Alterations of blood composition in 5-HT_2BR mutant mice. (A) In total blood, the number of white blood cells (WBCs), red blood cells (RBCs), the mean red blood cell volume (MRBCV), the hematocrit, and the hemoglobin content were not different among genotypes, whereas the number of platelets was significantly reduced and the percentage of neutrophil/granulocyte (% N/Gr) significantly increased in the blood of 5-HT_2B^−/− mice. Values are means ± SEM (n = 20, P < .05). Any statistical difference by unpaired t test versus control is indicated by ***P < .001 and **P < .01. (B) Flow cytometric analysis of blood cells using cell-surface markers allows identification of different populations. Mac-1 (CD11b) for monocytes/macrophages, Gr-1 (Gr+) for Granulocytes, PECAM (CD31) for endothelial cells/platelets/macrophages antibodies were used to identify lin⁺ cells or late multipotent progenitors. In blood, although the number of c-Kit⁺Lin⁻ cells was unaffected, the 5-HT_2B^−/− mice had reduced number of CD11b⁻/CD31⁺, immature endothelial progenitors. Values are means ± SEM (n = 10, P < .05). Any statistical difference by unpaired t test versus control is indicated by ***P < .001. (C) By flow cytometric purification of peripheral blood mononuclear cells, we first identified expression of 5-HT_2BRs in c-kit⁺ cells but not in c-kit⁻ cells (28.4 ± 7.9 fmoles/mg protein vs 4.6 ± 3.2, P < .05, n = 14). Any statistical difference by unpaired t test versus control is indicated by ***P < .001.