Changes in follicle-stimulating hormone and follicle populations during the ovarian cycle of the common marmoset

The common marmoset (Callithrix jacchus) belongs to the family Callitrichid ae, the only anthropoid primates with a high and variable number of ovulati ons tone to four). An understanding of folliculogenesis in this species may provide some insight into factors regulating multiple follicular growth in primates. The aims of this study were to characterize in detail changes in the antral follicle population at different stages of the ovarian cycle, t o characterize the marmoset FSH profile, and to relate cyclic changes in FS H to changes in follicle sizes and circulating estradiol concentrations. Fi fty-five pairs of ovaries were collected (32 of which were at five distinct stages of the cycle) from adult marmosets, and antral follicles were manua lly excised and separated into four size groups. Daily urinary FSH and plas ma estradiol and progesterone concentrations from Day 0 of the follicular p hase to 2 days postovulation were measured in 22 marmosets using enzyme imm unoassays, The FSH profile revealed two distinct peaks, on Days 2 and 6, du ring the 10-day follicular phase, with a marginal periovulatory increase on Days 9 and 10. Estradiol levels rose significantly (P < 0.05) above baseli ne (Days 1-4) on Day 5 and continuously increased to a peak on the day prec eding ovulation (Days 8 and 9). Follicle dissection revealed a high (mean = 68) and variable (range, 14-158) total number of antral follicles >0.6 mm, The number of antral follicles significantly declined (P < 0.001) with age . The number of preovulatory follicles (>2 mm) was positively correlated wi th the number of antral follicles (P < 0.001) and tended to be negatively r elated to age (P = 0.06). The number of antral follicles did not vary signi ficantly with stage of the ovarian cycle, although the follicle size distri bution was cycle-stage dependent (P < 0.05). Follicles >1.0 mm appeared onl y in the follicular phase, and preovulatory follicles (>2.0 mm) appeared on ly at the end of the follicular phase (Days 7-9), The Day 2 FSH peak corres ponded to emergence of a population of medium-size antral follicles, and th e Day 6 peak was consistent with rising estradiol levels and appearance of the preovulatory follicles. These results suggest that some aspects of marm oset folliculogenesis are comparable to those in Old World primates, includ ing the absence of multiple follicular waves and the appearance of an ident ifiable dominant follicle in the midfollicular phase. However, the midphase FSH peak, multiple dominant follicles, and abundance of nonovulatory antra l follicles differ strongly from the pattern in Old World primates and huma ns. The findings are discussed in relation to the regulation of growth of m ultiple ovulatory follicles and provide the basis for further studies on fa ctors influencing the dynamics of follicular growth and development in this species.