Live imaging of emerging hematopoietic stem cells and early thymus colonization.

We recently demonstrated in zebrafish the developmental migration of emerging hematopoietic stem cells (HSCs) that is thought to occur in mammalian embryos, from the aorta-gonad-mesonephros (AGM) area to the successive hematopoietic organs. CD41 is the earliest known molecular marker of nascent HSCs in mammalian development. In this study, we show that in CD41-green fluorescent protein (GFP) transgenic zebrafish embryos, the transgene is expressed by emerging HSCs in the AGM, allowing us for the first time to image their behavior and trace them in real time. We find that the zebrafish AGM contains no intra-aortic cell clusters, so far considered a hallmark of HSC emergence. CD41GFP(low) HSCs emerge in the subaortic mesenchyme and enter the circulation not through the dorsal aorta but through the axial vein, the peculiar structure of which facilitates their intravasation. The rise in CD41-gfp expression among c-myb(+) HSC precursors is asynchronous and marks their competence to leave the AGM and immediately seed the caudal hematopoietic tissue (which has a hematopoietic function analogous to that of the mammalian fetal liver). Imaging the later migration of CD41-GFP(+) precursors to the nascent thymus reveals that although some reach the thymus by extravasating from the nearest vein, most travel for hours through the mesenchyme from surprisingly diverse and remote sites of extravasation.