Preovulatory changes in the ovaries during the first spontaneous pro-oestrus in the rat

Events in the ovaries during the first spontaneous pro-oestrus were compared with those in the adult rat. The occurrence of first spontaneous pro-oestrus was determined from vaginal smears after surgical opening of the vagina. Pubertal rats showed a remarkable increase in ovarian weight from 10.00 to 15.00 h on the day of pro-oestrus, which was absent in the adults. They also showed an earlier and more pronounced interstitial oedema. Dispersion of cumulus cells, resumption of meiosis and ovulation also occurred slightly earlier in pubertal rats. A distinct difference was found in the growth rate of follicles of ovulatory size, i.e. follicles of larger than or equal to 500 X 10(5) mum3. At the first pro-oestrus these follicles showed a slower and less pronounced increase in size from 10.00 to 17.00 h compared with the adult follicles at this time, although this disparity was made up by a marked increase in growth from 17.00 to 24.00 h. The development of a new crop of follicles progressed similarly in the pubertal and adult animals. Follicular atresia seemed to progress more rapidly from 10.00 to 15.00 h in the pubertal animals compared with adults but from 15.00 h onwards no further differences were observed. The possible causal factors underlying the observed differences between the pubertal and adult animal are discussed.