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Abstract

In the mouse embryo, hematopoietic progenitor cells migrate to the fetal liver (FL) between gestational days (E) 9.5 and 10.5, where they rapidly expand to form the main fetal reservoir of hematopoietic cells. The embryonic megakaryocyte progenitors (MKPs) in the E11.5 FL were identified as CD49fHCD41H (and c-KitDKDR+CD42+CD9++CD31+) cells, expressing several hepato-specific proteins. Unlike adult bone marrow megakaryocytes (MKs), embryonic MKPs were CD45 and represent an abundant population in the FL. The CD49fHCD41H MKPs purified by cytometry differentiated in vitro to produce proplatelets, independent of thrombopoietin stimulation, and they responded to stimulation with adenosine diphosphate, thrombin, and the PAR4 thrombin receptor-activating peptide. Moreover, after removing CD49fHCD41H MKPs from purified E11.5 FL hepatoepithelial-enriched cell preparations (c-KitDCD45Ter119), the remaining CD49fD cells neither differentiated nor survived in vitro. Indeed, direct cell-to-cell contact between the CD49fHCD41H and CD49fD populations was required to promote the hepatocyte differentiation of CD49fD cells. The addition of vascular endothelial growth factor A (VEGF-A) and medium conditioned by E11.5 CD49fHCD41H MKPs produced a partial effect on CD49fD cells, inducing the formation of hepatoepithelial layers. This effect was abolished by anti-VEGF-A antibodies. Together, these findings strongly suggest that CD49fHCD41H MKPs are fundamental to promote FL development, as proposed in adult liver regeneration. Conclusion: The cells of the MK lineage present in the developing mouse embryo liver promote the growth of hepatoepithelial cells in vitro through VEGF-A signaling and may play a role in liver development in vivo. (HEPATOLOGY 2012;56:1934–1945)