PubMed · 5549782
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J A Hutch. 1971. The internal urinary sphincter: a double-loop system.. https://doi.org/10.1016/s0022-5347(17)61530-1
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In the present study a comparison was made between human yolk sacs and yolk sac tumors. Tubules surrounded by several to as many as 10 endodermal cells and intracellular tubules in one endodermal cell were frequently observed. The tubules were seen abundantly in the yolk sac of a 4-week pregnancy, and they resembled the reticular pattern of the yolk sac tumor. It was also observed that the papillary endoderm, which contained blood cells in the center and protruded into the endodermal tubules, resembled the Schiller-Duval body of yolk sac tumor. Ultrastructurally the tumor cells were similar to the yolk sac endoderm. alpha-Fetoprotein was positive in the yolk sac endoderm until approximately the seventh week of pregnancy. Yolk sac tumor was also alpha-fetoprotein-positive. In other words, our study of human yolk sacs of 4- to 11-week pregnancies presumes that the yolk sac tumor resembles the endoderm of 4- to 7-week pregnancies.
Haemopoiesis in human yolk sacs was examined using tissues obtained from a total of 27 cases in various stages of development from the fourth to eleventh week of pregnancy. In the early stages of development, the yolk sac was observed to be connected to the midgut by the vitelline duct, which became slender with later growth of the embryo. In the early stages of pregnancy, endodermal tissues were found to be a predominant component, whereas in the later stages, the mesenchymal tissues increased. The most immature blood cells and their mitotic figures were observed in the endodermal tissue. Haemopoiesis was found in endodermal tissue before mesenchymal tissue had developed. Electron microscopy revealed that maturation of the blood cells proceeded as the cells were formed in mesenchymal tissue and in blood vessels.
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.