Ultrastructure of the “germinal plasm” during maturation and early cleavage in Rana pipiens

Abstract

Examination of 1 μ-thick sections of fertilized Rana pipiens eggs revealed clear yolk-free areas, situated only within the subcortical region of the vegetal pole cytoplasm. During early cleavage, these areas appear to merge and migrate up the cleavage furrows. Their size and distribution are identical to areas previously identified as germinal plasm in fertilized anuran amphibian eggs. Electron microscopic examination of these germinal plasm areas revealed electron dense bodies, 0.17 – 0.25 μ in diameter, surrounded by particles 150 Å in diameter, and interspersed within large aggregations of mitochondria. These electron dense bodies have been observed only within the regions identified as germinal plasm in the light microscope. We have suggested that they be referred to as germinal granules. Germinal plasm regions, as seen with the light microscope, were not observed prior to fertilization. However, ultrastructural examination of the vegetal pole of unfertilized uterine eggs did reveal typical germinal granules located within the subcortical cytoplasm. Additional observations suggested the possibility that precursors of these granules are present already by the end of oocyte growth. Full-grown oocytes dissected from their ovarian follicles contained small electron-dense areas, situated at the vegetal pole within subcortical aggregations of mitochondria. After the induction of maturation with progesterone in vitro, progressively larger electron dense regions are observed until, by the time of germinal vesicle dissolution, typical germinal granules are present. These granules appear at about the same time in oocytes from which germinal vesicles were removed prior to progesterone treatment. Germinal granules were also observed in fertilized but uncleaved axolotl eggs. In this case, however, the granules were seen only in the marginal zone, and not within the vegetal hemisphere cytoplasm. The germinal granules we have described are essentially identical in appearance to Drosophila polar granules. This observation, coupled with the restriction of germinal granules to germinal plasm regions, both in anuran and urodele amphibians, leads to the suggestion that they represent the germ cell determinants.