| Abstrakt: |
Changes in DNA synthesis rate were studied during the S-phase in the rat brain subependymal cells. The experimental data related to the distribution of cells depending on the silver grain count over nuclei were obtained from autoradiographs of the rat brain histological sections (5 mcm) during the first 4 hours after the intraperitoneal injection of 3H-thymidine. Recently it has been established that after the whole body X-irradiation of the rats subependymal cells were blocked independently on the irradiation dose in one of the three points along the S-phase (Gracheva, 1982b). It was postulated that these three points were boundaries of the four subphases into which the S-phase was divided. The durations of subphases SI-SIV were found to be 25, 40, 20 and 15% of ts values. The experimental cell distribution curve, in dependence of the grain count over the nuclei, was reproducible theoretically in condition of lineal and stairway increase of DNA synthesis rate during the S-phase in the boundaries of the four subphases. Among the cell population in the S-phase seven cell fractions were found, recognized by their decreasing levels of DNA synthesis and by the progressively (1 : 2 : 4 : 8 : 16: 32 : 64: increasing portions of cells from the 1st towards the VIIth level. DNA synthesis rates of the adjacent levels were overlapping. The pauses in DNA synthesis at each subphase of the S-phase in the cells of the I-VI levels are regarded as a compensation for the high rate of DNA synthesis. The results obtained are discussed in terms of cell subordination to the DNA synthesis levels, within the following sequence: stem cells--semistem cells--differentiating cells from the I towards the VII level. |
