Potential intraembryonic hemogenic sites at pre-liver stages in the mouse.

In the course of a previous experimental study on the early development of the mouse embryo hemopoietic system, we found that, at the 10-25 pairs of somite stages, the para-aortic splanchnopleure contains hemopoietic progenitors. Trying to discover a structural basis for this potentiality, we have looked for cytological signs of hemopoiesis in the embryo proper between 8.5 and 12 days post-coitum, i.e. prior to full-blown fetal liver hemopoiesis. Two suggestive findings are reported: (1) intra-arterial hemopoietic cells aggregates are present in the omphalomesenteric and umbilical arteries and to a lesser degree in the dorsal aorta; (2) cells groups resembling yolk sac blood islands are observed in the mesentery. The intra-arterial aggregates are strikingly similar to the intra-aortic clusters of avian embryos. These cytological aspects provide the anatomical basis underlying recent functional data that revealed the hemogenic capacity of the para-aortic splanchnopleure.