Germ cell kinetics during early ovarian differentiation: An analysis of the oogonial cell cycle and the subsequent changes in oocyte development during the onset of meiosis in the rat

Abstract

The aim of this study was the comparison between the mitoses of oogonia and the initial stages of oocyte meiosis. The structural alterations that the germ cell chromatin undergoes during the oogonial mitosis have been compared with those occurring during the G1- and S-phase just before meiosis. Using plastic embedded 1-microm sections of fetal rat ovaries (embryonic days = ED 14-20) labeled with 3H-thymidine and re-embedded for electron microscopy, a study of the structural conditions of the nuclear chromatin has been combined with a kinetic analysis of the oogonial cell cycle and the transitional period into the meiotic prophase. After ovarian differentiation (ED 14) the oogonia show a non-clonal, but strong proliferation. On ED 16, proliferation changes to a clonal pattern and decreases during ED 17. A final increase in 3H-thymidine incorporation on ED 18 characterizes the meiotic S-phase. On ED 19 the nuclear labeling drops to zero. The mitotic cycle of the oogonia lasts 16.5 hr and can be divided into 11 stages according to the concept of El-Alfy and Leblond [(1988) Am. J. Anat., 183:45-56] on the basis of the chromatin pattern. The S-phase (10.0 hours) extends from the telophase-interphase transition through the interphase to early prophase. The postmitotic G1- and S-phases show a more extensive duration, respectively 10 and 11.5 hours, and differ from their oogonial counterparts by the spherical shape of the nuclei from the very beginning. The chromatin pattern is similar until the end of the S-phase and lacks any prophase-like, preleptotenal chromatin condensation before the oocytes exhibit (pre-) leptotenal structures. Once the germ cell has completed a sequence of clonal mitotic divisions, it irrevocably progresses into meiosis. During an extended postmitotic period, the structural characteristics of meiosis emerge stepwise.

Mitotic arrest of female germ cells during prenatal oogenesis. A colcemid-like, non-apoptotic cell death

“…Additionally, mitotic arrest shows identities with a preleptotene chromosomal condensation, a change of the nuclear chromatin of germ cells which have passed the G2-preleptotene transition (see page ##). Since on ED 18 less than 6% of the germ cells are proliferating oogonia (Wartenberg et al 1998) the majority of the germ cells are "postmitotic" oocytes (germ cells that have passed the last oogonial mitosis: Hilscher et al 1974). Thus the term "arrested mitosis" does not fit exactly the postmitotic condition of these cells.…”

“…Table 1 presents the percentage of labelled germ cells (column 3), the number and percentage of arrested mitoses (columns 5 and 6) and the percentage of degenerating germ cells (destined for cell death: column 11). The maximum of 3 H-thymidine-labelled germ cells on ED 18 represents the peak during premeiotic DNA synthesis (36.3% in column 3: Wartenberg et al 1998), which contrasts to the strong decline of the number of oogonial mitoses (column 4). With a delay of 24 h about 16% of the germ cells enter mitotic arrest on ED 19.…”

“…The 3 H-thymidine labelling procedure as well as the tissue fixation, plastic embedding and semithin sectioning were carried out as specified recently (Wartenberg et al 1998). Quantitative analysis of the labelling indices and the counts of the mitotically arrested and degenerating germ cells were realized in 1 µm sections coated with K-2-emulsion (Ilford, Essex).…”

“…Arrested mitoses were counted in 1-µm semithin sections and compared with the 3 H-thymidine labelled germ cells in adjacent sections prepared for histoautoradiography (see technique: Wartenberg et al 1998). Table 1 presents the percentage of labelled germ cells (column 3), the number and percentage of arrested mitoses (columns 5 and 6) and the percentage of degenerating germ cells (destined for cell death: column 11).…”

“…During the study of proliferative behaviour of female germ cells by 3 H-thymidine autoradiography before and after onset of the meiotic prophase (Wartenberg et al 1998) we came across a very peculiar relationship between oocytes that have passed into (pre)leptotene and those germ cells that have entered the interphase after their last oogonial mitosis (postmitotic oocytes according to Hilscher et al 1974). This finding prompted us to present a concept that may explain at least two of the germ cell degenerative waves: postmitotic oocytes become arrested after their final mitosis when they lag behind those germ cells that have already entered meiosis.…”