ABSTRACT
Blood cells are constantly produced from a pool of progenitors that ultimately derive from hematopoietic stem cells (HSC). In vertebrates, the hematopoietic system develops from two distinct waves or generation of precursors. The fi rst occurs in the yolk sac, or equivalent embryonic structure, and produces nucleated primitive erythrocytes, that provide the embryo with the fi rst transporter of oxygen and are therefore essential for the viability of the early embryo. The yolk sac also produces myeloid cells that migrate to the central nervous system and to the skin, to form the microglia and skin specifi c macrophages, the Langerhans cells. After a fi rst period of expansion, these populations become resting and are found throughout adult life. The second generation occurs in the dorsal aorta and produces HSC. These are generated once in the lifetime, from mesoderm
1Unit of Lymphopoiesis, Department of Immunology, INSERM U668, Pasteur Institute, rue du Docteur Roux 75015 Paris. aEmail: paulo.vieira@pasteur.fr bEmail: ana.cumano@pasteur.fr 2Univ. Paris Diderot, Sorbonne Paris Cité (Cellule Pasteur), rue du Docteur Roux 75015 Paris. Email: rachel.golub@pasteur.fr *Corresponding author
List of abbreviations after the text.
