Microarray using CD31+/CD41-/CD45- cells from E9.5 mouse heart tube, caudal half and yolk sac

Hematopoietic cells arise from spatiotemporally restricted domains in the developing embryo. Although studies of non-mammalian animal and in vitro embryonic stem cell models suggest a close relationship among cardiac, endocardial, and hematopoietic lineages, it remains unknown whether the mammalian heart tube serves as a hemogenic organ akin to the dorsal aorta. Here, we examined the hemogenic activity of the developing endocardium. Mouse heart explants generated myeloid and erythroid colonies in the absence of circulation. Hemogenic activity arose from a subset of endocardial cells in the outflow cushion and atria earlier than in the aorta-gonad-mesonephros region, and was transient and definitive in nature. Interestingly, key cardiac transcription factors, Nkx2-5 and Isl1, were expressed in and required for the hemogenic activity of the endocardium. Together, these data suggest that a subset of endocardial and yolk sac endothelial cells expressing cardiac markers serve as a de novo source for transient definitive hematopoietic progenitors. Two independent biological duplicates of freshly isolated mouse tissues (caudal half, heart tube, yolk sac) were sorted for CD31+/CD41-/CD45- cells.