Development of ovarian follicles during lactation in rats

Ovarian follicular development was studied during lactation in rats. At an early stage of lactation (day 7) the ovaries showed only small follicles in agreement with the expected 'follicular quiescence' during lactation. However, at a more advanced stage of lactation (day 14), there were large follicles which were capable of ovulation in response to exogenous gonadotropins. Unilateral ovariectomy early during lactation (day 2) resulted in compensatory follicular development in the remaining ovary. However, doubling of the number of large follicles per ovary had not yet occurred by day 13. Unilateral ovariectomy caused a significant prolongation of the delay of embryonic implantation in pregnant lactating rats: this probably reflected delay of the development of sufficiently large numbers of oestrogen-producing follicles for this process. Unilateral ovariectomy did not affect the length of lactational pseudopregnancy. The findings indicate that 'follicular quiescence' during lactation in rats is limited to the very first period after parturition. This limitation may result from the relative ineffectiveness of suckling to suppress the secretion of FSH. In this respect, ovarian follicular development in rats differs from that in many other species, including primates and man.