Primordial germ cell development: is the urodele pattern closer to mammals than to anurans?

All animals can be classified into three types depending on their modes of germ cell formation; epigenetic, intermediate and preformistic. In urodeles, which show the intermediate mode, primordial germ cells (PGCs) are morphologically recognized at first in early tailbud embryos. The PGCs, which are located within the lateral plate mesoderm, are induced as part of the regional induction of the mesoderm by the vegetal yolk endoderm. No cytologically distinctive, germ cell-specific germ plasma can be detected during early development of urodeles. 'Nuage' materials, which are specific to germ line cells in almost all animals, do, however, appear in the cytoplasm of the urodele PGCs during later embryogenesis. In contrast, PGCs in anurans are preformistically established under the influence of germ plasma. Because all germ cells, once established, show virtually identical behavior, regardless of whether different modes of germ cell formation are employed, the basic mechanism of germ cell formation and differentiation in all animals could be similar at the molecular levels. Although the molecules involved in germ cell formation in amphibians have not been identified, many aspects of germ plasm formation in anurans are similar to Drosophila, in which three classes of genes involved in germ cell formation have been identified: Class I genes are necessary for pattern specification during germ cell formation, Class II for the assembly of germ plasm components, and Class III for germ cell segregation. Assuming that germ cell formation in all animals requires the expression of all such genes, the three modes of germ cell formation mentioned above could be explained in terms of spatio-temporal expression of genes which are similar to those that have been identified in Drosophila. A tentative model of gene regulation for the three different modes of germ cell formation has been proposed in terms of temporal expression of these three classes of genes.