TPO treatment (100 μg/kg per day for 5 days) increased platelet counts and restored platelet morphology in the blood of GATA-1^low mice. (A) Platelet counts (as absolute numbers, top; percent change, with respect to values measured in each mouse 3-5 days before TPO treatment, bottom) in wild-type (▪) and GATA-1^low (⋄) mice (both 12 months old) treated with 100 μg TPO/kg per day for 5 days, as indicated. Data are presented as the mean (± SD) of independent determinations for a total of 18 GATA-1^low and 14 normal littermates analyzed per experimental point. Values statistically higher than those from untreated animals are indicated: ^* indicates P < .05; §, P < .01. (B) May-Grünwald-Giemsa staining of a representative blood smear from a GATA-1^low mouse at the peak (day 14) of its response to TPO, showing the presence in these samples of both megathrombocytes (arrowhead) and normal-sized platelets (arrows). As shown by the open arrow in the insert, normal-sized platelets from TPO-treated GATA-1^low mice formed discrete aggregates virtually absent on blood smears from untreated mutant littermates. (Original magnification × 100/1.32 numerical aperture.)