Hemopoietic Cell Progenitors in the Avian Embryo: Origin and Migrations

The cellular events involved in the formation of hemopoietic progenitors during early embryogenesis are critical for the lifelong function of the blood‐forming system if, as is supposed, these progenitors arise during ontogeny and thereafter maintain their stock by self renewal. Several aspects of these events have been unravelled in recent years. Basic data on the interactions between stem cells and hemopoietic microenvironments have emerged from studies of the avian embryo by experimental approaches that are possible only in that class at the time of organogenesis. Interestingly, when it has been possible to devise appropriate experimental models, similar mechanisms have often been detected in mammals. When early developmental processes are investigated, difficulties arise from the small contingent of cells available. Cell markers have made it possible to trace individual cells and thus to follow lineages through their phenotypic changes and their extensive migrations. In the avian embryo, a cell‐marking technique devised by N. Le Douarin in 19691 has become widely used for the study of various lineages. It consists in associating cells (or tissues, rudiments, embryonic territories) from two species, the chicken and the quail. Cell nuclei have a different appearance in these two species (Figure 1). In recent years, the distinction of blood cells according to species in this heterospecific system has become refined by the availability of two monoclonal antibodies with affinity restricted to the quail species and which recognize the hemangioblastic lineage, MB12 and QH13 (Figure 2). The hemangioblastic lineage comprises, on the one hand, endothelial cells, and on the other, blood cells and their progenitors. The points to consider about the ontogeny of the blood system are the relationships between stem cells and hemopoietic organ rudiments, the spatial and chronological origins of stem cells during ontogeny, the mechanisms through which they settle into the stromas of hemopoietic organs and the nature of the relationship between endothelial and hemopoietic cells.