[Electron microscopic study of endopolyploid nuclei in rat trophoblast giant cells. III. Nucleolus and its fibrillary center at different stages of the endoreplication cycle].

The fine structure of the nucleolus and nucleolus-forming chromaosomes has been studied in the course of the endomitotic cycle of the trophoblast giant cells on the 12th and 13th days of the foetus development. The nucleolar ultrastructure was regarded in relation to the degree of chromatin condensation in the nucleus. In the nucleus with dispersed chromatin (endointerphase), the nucleoli have the nucleoneme structure and involve the four components: fibro-granular threads and accumulations (with granules 20 nm in diameter), round zones of fibres with moderate electron density surrounded by dense fibrillar material (fibrillar centres), and lacunar spaces. In contact with the nucleolus are thin chromatinous fibres of the karyoplasm which penetrate into the open lacunae on the periphery of the nucleus to be running, presumably, through the whole nucleus. The fibrillar centres either run deep into the lacunar spaces of the nucleolus, or attach to it. In the nuclei with condensed chromtin (endoprophase), the nucleoli are surrounded with a well developed layer of perinucleolar chromatin running along lacunae deep into the nucleolus. The nucleoli here made of the same components as nucleoli in the nuclei with dispersed chromatin, but are the prominent electron dense fibrillar component is getting more obvious, the number of granules in the fibro-granular component is reduced, it lacunae display regions of the nucleolus-organizer not only as fine scattered chromatinous fibres, but also as small compact blocks. A question is discussed of the fibrillar centres of the nucleolus as being places of the nucleolus organizer, and of its changes within the cycle of mitosis and endomitosis.