[Characteristics of karyosphere formation in the lake frog. Light microscopy data]

Morphological peculiarities of the oocyte nuclear organization were examined in R. ridibunda during winter and spring (February-March). Numerous nucleoli were seen to be assembled around regressive lampbrush chromosomes in the centre of the nucleus, and a central body was formed to which the chromosomes were attached. As result, a structural complex is constituted that involves a karyosphere and a capsule. Nucleoli are characterized by segregation and intensive fragmentation of their material. In result, a considerable part of nucleolar DNA is eliminated in the form of ring and polymorphous structures (micronucleoli). Besides the membranous component of nucleoli (nucleolar threads or tails) is lost. Towards the end of this period, nucleoli with complicated morphology become spherical again. The formation of the central body is started from the appearance of some small optically-light protein structures 5-20 nm in diameter (central body precursors-CBP). CBP are closely surrounded with ring micronucleoli to make intimate contact with the chromosomes and nucleolar threads. CBP commonly lie in one region of the nucleus not far from each other. The formation of a definitive central body obviously occurs due to a fusion of some small CBP. A conclusion is made of the nucleolar origin of the ring and polymorphous structures and of their essential role in the central body formation. The participation of chromosomal and eliminated nucleolar DNA in this process is discussed.