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Summary On Day 7 of pregnancy, when the inner cell mass coalesces to one side of the blastocyst, a discontinuous layer of cells is present underlying the inner cell mass and isolated cells were found around the mural trophoblast. As early as Day 8, these cells that were scattered around the blastocyst cavity unite to form a continuous endodermal layer, resulting in a bilaminar blastocyst. The endodermal cells have slightly bulging apical surfaces towards the cavity of the blastocyst, but in many areas have flanges and projections that form complex basolateral compartments within the endoderm and especially between the endoderm and trophoblast. These cells also develop an extensive series of unusual apical ridges. With the development of the primitive streak and the trilaminar blastocyst, angiogenesis proceeds from the forming embryo into the yolk sac. At first the anastomotic vessels form a simple mesh, but by Day 20, erythroblasts are present in the newly formed sinus terminalis. The loss of the capsule (Day 21) and the development of the embryonic circulation establish the relationships of the definitive yolk sac. There is a bilaminar region with endodermal cells showing basolateral compartmentalisation and a specialised sinus terminalis region bordering the choriovitelline placenta. The largest endodermal cells are associated with the choriovitelline placenta, but a substantial cuboidal lining also covers the splanchnopleuric portion of the vascular yolk sac. The allantochorion rapidly assumes predominance in respiratory exchanges, and the choriovitelline placenta not only decreases in relative area but increases in thickness. However, the 3 major regions of the yolk sac persist for several weeks after their formation, and both the vasculature and especially the endodermal cells remain robust despite the decrease in size of the yolk sac. It is concluded that the pleomorphic endodermal cells of the equine yolk sac differentiate in response to local signals. By Day 25 the allantochorion is an increasingly significant exchange unit, and the continued robustness of the vascular portions of the yolk sac and especially its endodermal layer suggests that synthetic and metabolic activities of the endodermal cells will be the dominant function of the yolk sac subsequently.

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