Fetal hematopoietic stem cells are making waves

Hematopoiesis first arises during early embryogenesis when passive diffusion of oxygen and nutrients becomes insufficient to support the developing organism. Early hematopoietic development is largely focused on the production of red blood cells and tightly linked to concomitant vascular development. As a result of their coordinated functions, both the hematopoietic and cardiovascular systems are essential for fetal survival, unlike other organ systems. Beyond these initial functions, the fetal hematopoietic system evolves to generate a diverse output of mature elements, including erythrocytes, platelets, macrophages, other myeloid cells, innate lymphocytes, as well as T and B lymphocytes arising from progenitors that have gained the capacity to rearrange antigen receptor genes. During fetal life, subsets of hematopoietic progenitors acquire long-term self-renewal potential as assessed after transplantation into lethally irradiated hosts, a functional property that has been used historically to define hematopoietic stem cells (HSCs) (1-4).