Cytologic, hormonal, and ultrasonographic correlates of the menstrual cycle of the New World monkey Cebus apella

Few reports on the reproductive physiology of Cebus apella have been published. In this study we characterized menstrual cycle events by means of vaginal cytology, ultrasonography (US), and hormonal measurements in serum during three consecutive cycles in 10 females, and assessed the probability that ovulation would occur in the same ovary in consecutive cycles in 18 females. The lengths and phases of the cycles were determined according to vaginal cytology. Taking the first day of endometrial bleeding as the first day of the cycle, the mean cycle length ± SEM was 19.5 ± 0.4 days, with follicular and luteal phases lasting 8.2 ± 0.2 and 11.3 ± 0.4 days, respectively. The follicular phase included menstruation and the periovulatory period, which was characterized by the presence of a large number of superficial eosinophilic cells in the vaginal smear. The myometrium, endometrium, and ovaries were clearly distinguished on US examination. During each menstrual cycle a single follicle was recruited at random from either ovary. The follicle grew from 3 mm to a maximum diameter of 8-9 mm over the course of 8 days, in association with increasing estradiol (E₂) serum levels (from 489 ± 41 to 1600 ± 92 pmol/L). At ovulation, the mean diameter of the dominant follicle usually decreased by >20%, 1 day after the maximum E₂ level was reached. Ovulation was associated with an abrupt fall in E₂, a decreased number of eosinophilic cells, the presence of leukocytes and intermediate cells in the vaginal smear, and a progressive increase in progesterone (P) levels that reached a maximum of 892 ± 65 nmol/L on days 3-6 of the luteal phase. The menstrual cycle of Cebus apella differs in several temporal and quantitative aspects from that in humans and Old World primates, but it exhibits the same correlations between ovarian endocrine and morphologic parameters.