Ultrastructure of late oocyte nuclei in Rana temporaria

ABSTRACT The ultrastructural organization of the nucleus during late oogenesis in Rana temporaria was examined in normal and in hormone-stimulated maturing oocytes in vitro. At this stage numerous nucleoli are assembled around a knot of highly contracted chromosomes (the karyosphere) making up a karyosphere capsule. The nucleoli are fibrillar. They bear no pro-ribosomal granules and do not synthesize RNA. This period is characterized by segregation of nucleolar material into core and cortex. The nucleoli are of irregular shape. The presence of micronucleoli and nuclear bodies indicates an intensive fragmentation of nucleolar material. In the central fibrous zone of the capsule separating the chromosomes from the nucleoli, abundant accumulations of annuli lacking any membranous component occur. The annuli are connected by fibrous material and are regularly packed, forming peculiar pseudomembranes which are connected directly to the chromatin. Pseudomembranes, though less abundant and less regularly packed, are also found in the middle zone of the capsule. Along with annuli, membranous areas of various sizes and shapes are found in the pseudomembranes. Pseudomembranes are linked to micronuclei by granular filaments 20–30 nm thick. A variety of membranous structures in the form of intranuclear annulate lamellae or membranous tubular formations was detected in the peripheral zone of the capsule and outside it. The conclusion is drawn that the fibrous component of the karyosphere capsule consists of different membranous and pseudomembranous structures. It is suggested that chromatin participates directly in the formation of pseudomembranes and intranuclear membranous structures.