Collagen-mediated changes in [Ca²⁺]_i in megakaryocytes from wild-type and syk −/− mice. Megakaryocytes from either (A) wild-type Balb/c mice or (B) radiation chimeric Balb/c mice repopulated with syk −/− fetal liver were isolated from bone marrow as described. After plating onto poly-L-lysine–coated coverslips, stage III/IV megakaryocytes were identified and injected with FURA-2 and then stimulated with collagen (100 μg/mL) as indicated. [Ca²⁺]_e levels were adjusted 2 minutes before the addition of agonist. FURA-2 fluorescence was measured as described and [Ca²⁺]_i was determined from a precalculated calibration curve. Representative traces in (A) and (B) are each taken from cells from a single animal, but a representative of at least 6 (A) or 3 (B) cells from a minimum of three mice.