Ultrastructural characteristics of primordial germ cells in the quail embryo.

An avian species, the quail has become a desirable animal model in experimental embryology and reproductive biology. To understand the ultrastructural characteristics of primordial germ cells (PGC) of this species, we studied PGC in the Japanese quail (Coturnix coturnix japonica) embryo at various developmental stages from their appearance in the germinal crescent through migration to settlement in the gonadal ridges by means of electron microscopy. The results were compared with those of another well-known avian species, the chick. Several ultrastructural characteristics of quail PGC not described previously in chick PGC were observed as follows: 1) No glycogen particles were detected in the cytoplasm at any stage examined. 2) Electron-dense and membrane-bounded granules were found in the PGC cytoplasm during the sexually indifferent gonadal stages. 3) Quail PGC were characterized by a prominent nucleolus associated with condensed chromatin (heterochromatin), and the developmental changes of the nucleus were noted; the nucleolus initially appeared as a compact mass at the germinal crescent stage and became dispersed at later stages during the colonization of the gonadal ridges. These findings suggest several physiological and functional differences in the cell cycle between these two avian species. This is the first report describing detailed ultrastructural characteristics of PGC in the quail embryo.