The primordial germ cells in early mouse embryos: light and electron microscopic studies

The migrating primordial germ cells in mouse embryos aged between 8.5 and 11 days were identified under the light and electron microscopes by means of localizing alkaline phosphatase activity, and their ultrastructure was studied. Most of the migrating primordial germ cells were round with smooth contour; they were invariably in close contact with irregularly shaped surrounding cells. The ultrastructural characteristics of early primordial germ cells included: prominent nucleoli, cytoplasmic protrusions into the nuclei, Golgi complexes with alkaline phosphatase activity, disappearance of granular endoplasmic reticulum with increasing age of the embryos, dense cytoplasm with extremely abundant ribosomes, and cytoplasmic dense granules. The significance of these findings is discussed in relation to the origin and migration of mouse primordial germ cells.