Detailed characterization of the human aorta-gonad-mesonephros region reveals morphological polarity resembling a hematopoietic stromal layer

The definitive long-term repopulating human hematopoietic stem cell, which seeds the adult blood system, was previously thought to derive from the ext ra-embryonic yolk sac, However, there is now considerable evidence that in both avian and murine systems, yolk sac hematopoietic cells are largely a t ransient, embryonic population and the definitive stem cell, in fact, deriv es from a distinct region within the embryonic mesoderm, the aorta-gonad-me sonephros region. In the human embryo, an analogous region has been found t o contain a cluster of cells distinct from, but closely associated with, th e ventral endothelium of the dorsal aorta, the appearance of which is restr icted both spatially and temporally. We have used antibodies recognising he matopoietic regulatory factors to further characterise this region in the h uman embryo. These studies indicate that all factors examined, including va scular endothelial growth factor and its receptor FLK-1, Flt-3 ligand and i ts receptor STK-1, and stem cell leukemia transcription factor, are express ed by both hematopoietic cells in the cluster and endothelial cells, Howeve r there is some discontinuity in cells directly underlying the cluster. Fur thermore, we have identified a morphologically distinct region of densely-p acked, rounded cells in the mesenchyme directly beneath the ventral wall of the dorsal aorta, and running along its entire length. In the preumbilical AGM region, directly underlying the hematopoietic cluster, but not at more rostral and caudal levels, this region of mesenchyme expresses tenascin-C, an extracellular matrix glycoprotein known to facilitate cell-cell interac tions and migration. This region of cells may therefore provide the microen vironmental support for the intraembryonic development of definitive hemato poietic stem cells, a process in which tenascin-C may play a pivotal role. Dev Dyn 1999;215:139-147, (C) 1999 Wiley-Liss, Inc.