An ultrastructural analysis of “cytoplasmic markers” in germ cells of Oryzias latipes

A sequential investigation of early embryonic stages of the teleost Japanese medaka, Oryzias latipes, was conducted to determine the origin, specific identification and relationship of primordial germ cells (PGCs) to other embryonic cell types. Two “cytological markers” have been identified in the cytoplasm of PGCs which provide a reliable method of following the translocation of germ cells throughout the maturation process. One marker, designated “mitochondria-associated granular material” (MAGM) may be analogous to granular material observed in mature germ cells of many other vertebrate species. The second cytological structure consists of a fenestrated sheet of distinctive endoplasmic reticulum which usually follows the curvature of the nucleus. Although the nature and function of these “markers” have not been determined, the ultrastructural observations, particularly those of MAGM, suggest some evolutionary significance regarding the phylogeny of germ cells. Moreover, the presence of this granular material in all stages of development illustrates a continuity of the germ cell line in Oryzias. A new improved method for removal of the chorion which facilitates a more rapid and adequate fixation of teleost eggs and embryonic tissues is also described.