In rats, the pool of primordial follicles is established within the first 3 days postnatal (dpn). Immediately after their differentiation, a subset of follicles begins to grow and constitutes the initial follicular waves. In this study, we have investigated the development of these early growing follicles after deletion of the primordial follicle pool induced by 1.5 Gy -irradiation at 5 dpn.

Within only 24 h, i.e. at 6 dpn, 99% of the primordial follicles disappeared while most of the growing follicles remained unaffected. The study of these surviving follicles throughout the immature period has shown that their subsequent growth proceeded normally, as assessed by proliferating cell nuclear antigen immunostaining and follicular counts. No modification in the process of follicular atresia, studied by TUNEL and Southern of DNA fragmentation analysis, was observed. Complementary analysis, by either in situ hybridization for inhibin subunits, P450-Aromatase and LH receptor mRNAs or plasma dosages of 17-estradiol and inhibin B, further showed that follicular maturation was unaltered. In line with these observations, pubertal onset was normal, regarding both age and ovulation rate. Nevertheless, as a consequence of the non-renewal of the growing pool, the follicular complement was practically exhausted at puberty, and 90% of the females evidenced sterility by 4 months.

Altogether, our results demonstrate that the deletion of the primordial follicle pool has induced no modification in the growth pattern of the early growing follicles which develop as their counterparts in control ovaries. Within the immature period, the initial follicular waves ensure the ovarian functionality and, thus, play a key role in the initiation of reproductive life.