| Abstrakt: |
The structure of the nucleoli and their chromatin from grass frog Rana temporaria oocytes during different stages of oogenesis has been studied. Light microscopic analysis of dispersed according Callan's method (Callan et al., 1987) nucleoli of third stage of oogenesis (Dumont, 1972) revealed that by decreasing ionic strength to 25 mM spherical nucleoli got the appearance of necklace-like structures. Their electronmicroscopic analysis following by Miller's technique (Miller, Beatty, 1969) showed that they represent rosette-like structures which are connected to each other by ribosomal transcriptional units. Loops of the rosettes also are formed by transcriptional units of ribosomal genes. By the decreasing of transcriptional activity of nuclei from oocytes of 5-6th stages of oogenesis from hibernating frogs the structure of dispersed chromatin has been changed. The number of beads along the necklace-like structure axis decreased. Electron microscopic analysis of spread nucleolar chromatin from oocytes of these stages revealed the increase of part of inactive nucleosomal chromatin and the decrease of the number of loops in rosettes. The study of nucleoli ultrastructure showed that their morphology depends on their chromatin structure. In nucleoli with higher activity in oocytes of third stage of oogenesis fibrillar centers have not been found. At the same time in partially inactive nucleoli of 6th stage of oogenesis (Winter season) it was possible to identify well developed typical fibrillar centers. In those cases when nucleoli were completely inactive during the same stage of oogenesis and were not stained with 3H-actinomycin D, fibrillar centers were absent. |
