Induction hematopoietic mesoderm

In nearly all vertebrate animals, embryonic blood development begins during gastrulation and results from the induction of extraembryonic mesoderm to form hematopoietic tissue. In the mouse, these events are initiated at around 6.5 days post coitum (p.c.) and lead to the formation of the yolk sac, a bilaminar membrane composed of adjacent mesodermal and primitive endodermal cell layers (reviewed in ref. 3). The yolk sac is an extraembryonic tissue which surrounds the entire embryo. Though it will not contribute cells directly to the fully formed animal, its fimction is essential to normal development (reviewed in refs. 3,6). 

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