Germ line of Tetrodontophora bielanensis (Insecta, Collembola). Ultrastructural study on the origin of primordial germ cells.

In the early embryo of Tetrodontophora bielanensis (up to the stage of 500 blastomeres) nuage granules occur in two different locations: (1) in areas where the invaginating cleavage furrows have pushed fragments of the oosome into the yolk mass, and (2) in the oosome proper. In the first areas the granules are few in number and certain cells that have enclosed them in their cytoplasm eventually degenerate. The remaining cells arising in these areas are devoid of any nuage granules and differentiate into yolk cells. A different situation is observed in the other areas, where certain cells resulting from tangential divisions of the superficial blastomeres contain many nuage granules and represent primordial germ cells (PGCs). The incipient PGCs differ from the other cells of the embryo in possessing nuage granules associated with mitochondria and in lacking any annulate lamellae.