P-selectin is intrinsically regulated in fetal hematopoietic progenitors. (A) Experimental scheme. Kit-positive hematopoietic progenitors were derived from E12.5 fetal liver or adult bone marrow and cultured for 3 days in the presence of TPO. Megakaryocytes were isolated by CD41 selection. For transplant studies, lin⁻sca⁻kit⁺ (HPC) cells were sorted from E14.5 fetal livers and from adult bone marrow of UBC-GFP mice and transplanted into GFP⁻ recipients. Whole blood from recipients was activated on days 5 and 10 posttransplant, with 0.5 U/mL thrombin in the presence of antibodies for activated αIIbβ₃ (JON/A) and surface P-selectin (CD62P). Donor (GFP⁺) and recipient (GFP⁻) platelet populations were evaluated individually for activated integrin αIIbβ₃ and P-selectin surface expression. (B) Transcript levels of P-selectin (Selp) relative to 18S in cultured megakaryocytes. Significance was determined using an unpaired 2-tailed Student t test *P < .05; n = 5. (C) Integrin activation (αIIbβ₃) in donor (GFP⁺) platelets. Analysis was performed using 2-way ANOVA with Bonferroni posttest, n ≥ 3. (D) P-selectin surface expression in donor (GFP⁺) platelets. Significance was determined using a 2-way ANOVA followed by a Bonferroni posttest, **P < .001; n ≥ 3. (E) Integrin activation (αIIbβ₃) in donor (GFP⁻) platelets. Analysis was performed using 2-way ANOVA with Bonferroni posttest, n ≥ 3. (F) P-selectin surface expression in donor (GFP⁻) platelets. No significance was detected using a 2-way ANOVA with Bonferroni posttest, n ≥ 3.