Intra-aortic hematopoietic cells

Abstract

Life-long function of the blood-forming system depends on a pool of self-renewable Hematopoietic Stem Cells (HSCs). During ontogeny, these cells seed the rudiments of hematopoietic and lymphoid organs, whether they are mesodermal (bone marrow, spleen, milky spots of the omentum, secondary lymphoid organs) or endodermal/mesodermal (thymus, bursa of Fabricius, fetal liver). The only exception is the yolk sac, which produces its own progenitors and stem cells. To grasp how the adult HSCs pool is maintained, it is important to understand how HSCs become committed and segregated during development. It was once thought that these cells emerged once for all, early in ontogeny, in the yolk sac (or, in amphibians, in the yolk sac-equivalent, the ventral blood island) [1]. However it was known that the cellular and molecular features of blood cells, notably red cells, changed along the course of development, a fact that might indicate either an environmental influence of the differentiation site or intrinsic properties of successive generations of HSCs. An experimental model, consisting of a quail embryo developing on a chicken yolk sac, then disclosed the existence of an intra-embryonic origin source of HSCs [2]. In these chimeras the definitive hematopoietic organs were colonized by HSCs from the embryo and hemoglobin switches could be related to the emergence of these intra-embryonic HSCs [3].