Fig. 2. Establishment of definitive hematopoietic stem cell (HSC) pools in mouse and human embryos. (A) Hematopoietic development starts as specification of primitive streak mesoderm (gray) into hematopoietic and vascular fates. Nascent HSCs undergo a maturation process (blue) that allows them to engraft, survive and self-renew in future hematopoietic niches. Subsequently, fetal HSCs expand rapidly, after which a steady state is established in which HSCs reside in a relatively quiescent state in the bone marrow. (B) The ages at which mouse and human hematopoietic sites are active. Gray bars, mesoderm; red bars, active hematopoietic differentiation; yellow bars, HSC genesis; blue bars, presence of functional adult-type HSCs. Broken yellow bars for yolk sac and placenta indicate that de novo HSC genesis has not been experimentally proven.