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 ca rry the embryotroph from the yolk sac into the rat embryo has been stu died by electron microscopy after perfusing the conceptus with a solut ion 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 vitel line vein and its tributaries are lined by a discontinuous endothelial layer, with no basement membrane or mural elements, and it is separat ed 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 sa c, the intercellular spaces between the yell; sac endodermal cells but not on their apical surfaces, the intercellular spaces between the ce lls lining the exocoelomic cavity, the exocoelomic cavity, the mesench ymal space around the umbilical vessels and the intercellular spaces b etween the ectodermal cells of the embryo, It has been demonstrated th at substances enter the exocoelomic cavity mainly through the intercel lular spaces of its lining cells via the mesenchymal space around the main vitelline vein and its tributaries, Whilst we were unable to demo nstrate gaps in the endothelial lining of the umbilical vessels? it se ems to be the likeliest explanation for the presence of lanthanum arou nd its extravascular space, The significance of the distribution of th e contents of he vitelline vasculature is discussed.