PECAM-1 null marrow exhibits excessive megakaryocytopoiesis and altered megakaryocyte localization. (A) Comparison of the numbers of megakaryocytes per high-power field in the femurs of WT, Het, and CD31KO littermates (n = 12). (B) Comparison of percentage of CD41a⁺ cells in WT and CD31KO BM (n = 11). (C) Comparison of Tpo concentration in WT and CD31KO peripheral blood serum (n = 3). (D-F) Representative low-power (10×) micrographs of hematoxylin and easin staining of WT, Het, and CD31KO femoral bone marrow sections. In WT (panel D) and Het (panel E) marrow, many WT megakaryocytes were shown to be associated with the sinusoidal vasculature (green asterisks), while most CD31KO megakaryocytes (F) were resident in the stromal compartment (red asterisks). (G) Representative high-power (20×) micrographs of WT, Het, and CD31KO marrow sections illustrating megakaryocytes (M) intimately associated with marrow vessels (V) in the top panels and megakaryocytes in the stromal compartment of the marrow in the bottom panels. (H) Quantification of megakaryocyte cell numbers in each × 20 high-power-field revealed significantly more megakaryocytes associated with sinusoidal vessels in the WT and Het marrows, while most CD31 KO megakaryocytes were observed to be resident in the stromal areas. Specifically, quantitation revealed a ratio of megakaryocyte localization in vasculture to stromal niches of 2:1 in WT and Het BMs and 1:4 in CD31KO BMs (n = 12). Vertical lines in the bar graphs represent standard deviations.