Origin of stem hemopoietic cells in embryonic development

The main hematopoietic organ of mature mammals is the bone marrow, in which hemopoietic stem cells (HSCs) are maintained throughout life, and the production of mature blood cells is regulated. The adult HSCs are characterized by polypotency, self-maintenance and a specific phenotype, as well as the ability to direct migration to the hematopoietic organs. The ability of the HSCs being transferring to another organism to restore hemopoiesis is the main functional criterion for their existence in tissue. However, functioning in postnatal ontogenesis and possessing these properties, HSCs represent the final stage of maturation of their precursors (pre-HSCs) which arise in prenatal development. In embryogenesis, hemopoiesis occurs in several transitional blood-forming organs - the yolk sac, aorto-gonadone-mesonephros (AGM), placenta, and liver. Despite the long history of hematopoietic system ontogeny studying, it is still not entirely clear in which anatomical site the first pre-HSCs originate, giving rise to the definitive HSCs line. The review considers modern concepts of the features of the hematopoietic cells formed in the yolk sac, AGM and placenta, and their contribution to embryonic liver colonization and in definitive hematopoiesis. Further study of the mechanisms of HSCs formation in embryogenesis is of undoubted importance not only for understanding the fundamental aspects of the functioning of the hematopoietic system, but also for improving the methods of treating hematological diseases.