PECAM-1 null marrow exhibits fewer functional megakaryocytic progenitors in short-term colony formation assay. (A) CFU assay comparing the number of functional hematopoietic stem cells (HSCs) between WT, Het, and CD31KO littermate BMs. Input cells (5 × 10⁴) for CFU assays were erythrocyte-depleted whole BM cells (n = 3). (B) CFU-MK assay comparing the functional megakaryocytic progenitor cell numbers between WT, Het, and CD31KO marrow. The 5 × 10⁴ input cells were erythrocyte-depleted whole BM cells. The megakaryocytic colonies were identified using acetylcholine esterase assays (n = 3). (C) The presence of CD31 on HSCs affects their function. The Lin⁻ c-kit⁺ WT cells were sorted into 2 populations: Lin⁻, c-kit⁺, CD31⁺ and Lin⁻, c-kit⁺, CD31⁻, followed by CFU assays using 10 000 sorted cells as inputs. The sorted Lin⁻ c-kit⁺ population from CD31KO mouse marrow was used as a control. In the absence of CD31, WT hematopoietic progenitor cells were less functional in terms of CFU capability (n = 3). Vertical lines represent standard deviations.