CD45 expression discriminates waves of embryonic megakaryocytes in the mouse.

Embryonic megakaryopoiesis starts in the yolk sac on gestational day 7.5 as part of the primitive wave of hematopoiesis, and it continues in the fetal liver when this organ is colonized by hematopoietic progenitors between day 9.5 and 10.5, as the definitive hematopoiesis wave. We characterized the precise phenotype of embryo megakaryocytes in the liver at gestational day 11.5, identifying them as CD41CD45-CD9CD61MPLCD42c tetraploid cells that express megakaryocyte-specific transcripts and display differential traits when compared to those present in the yolk sac at the same age. In contrast to megakaryocytes from adult bone marrow, embryo megakaryocytes are CD45 until day 13.5 of gestation, as are both the megakaryocyte progenitors and megakaryocyte/erythroid-committed progenitors. At gestational day 11.5, liver and yolk sac also contain CD41CD45 and CD41CD45 cells. These populations, and that of CD41CD45CD42c cells, isolated from liver, differentiate in culture into CD41CD45CD42c proplatelet-bearing megakaryocytes. Also present at this time are CD41CD45CD11b cells, which produce low numbers of CD41CD45CD42c megakaryocytes , as do fetal liver cells expressing the macrophage-specific Csf receptor-1 (Csf1r/CD115) from MaFIA transgenic mice, which give rise poorly to CD41CD45CD42c embryo megakaryocytes both and In contrast, around 30% of adult megakaryocytes (CD41CD45CD9CD42c) from C57BL/6 and MaFIA mice express CD115. We propose that differential pathways operating in the mouse embryo liver at gestational day 11.5 beget CD41CD45CD42c embryo megakaryocytes that can be produced from CD41CD45 or from CD41CD45 cells, at difference from those from bone marrow.