[Nuclear ultrastructure of the oocytes from human antral follicles. Formation of the karyosphere].

The organization of the nucleus in the oocytes from human antral follicles was examined at the electron microscopic level. At this time all the chromosomes are aggregated around an inactivated nucleolus forming a karyosphere 5-7 micron in diameter. The nucleolus bears no granular component and consists of densely packed delicate fibrillar material. The peripheral zone resembling a ring 0.5 micron thick is separated in the nucleolus. Nucleolus-like bodies (NLB), consisting of granules 20 nm in diameter embedded in finely fibrillar material, are constantly observed in contact with the chromatin. The eventually formed karyosphere is a complex of intimately interconnecting structures--the nucleolus, chromosomes and NLB. However, the chromatin surrounding the nucleolus does not form a continuous (compact) mass as it is observed at the light microscopic level. It is determined that the human karyosphere is formed during the preovulatory period when the connection between oocyte and follicular cells of cumulus oophorus is lost. The duration of karyosphere existence in the human oocytes, and relation of the karyosphere to the processes of antral follicle atresia are discussed.