Fig. 2. Principle scenarios for vertebrate hematopoietic development. A schematic representation of hematopoietic system generation during embryonic development. (A) Prior to the avian yolk sac chimera experiments of Dieterlen-Lièvre, the hematopoietic system was thought to emerge once during development and to originate from a single cohort of mesenchymal/endothelial-type precursors or hemangioblasts (stellar-like cells). Primitive erythroid cells (red punctate areas) first appear and are followed by the appearance of myeloid or mixed erythroid-myeloid hematopoietic progenitors (yellow region transiting into the region of graded red). (B) In the JEEM publication, Dieterlen-Lièvre showed that two independent waves of hematopoietic generation exist during avian development: the transitory, embryonic hematopoietic cell hierarchy coming from the yolk sac (left curve containing red-punctate and yellow regions); and the adult definitive hematopoietic cell hierarchy coming from the body of the embryo (right curve containing graded red region). The hierarchies emerge independently from at least two distinct types of embryonic precursors (indicated by two stellar cells) and overlap in developmental time. This scenario became the prevailing model in the field of developmental hematopoiesis, and was followed by the finding in mammals that immature adult-type progenitors and HSCs develop with a significant delay in the conceptus (Medvinsky and Dzierzak, 1996; Medvinsky et al., 1993; Muller et al., 1994).