[Nuclear structures in the late oogenesis of the common frog. I. Light microscopic observations]

The dynamics of the Rana temporaria karyosphere formation throughout different seasons was studied in addition to morphological changes in the karyosphere components with and without hormonal stimulation of oocyte maturation in vitro. Towards the end of summer, the nuclear structure (lampbrush chromosomes and multiple nucleoli) were shown to assemble in the centre of the nucleus forming a karyosphere. The completely formed karyosphere (up to 150--200 mu in diameter) is observed during autumn and winter to consist of the central chromosome part and a surrounding capsule. The capsule consists of numerous nucleoli and a distinct fibrous component. Filaments penetrating the whole nucleolar zone of the capsule are tightly packed around the chromosomes, thus forming the inner fibrous zone of the capsule, separating the chromosomes from the nucleoli. On the periphery of the karyosphere, threads attached to the nucleoli are clearly seen. This pattern of the karyosphere construction is conserved through winter till spring. In winter, spherical nucleoli change into rings and necklaces. In spring, nucleoli become spherical again and lose their threads. In the period of oocyte maturation and disappearance of nuclear envelopes, numerous nucleoli fuse into a single conglomeration which later disappears. Only the inner fibrous zone is conserved which separates the chromosomes from the cytoplasm and yolk platelets. In this state, the karyosphere persists up to the first meiotic metaphase. The nature of the fibrous component of the karyosphere capsule, its origin and function are discussed.