The problem of the origin of primordial germ cells (PGCs) in vertebrates: historical review and a possible solution.

A concise review of the notices about the origin of primordial germ cells (PGCs) in vertebrates is provided. Differences among various taxa about the PGCs origin, not easily understandable on the base of the traditional knowledge, are pointed out. All those differences can be explained taking into consideration the recent “theory of the endoderm as secondary layer”. That theory allows to understand that those differences are only apparent, being related to modifications of stages of the embryogeny consequent, overall, to a different yolk amount in the egg. Eggs very rich in yolk became meroblastic and the portion of primordial ectomesenchyme destined to give rise to a part of mesoderm and the PGCs separates early from the part destined to give rise to the rest of the mesoderm and to the digestive endoderm, in order to form the vitelline hypoblast lamina. To this lamina, differently from the traditional interpretation, a mesodermal, not endodermal, meaning has to be attributed. The misunderstanding regarding the meaning of this lamina clarified, all the differences about the origin of the PGCs appear seeming; as a matter of fact, also in taxa where the PGCs were considered of endodermal origin, they have a mesodermal origin. Considering that a mesodermal origin of PGCs has been demonstrated also in all sponges and cnidarians, a unique, mesodermal origin of the germinal cells in all pluricellular animals comes out.