Primordial germ cell development in avians.

The origin of the germline is studied in avians by tracing primordial germ cells from the stage of the germinal crescent backwards to earlier developmental stages. It has been demonstrated that during primitive streak formation, the germline has already been segregated. However, during this stage, the cells are seen gradually migrating from the epiblast to the hypoblast. The vertical migration is followed by a horizontal translocation to the extra-embryonic germinal crescent, being carried out by the hypoblast that is pushed anteriorly by the invading endodermal cells. In contrast, it has been shown that in the mouse the germ cells are allocated during gastrulation in a cluster of cells. Our results demonstrate that in avians the allocation takes place according to a different mode. The close association between the germ cells and the extra-embryonic mesoderm indicates that the germline in avians, as in the mouse, develops from a subset of cells that have already segregated from the epiblast as extra-embryonic mesoderm.