Morphometric analysis of the visceral yolk sac endoderm in the rat in vivo and in vitro.

Rat embryos together with their visceral yolk sac were grown in vitro during the early period of organogenesis from days 9.5 to 11.5 and the ultrastructural morphology od the visceral yolk sac cells was compared with that in vivo at the beginning and at the end of the culture period. Identical areas of the visceral yolk sac endodermal cells were analysed morphometrically. A measure of the functional activity was obtained by comparison of the volume density of surface density of the vacoular system. At 9.5 and 11.5 days, the values obtained were virtually identical for yolk sacs in vivo and in vitro. At 9.5 days, the volume and surface density of the endocytotic vacuoles in the embryonic endoderm was significantly lower than in the visceral yolk sac endoderm. It is concluded that the digestive function of the yolk sac is almost certainly identical in culture and in vivo and that the cells of the embryonic endoderm do not take a significant part in embryonic nutrition.