Germinal Plasm and Differentiation of the Primordial Germ Cells in Amphibians

Abstract

Primordial germ cells (PGCs) in amphibians were defined and characterized by light and electron microscopy. The process of PGC formation in the South African clawed toad, Xenopus laevis, was explained relative to developmental behavior of the germinal plasm. Experiments demonstrating roles of the germinal plasm in PGC formation in anurans were reviewed, and the contribution of our studies was considered in this context. It has been reported that the animal pole cytoplasm inhibits PGC formation in Xenopus when injected into the vegetal pole region of UV-irradiated or unirradiated intact eggs, thus throwing some doubt on the exclusive role of the germinal plasm in PGC determination. In addition. PGC formation from the presumptive somatic cells was discussed based on data in urodeles. Two different theories underlying the studies of PGC formation were revealed. One is designated as 'determinant' theory, the other induction theory. The former, favorable in anurans, proposes that the presumptive PGCs contain the germinal determinant factors, while cells lacking these factors are differentiated into the somatic cells. The latter, favorable in urodeles, indicates that the 'determinant' specific to PGC formation does not exist, instead PGCs differentiate under some inductive influence from the neighboring embryonic tissues. A possible model applicable to PGC formation both in anurans and urodeles was presented. Studies to obtain conclusive evidence for each of the two working hypotheses, the 'determinant' and induction theories, were described.