Transmission of substances through the exocoelomic cavity after introduction into the vitelline circulation by cannulation: What route?

The morphology of the main vitelline vein and its tributaries which carry the embryotroph from the yolk sac into the rat embryo has been studied by electron microscopy after perfusing the conceptus with a solution of lanthanum nitrate in Karnovsky's fixative. The distribution of the contents of these vessels and the routes taken into and out of the various embryonic compartments have also been investigated. The vitelline vein and its tributaries are lined by a discontinuous endothelial layer, with no basement membrane or mural elements, and it is separated from the exocoelomic cavity by a continuous layer of squamous cells. In addition to the lumina of the vessels of the conceptus, lanthanum nitrate was observed in the mesenchymal space surrounding the yolk sac, the intercellular spaces between the yolk sac endodermal cells but not on their apical surfaces, the intercellular spaces between the cells lining the exocoelomic cavity, the exocoelomic cavity, the mesenchymal space around the umbilical vessels and the intercellular spaces between the ectodermal cells of the embryo. It has been demonstrated that substances enter the exocoelomic cavity mainly through the intercellular spaces of its lining cells via the mesenchymal space around the main vitelline vein and its tributaries. Whilst we were unable to demonstrate gaps in the endothelial lining of the umbilical vessels, it seems to be the likeliest explanation for the presence of lanthanum around its extravascular space. The significance of the distribution of the contents of he vitelline vasculature is discussed.