POTENTIAL INTRAEMBRYONIC HEMOGENIC SITES AT PRE-LIVER STATES IN THE MOUSE

In the course of a previous experimental study on the early developmen t of the mouse embryo hemopoietic system, we found that, at the 10-25 pairs of somite stages, the para-aortic splanchnopleure contains hemop oietic progenitors. Trying to discover a structural basis for this pot entiality, 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 omph alomesenteric and umbilical arteries and to a lesser degree in the dor sal aorta; (2) cells groups resembling yolk sac blood islands are obse rved in the mesentery. The intra-arterial aggregates are strikingly si milar to the intra-aortic clusters of avian embryos. These cytological aspects provide the anatomical basis underlying recent functional dat a that revealed the hemogenic capacity of the para-aortic splanchnople ure.