Hemangioblast commitment in the avian allantois: Cellular and molecular aspects

We recently identified the allantois as a site producing hemopoietic and en dothelial cells capable of colonizing the bone marrow of an engrafted host. Here, we report a detailed investigation of some early cytological and mol ecular processes occurring in the allantoic bud, which are probably involve d in the production of angioblasts and hemopoietic cells. We show that the allantois undergoes a program characterized by the prominent expression of several "hemangioblastic" genes in the mesoderm accompanied by other gene p atterns in the associated endoderm. VEGF-R2, at least from stage HH17 onwar d, is expressed and is shortly followed by transcription factors GATA-2, SC L/tal-1, and GATA-1. Blood island-like structures differentiate that contai n both CD45(+) cells and cells accumulating hemoglobin; these structures lo ok exactly like blood islands in the yolk sac. This hemopoietic process tak es place before the establishment of a vascular network connecting the alla ntois to the embryo. As far as the endoderm is concerned, GATA-3 mRNA is fo und in the region where allantois will differentiate before the posterior i nstestinal portal becomes anatomically distinct. Shortly before the bud gro ws out, GATA-2 was expressed in the endoderm and, at the same time, the hem angioblastic program became initiated in the mesoderm. GATA-3 is detected a t least until E8 and GATA-2 until E3 the latest stage examined for this fac tor. Using in vitro cultures, we show that allantoic buds, dissected out be fore the establishment of circulation between the bud and the rest of the e mbryo, produced erythrocytes of the definitive lineage. Moreover, using het erospecific grafts between chick and quail embryos, we demonstrate that the allantoic vascular network develops from intrinsic progenitors. Taken toge ther, these results extend our earlier findings about the commitment of mes oderm to the endothelial and hemopoietic lineages in the allantois. The det ection of a prominent GATA-3 expression restricted to the endoderm of the p reallantoic region and allantoic bud, followed by that of GATA-2, is an int eresting and novel information, in the context of organ formation and endod erm specification in the emergence of hemopoietic cells. (C) 2001 Academic Press.