The Problem of the Origin of Germ Cells

Abstract

A critical survey of the literature is made. More recent investigations are discussed in 4 groups (1) those which deny early segregation of germ cells; (2) those which admit early segregation but bring evidence that segregated cells are not definitive and degenerate to be replaced by epithelial proliferations; (3) investigations which have led to the conclusion that germ cells are segregated early and migrate to the site of the developing gonad to persist as definitive cells, but that their numbers are increased periodically by a proliferation from the epithelium; (4) those which indicate that the definitive germ cells are set aside at an early stage in embryonic development not to be replaced later by transformation of differentiated peritoneal cells, their numbers being increased only by mitotic divisions. The author''s experiments on ovarian regeneration in rats consisted of 2 series: (1) 105 doubly-ovariotomized (210 operations), ranging in age from 10 to 200 days and (2), 108 rats (216 operations), all under 40 days of age. An attempt was made to determine the conditions under which the operator can be sure of complete removal and what bearing the age of the animal at operation has upon the certainty of complete removal. Autopsies of the 1st series showed 2.88% of regeneration among rats beyond 40 days of age at operation. In 2 cases histological study of the excised ovary demonstrated incomplete removal. In the 2nd series of operated rats (under 40 days) autopsy findings were entirely negative. The success of complete extirpation depends primarily upon the age of the animal at the time of excision. It was easily possible to be certain of complete removal of the ovary in rats under 40 days of age, and upon complete removal no regeneration occurred. The small amount of regeneration observed in these experiments was due to the difficulties inherent in a perfect ovariotomy, age being the determining factor, because of the tabulated condition of adult ovaries. In operations performed on very young rats when the capsule as well as the stalk of the ovary remained in place, ample opportunity was afforded for proliferation from the epithelium. If no regeneration occurs under these conditions, it follows that the epithelium cannot replace the excised ovarian tissue. In so far as these experiments bear on the problem of the definitive germ cells, they lend no support to origin from a peritoneal source.