Visceral endoderm hematopoiesis

Visceral Endoderm Provides a Signal(s) Required for Embryonic Hematopoiesis... 

To establish more directly that visceral endoderm is required for induction of hematopoiesis in the gastrulating embryo, tissue recombination experiments were performed. A semi-quantitative RT-PCR protocol was used to assay for activation of the endogenous mouse embryonic p-like globin genes (15), Ectoderm and VE layers from individual embryos were cultured separately or in combination for two to four days. RNA was prepared from individual explants and analyzed for embryonic globin gene activation using the RT-PCR assay. 

We have demonstrated 4) that primitive (visceral) endoderm secretes diffusible signal(s) that are required for the initiation of hematopoiesis and vasculogenesis during embryogenesis. Induction of mesoderm precedes the cellular and molecular events reflected in the explant cultures (our unpublished results). A number of important questions remain unresolved ... 

Might the molecules responsible for respecification of cell fate observed in the reprogramming assay be distinct from those involved in activation of hematopoiesis from nascent posterior mesoderm in vfvol The induction (epiblast explant) assay presumably reflects what occurs in vivo, while the reprogramming (anterior ectoderm) assay reflects the potential activity of visceral endoderm signals. Anterior ectoderm is normally fated to form neurectoderm, not blood or endothelial cells. 

Do any of the signaling molecules produced by visceral endoderm play a role in adult hematopoiesis While primitive (visceral) endoderm cells are not thought to contribute directly to tissues wifliin the developing animal (those endodermal derivatives form from a different, definitive lineage), they express many of the same genes as endodermal cells of the liver or gut (5,6). 

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