Role of the spleen in production of thrombocytopenia in p45 NF-E2 knockout mice. (A) Platelet counts from p45 NF-E2 heterozygote (+/−; N = 17) and homozygous mutant (−/−; N = 6) fetuses at embryonic day 15, before functional development of the spleen. Platelet counts, performed manually, likely reflect some dilution of blood samples by tissue fluid. Statistical significance was established using the Student’s t-test (P < .001). (B) Serial platelet counts from p45 NF-E2 knockout (KO; N = 4) and littermate control adult (N = 8) mice before and after splenectomy. Although platelet counts were obtained for up to 50 days after splenectomy, results are shown only for the initial 32 days, after which no significant changes were noted.