, α₂^+/+; n = 11, α₂^-/-], or 3 μg [n = 3, α₂^+/+; n = 3, α₂^-/-]). (C) Representative histologic sections of the lungs of α₂^+/+ mice injected with saline alone, or α₂^+/+, α₂^-/-, or F_cγR^-/- mice injected with 25 μg collagen. Shown at × 200 original magnification. Thrombi were not observed in α₂^+/+ mice injected with saline. In contrast, many thrombi were identified in the lungs of α₂^+/+ and α₂^-/- mice injected with collagen. Only rare thrombi were observed in F_cγR^-/- mice.