Kivonat:
We recently identified the allantois as a site producing
hemopoietic and endothelial cells capable of colonizing the bone
marrow of an engrafted host. Here, we report a detailed
investigation of some early cytological and molecular processes
occurring in the allantoic bud, which are probably involved 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 patterns in the associated
endoderm. VEGF-R2, at least from stage HH17 onward, is expressed
and is shortly followed by transcription factors GATA-2,
SCL/tal-1, and GATA-1. Blood island-like structures
differentiate that contain both CD45(+) cells and cells
accumulating hemoglobin; these structures look exactly like
blood islands in the yolk sac. This hemopoietic process takes
place before the establishment of a vascular network connecting
the allantois to the embryo. As far as the endoderm is
concerned, GATA-3 mRNA is found in the region where allantois
will differentiate before the posterior instestinal portal
becomes anatomically distinct. Shortly before the bud grows out,
GATA-2 was expressed in the endoderm and, at the same time, the
hemangioblastic program became initiated in the mesoderm. GATA-3
is detected at least until E8 and GATA-2 until E3 the latest
stage examined for this factor. Using in vitro cultures, we show
that allantoic buds, dissected out before the establishment of
circulation between the bud and the rest of the embryo, produced
erythrocytes of the definitive lineage. Moreover, using
heterospecific grafts between chick and quail embryos, we
demonstrate that the allantoic vascular network develops from
intrinsic progenitors. Taken together, these results extend our
earlier findings about the commitment of mesoderm to the
endothelial and hemopoietic lineages in the allantois. The
detection of a prominent GATA-3 expression restricted to the
endoderm of the preallantoic region and allantoic bud, followed
by that of GATA-2, is an interesting and novel information, in
the context of organ formation and endoderm specification in the
emergence of hemopoietic cells.