Aorta-associated CD34+ hematopoietic cells in the early human embryo.

Hematopoiesis is established from circulating blood stem cells that seed the embryonic rudiments of blood-forming tissues, a basic notion in developmental hematology. However, the assumption that these stem cells originate from the extraembryonic mesoderm, where primitive hematopoiesis is initiated by intrinsic precursors, has been reconsidered after analysis of blood cell development in avian embryo chimeras: yolk-sac-derived stem cells do not contribute significantly to the definitive blood system, whose first forerunners develop independently along the ventral aspect of the embryonic aorta. Recently, the homologous intraembryonic tissues of the mouse have been submitted to sensitive in vivo and in vitro assays, which showed that they also harbor multipotential hematopoietic stem cells. We have now identified a dense population of hematogenous cells, marked by the surface expression of the CD34 glycoprotein, associated with the ventral endothelium of the aorta in the 5-week human embryo. Therefore, we extend to the human species the growing evidence that intraembryonic hematopoietic cells developing independently of the yolk sac might be the real stem of the whole blood system.

In vivo diversification and migrations of chick embryo heart muscle cells: a morphometric analysis with ALV- and SNV-based non-replicative vectors.

By means of a reporter gene we previously demonstrated that non-replicative Avian Leukemia Virus- and Spleen Necrosis Virus-based retroviral vectors were preferentially expressed in the heart of avian embryos from different species. Using a computer-assisted approach, we now compare clones tagged by the two types of vectors, for volume, anatomical and subanatomical localisation, number of Hoechst-stained cell nuclei and mean cell division time during the period of heart morphogenesis, i.e. from stages 17-19 to 34 of Hamburger and Hamilton (1951). This analysis demonstrates that clones labelled by the two types of viruses display similar features and bring about new insights on the relationships between mitotic and migratory properties of the myocardial cells and histogenesis of the heart. Since only exteriormost cells were tagged with our inoculation procedure, our analysis shows that: (1) at stages 17-19, the myocardium is composed of cells with diverse potentials; some cells still retain the capacity to divide extensively and participate to different heart muscle layers, whilst most are restricted in their multiplication potential and contribute to single muscle layers; (2) about half of the clones are located deep in the heart wall, revealing extensive cell migrations from the heart surface to the ventricular trabeculae, the first migrating cells tagged being detected 20 h after viral inoculation. The presence of these cells is consistent with the finding of a large number of compact trabecular clones 5 days later suggesting that these cells divide mainly after completing migration. Our approach provides new insights as well as quantitative data on the different processes involved in heart morphogenesis, namely multiplication, migration and localisation of heart muscle cells.