804 



SPERM, OVA, AND PREGNANCY 



total number reaching the gonads is small. 

 These findings were in strong contrast to 

 the behavior of germ cells of the normal 

 mouse. 



By use of a genetical marker, further 

 experimental proof of extragonadal origin 

 of germ cells was obtained. From theoretic 

 expectations, experimental matings using 

 heterozygotes should yield 25 per cent de- 

 fective offspring. The actual frequency of 

 embryos with gonads containing few germ 

 cells was 28 to 29 per cent. The observations 

 of Mintz and Russell give significant veri- 

 fication of the initial extragonadal origin 

 of primordial germ cells in the mouse. Their 

 work demonstrates further that mice of 

 different strains lose oocytes at different 

 rates depending on their genetical charac- 

 teristics. 



In some of the mutant mice, there is a 

 complete absence of ovocytes in the ovaries 

 of the adults. Russell and Fekete (1958) 

 have shown that when chimeric organ cul- 

 tures were made in vitro, combining one- 

 half of a fetal ovary from the mutant strain 

 with one-half of an ovary from a normal 

 animal, no germ cell differentiation occurred 

 despite active proliferation of the germinal 

 epithelium. 



The sterility pattern described for the 

 female has been observed also in the male 

 mouse. Primordial germ cells are very 

 poorly represented in the testes of WW, 

 WW" and W^'W"' embryos and newborn. 

 The mature males of these strains are in- 

 variably sterile. Veneroni and Bianchi 

 (1957) reported some success in treating 

 such sterile males with follicle stimulating 

 hormone and testosterone propionate. They 

 conclude that the problem of sterility is re- 

 lated not only to the reduction in the num- 

 ber of primordial germ cells but also to an 

 endocrinologic deficiency. 



Willier (1950) studied the developmental 

 history of the primordial germ cells in the 

 chick by preparing chorio-allantoic grafts 

 of the blastoderm at certain critical stages, 

 namely, (1) at the time the germ cells were 

 still near the site of their origin, (2) during 

 their migration, and (3) when they had 

 arrived in the prospective gonadal areas. He 

 found that under these experimental condi- 

 tions the ovarian cortex never forms; he 

 attributed this deficiency, at least in part, 



to a failure of the development of a mecha- 

 nism in the graft for transporting the pri- 

 mordial germ cells to the areas of the devel- 

 oping gonad. Swift (1914), Dantschakoff, 

 Dantschakoff and Bereskina (1931 ) , Willier 

 (1950), and Weiss and Andres (1952), sug- 

 gested that the primary germ cells are car- 

 ried to the primitive sex glands of the chick 

 embryo by way of the blood stream. Thus 

 the cells are originally distributed at ran- 

 dom, but they accumulate and persist only 

 in the gonadal primordium. 



Recently, Simon (1957a, b) confirmed the 

 vascular transport of the germ cells in the 

 chick by the application of several ingenious 

 experimental embryologic techniques of 

 transplantation and parabiosis. In the de- 

 veloping chick of less than 10 somites the 

 primitive germ cells are localized in the 

 germinal crescent zone in the anterior part 

 of the yolk sac. The caudal part of the em- 

 bryo containing the future genital ridge was 

 severed and moved some distance from the 

 original embryo. Vascularity of both parts 

 was interfered with as little as possible. 

 Stained sections of embryos examined on the 

 4th day of development revealed that the 

 gonads had been populated by germ cells 

 which could have reached them only by way 

 of the vascular stream. In other experiments 

 the caudal areas of 10 somite embryos, 

 where gonads were not seeded by germ cells, 

 were transplanted to the area vasculosa 

 of other 10 somite embryos. The developing 

 gonads in the transplants were colonized by 

 germ cells. In still another experiment chick 

 embryos were placed in parabiosis. In one 

 of the transplanted embryos the anterior 

 crescent containing the primordial germ 

 cells was cut away. In cases of successful 

 parabiosis the gonads of both embryos were 

 seeded by germ cells. 



Even though it is recognized that in many 

 mammals and the chick the germ cells of 

 the primitive sex glands are derived from 

 migratory primordial germ elements, a 

 more difficult problem remains of a possible 

 second source of germ cells arising from so- 

 matic cells in the gonad of embryos, fetuses, 

 and mature animals. It has been proposed 

 that the original germ cells degenerate after 

 having reached the gonads and having ef- 

 fected their inductive roles, and that new- 

 cells arise secondarily by proliferation of 



