[Hematopoietic stem cell emergence and stem cell-independent hematopoiesis in the mouse embryo]

All blood cells are produced by hematopoietic stem cells (HSCs) in the adult bone marrow. However, prior to the emergence of the first HSCs at embryonic day (E) 11, mouse embryos present several hematopoiesis waves, such as primitive hematopoietic cells, definitive erythro-myeloid progenitors, lymphoid potent cells, and multi-potent progenitors. Non-HSC-derived hematopoiesis is called HSC-independent hematopoiesis and has been considered a transient wave that is diminished after birth. Recent reports have shown that tissue-resident macrophages are derived from the yolk sac (YS) and are HSC-independent. Similarly, the presence of a developmental pathway for innate-like B-1a cells, independent of fetal liver HSCs, has been shown, although the question of whether B-1a cells are produced by fetal liver HSCs remains controversial. The present review focuses on the process of HSC development and introduces recent information regarding HSC-independent tissue macrophages and B-1 lymphocytes.