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The socalled primordial germ cell in human embryos: Its nature and significance on the origin of germ cells


The socalled primordial germ cell in human embryos: Its nature and significance on the origin of germ cells



J Biol Osaka City Univ 14: 1-12



Thirty human embryos whose developmental stages ranged from Streeter Horizon XII (Witschi Stage 17) to Horizon XXI (Witschi Stage 33) were serially sectioned at 10 microns and their processes of gonadogenesis and the origin of the socalled germ cells were studied. Alkaline phosphatase activity of the socalled primondial germ cell was examined employing an embryo of Streeter Horizon XVII (Witschi Stage 28) and the findings were used to estimate the physiologic meaning of the said cells. The socalled primordial germ cells are generally recognized in the gonadal region, in the celomic angle, in the mesentery, and in the gut mesenchyme of the embryos corresponding to Streeter Horizon XIIIXV. They are also detected in the epithelium of the gut tract in a few cases. In the subsequent stages of the development a number of primordial germ cells are found chiefly in the gonad area, and in the embryos after Horizon XVIII their location is confined in the developing gonad. These findings come into line with the observation made by Witschi (1948). Strong alkaline phosphatase activity was recognized in the socalled germ cells, however, this activity is not confined in these cells: a strong activity is also observed in the cell of the neural tube, in the adrenal blastema cell, in the mesonephric epithelial cell, in the liver cell, and in the limbbud mesenchyme. This result is in accordance with the findings obtained by McKay et al. (1953 and 1955). Therefore, the alkaline phosphatase activity can not be regarded as a positive identifying criterion of the germ cell. It is an expression of the most actively growing cell. The morphological criteria of the primordial germ cell adopted by the former investigators are also recognized in other actively growing embryonic cells. Hence, these are also the general cytological characteristics of the cell which undergoes an active nucleoprotein synthesis. Considering the physiologic significance of the socalled primordial germ cell, it has been concluded that the shift of the location of the socalled germ cells does not necessarily mean the migration of the prospective germ cells, but it indicated the shift of the locus of the metabolic center of the developing tissue with the proceeding of the embryonic development. The early segregation, the extragonadal origin, and the migration of the germ cell toward the gonadal region has not been proved corroboratively, then, the epigenetic view on the origin of the germ cells is necessitated in order to proceed to future study on the mechanism of the germ cell differentiation in human embryos.

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