Minoru Kotani

ULTRASTRUCTURE OF THE ‘GERMINAL PLASM’ IN XENOPUS EMBRYOS AFTER CLEAVAGE

The endodermal location of ‘germinal plasm’‐bearing cells (GPBCs) and the ultrastructure of the ‘germinal plasm’ were studied in Xenopus laevis embryos at gastrula, neurula, tailbud and younger tadpole stages. Primordial germ cells (PGCs) of feeding tadpoles were also observed ultrastructurally.

Ultrastructural changes associated with UV irradiation in the "germinal plasm" of Xenopus laevis.

To detect structural changes following UV irradiation in the “germinal plasm,” ultrastructure of the “germinal plasm” was studied in normal and UV-irradiated eggs of Xenopus laevis at the following stages: prior to fertilization, early 2-cell, 32-cell, and late blastula. It was revealed that ultrastructural features of the “germinal plasm” were essentially common between Xenopus laevis and Rana pipiens. That is, the “germinal plasm” is composed primarily of a large aggregation of mitochondria and distinctive electron dense bodies (germinal granules). Irregularly shaped cylinderlike granules (giant germinal granules), having the same internal characteristics as the germinal granules, were found in the “germinal plasm” of all eggs examined. Comparison between normal and UV-irradiated eggs has demonstrated that UV irradiation causes swelling and vacuolation of mitochondria and fragmentation of germinal granules. The suggestion is that the integrity of certain UV-sensitive factor(s), which is involved in maintaining normal structure of germinal granules, is indispensable for the determination of the primordial germ cells.

Cortical granules remaining after fertilization in Xenopus laevis

Abstract Eggs of Xenopus laevis were examined in an electron microscope at unfertilized egg, 1-cell, 2-cell, 32-cell, and blastula stages. Granules closely resembling cortical granules were observed within the “germinal plasm” as well as in the peripheral cytoplasm of all the eggs examined. A staining method was developed that makes it easier to count cortical granules in thick Epon sections. Light and electron microscope examinations revealed that granules remaining after fertilization possessed morphological characteristics wholly consistent with those of cortical granules of unfertilized eggs. These granules were confirmed to be true cortical granules.

Ultraviolet effects on presumptive primordial germ cells (pPGCs) in Xenopus laevis after the cleavage stage.

Abstract The presumptive primordial germ cell (pPGC) number with development after the cleavage stage and the fate of pPGCs damaged by uv irradiation were studied in successive Epon sections (0.5 μm thick) with the light microscope in both uv-irradiated and unirradiated Xenopus embryos. taking survival rate and sterility into consideration. The pPGCs of the uv-irradiated embryos occupy nearly the same location in the embryos as those of the unirradiated embryos at stages 12, 17, 23, and 28 [see Ikenishi, K., and Kotani, M. (1975). Develop. Growth Different. 17, 101–110 ]. At stage 33 34 they are found in the central part of the endoderm cell mass in the uv-irradiated embryos, while they are situated in the lateral or dorsal part of the endoderm cell mass in the unirradiated. In the uv-irradiated embryos, a cavity which was never found in the unirradiated embryos was observed in the endoderm cell mass beneath the archenteron cavity and in the almost-median part of the posterior endoderm cell mass at stages 17 and 23, respectively, and some vacuoles in pPGCs as well as in somatic cells around those pPGCs were noticed at stages 17– 33 34 . The number of pPGCs of the unirradiated enbryos increases about three- or fourfold during stages 12–46, while the pPGCs of the uv-irradiated embryos slowly increase in number from stage 17 to stage 28, indicating that the division occurs in pPGCs, then decrease with development and finally disappear from the tadpole.

Presumptive Primordial Germ Cells (pPGCs) and PGCs in Tadpoles from UV‐irradiated embryos of Xenopus

In order to determine whether or not tadpoles that once lacked primordial germ cells (PGCs) in the genital ridges and dorsal mesentery as a result of ultraviolet (UV) irradiation subsequently contained germ cells at more advanced stages of larval development, the numbers of presumptive PGCs or PGCs were carefully examined in Xenopus tadpoles at Nieuwkoop and Fabers stage 35/36–52 that developed normally from UV‐irradiated eggs.

‘NUAGE’-CHROMOSOME ASSOCIATION IN METAPHASE SPERMATOGONIA OF THE NEWT, CYNOPUS PYRRHOGASTER

The ‘nuage’, an electron dense cytoplasmic component specific to the germ line cell, was found to be closely associated with a metaphase chromosome in the spermatogenic cell of the newt, Triturus (Cynops) pyrrhogaster. This finding suggests a possible relationship between both components at a certain stage of spermatogenesis.