Surface morphology of the human yolk sac: endoderm and mesothelium.

The yolk sac of human embryos from the 5th to 7th week of gestation was first revealed under the scanning electron microscope (SEM) with complementary observations under the transmission electron microscope (TEM). The inner and outer surfaces of the human yolk sac basically showed profiles similar to those of other mammalian yolk sacs reported by previous workers. The free surface of the endodermal cell measured 10-15 microns in diameter and was only slightly swollen in its earlier stages, possessing microvilli of 100-600 nm in length. On the other hand, the outer surface of mesothelial cells was swollen, but much smaller in size (8-10 microns). The mesothelial microvilli were much longer than those of the endoderm, measuring about 1.5 micron in length. On the endodermal surface, ruffles and various sizes of holes with short microvilli were occasionally found. The latter became larger with development, and seemed to be continuous with the endodermal tubules.