GERM-CELL KINETICS DURING EARLY OVARIAN-DIFFERENTIATION - AN ANALYSISOF THE OOGONIAL CELL-CYCLE AND THE SUBSEQUENT CHANGES IN OOCYTE DEVELOPMENT DURING THE ONSET OF MEIOSIS IN THE RAT

The aim of this study was the comparison between the mitoses of oogoni a and the initial stages of oocyte meiosis. The structural alterations that the germ cell chromatin undergoes during the oogonial mitosis ha ve been compared with those occurring during the G1- and S-phase just before meiosis. Using plastic embedded 1-mu m sections of fetal rat ov aries (embryonic days = ED 14-20) labeled with H-3-thymidine and re-em bedded for electron microscopy, a study of the structural conditions o f the nuclear chromatin has been combined with a kinetic analysis of t he oogonial cell cycle and the transitional period into the meiotic pr ophase. After ovarian differentiation (ED 14) the oogonia show a non-c lonal, but strong proliferation. On ED 16, proliferation changes to a clonal pattern and decreases during ED 17. A final increase in H-3-thy midine incorporation on ED 18 characterizes the meiotic S-phase. On ED 19 the nuclear labeling drops to zero. The mitotic cycle of the oogon ia lasts 16.5 hr and can be divided into 11 stages according to the co ncept of El-Alfy and Leblond [(1988) Am. J. Anat., 183:45-56] on the b asis of the chromatin pattern. The S-phase (10.0 hours) extends from t he telophase-interphase transition through the interphase to early pro phase. The postmitotic G1- and S-phases show a more extensive duration , respectively 10 and 11.5 hours, and differ from their oogonial count erparts by the spherical shape of the nuclei from the very beginning. The chromatin pattern is similar until the end of the S-phase and lack s any prophase-like, preleptotenal chromatin condensation before the o ocytes exhibit (pre-) leptotenal structures. Once the germ cell has co mpleted a sequence of clonal mitotic divisions, it irrevocably progres ses into meiosis. During an extended postmitotic period, the structura l characteristics of meiosis emerge stepwise. (C) 1998 Wiley-Liss, Inc .