Effects of Successive P4 Implant Treatments on Subsequent Patterns of Estrous Cyclicity during Aging
Figure 4 depicts the incidence of regular estrous cyclicity in female rats treated with P4 implants, as compared with virgin controls. At 9 mo of age, only 21% of control females were regularly cyclic, the remainder displaying irregular cycles (14%) or persistent estrus (65%; data not shown). In contrast, rats previously treated for over 4 mo with successive P4 implants subsequently exhibited a significantly greater incidence of regular cyclicity (58%) at 9 mo of age than controls (p < 0.05). The maintenance of regular cyclicity among P4-treated rats was reflected by a decreased incidence of persistent estrus (26%; data not shown) compared to that observed in controls (65%). ventolin inhaler
Influences of P4 Implant Treatments on the Ovarian Follicular Reserve
To examine possible influences of steroid implant treatments on the age-related loss of ovarian follicles, histological analyses were performed on ovaries obtained during the last implant treatment (8 mo of age). Ovaries from cyclic controls exhibited follicles at various stages of development and showed prominent corpora lutea (data not shown). In contrast, although P4-implanted females also displayed numerous nonatretic follicles in early stages of development (up to 250 ^m in diameter), no corpora lutea were evident, consistent with the absence of ovulation during P4 treatment. As shown in Figure 5, ovaries from 8-mo-old cyclic control females had 713 ± 226 resting (primordial and small primary) follicles (< 50 ^m in diameter) per ovary (n = 7). In contrast, P4-implanted rats had 2012 ± 297 resting follicles per ovary (n = 5), significantly more than in controls (p < 0.05). Thus, the effects of successive P4 implants on the subsequent maintenance of regular cyclicity were associated with the absence of corpora lutea formation and delayed loss of follicles from the ovarian follicular reserve during P4 implant treatment.
FIG. 4. Percentage incidence of regular estrous cyclicity observed in virgin female rats previously treated with blank implants (control) or implants containing P4 (P-implanted). The pattern of estrous cyclicity was determined by daily vaginal lavage for 4 wk after implant removal at 8 mo of age. Rats exhibiting at least three consecutive 4- to 5-day-long cycles were considered to be regularly cyclic (n = 50-52 rats per group).
FIG. 5. Influence of P4 implant treatments on the ovarian follicular reserve. At 8 mo of age, ovaries were obtained from implanted (n = 5) and from regularly cyclic control rats (n = 7) to determine the numbers of resting follicles (< 50 ^m in diameter; primordial and small primary follicles) per ovary.