Embryonic development of the human hematopoietic system

Abstract

Human hematopoiesis is initiated in the yolk sac during the third week of development. At the same time the capacity to produce blood cells also arises in the embryo, within the splanchnopleura, but this potential is not expressed before day 27, when clustered hematopoietic stem cells emerge from the ventral wall of the aorta and vitelline artery. Budding of hematopoietic cells from vessel walls reflects the re-differentiation of local endothelial cells, which are likely derived from angio-hematopoietic mesodermal ancestors emigrated from the splanchnopleura. Yolk sac-derived stem cells are limited to myelo-erythroid development, whereas those born in the embryo are, in addition, lymphopoietic and therefore represent the first multi-potent, adulttype blood progenitors that appear in human ontogeny, preceding shortly the onset of liver hematopoiesis. These results allowed the establishment of a novel hierarchy of blood-forming tissues in human development and induced an in depth reconsideration of the very origin of definitive human hematopoiesis. These results also fully corroborate the outcome of experiments performed in parallel in avian and mouse embryos and point to the conservation in all higher vertebrates of an ancestral route of blood cell production via embryonic vessel walls.

The characteristics of vessel lining cells in normal spleens and their role in the pathobiology of myelofibrosis

“…33 As a secondary lymphoid organ, the adult spleen supports the proliferation and differentiation of phagocytic and antigenpresenting cells in response to inflammation from precursors traveling through circulation. 27,[34][35][36][37] Very limited numbers of hematopoietic stem cells/progenitor cells (HSC/PCs) have been detected in adult normal human spleens.…”

Osteopontin: a bridge between bone and blood

“…Initially few, if any CD34 + cells are found within these logettes and haemopoiesis takes place at a distance from ossifying cartilage trabeculae and osteoblasts. Beyond 15 weeks, haemopoietic stem cell (HSC) numbers increase within the developing BM cavities and during the remainder of gestation haemopoiesis completes transition from the fetal liver to become established within the cavities of bones at many locations within the developing skeleton (Tavian & Peault, 2005). Thereafter in the adult, under steady-state conditions, haemopoiesis is restricted to the BM and is responsible for the ongoing production of vast numbers of maturing blood cells throughout life.…”

Embryonic origin of human hematopoiesis

“…Indeed, cells expressing markers of hematopoietic progenitors are present in the dorsal aorta (reviewed in Cumano and Godin, 2007). We have participated in this discovery by identifying in the human embryo a dense population of HSC emerging in association with the ventral side of the aortic endothelium, between the 27 th and the 40 th day of development (reviewed in Tavian and Peault, 2005a).…”