Characterization of ovarian aging and reproductive senescence in vervet monkeys (Chlorocebus aethiops sabaeus)

Abstract

Female vervet monkeys (Chlorocebus aethiops sabaeus) are used as an experimental model for chronic diseases relevant to women's health. However, reproductive senescence (menopause) has not yet been characterized for vervet monkeys. Here we describe the histologic, hormonal, and menstrual markers of reproductive senescence in vervet monkeys from the Wake Forest Vervet Research Colony. Ovaries from monkeys (age, 0 to 27 y) were serially sectioned (5 μm), stained, and photographed. In every 100th section, the numbers of primordial, primary, and secondary follicles were determined, and triplicate measurements were used to calculate mean numbers of follicles per ovary. Antimüllerian hormone (AMH), follicle stimulating hormone, and menstrual cycle length were measured in additional monkeys. Primordial follicles and AMH decreased significantly with age, and significant correlations between numbers of primordial and primary follicles and between numbers of primary and secondary follicles were noted. Histologic evaluation revealed that ovaries from 4 aged monkeys (older than 23 y) were senescent. One aged monkey transitioned to menopause, experiencing cycle irregularity over 4 y, eventual cessation of menses, and plasma AMH below the level of detection. Finally, with increasing age, the percentage of female vervets with offspring declined significantly. The present study provides insight into ovarian aging and reproductive senescence in vervet monkeys. Results highlight the importance of considering this nonhuman primate as a model to investigate the relationships between ovarian aging and chronic disease risk.

Figure 1.

(A) Comparison of numbers of primordial, primary, and secondary follicles for monkeys in the following age categories: 6 d to 9 y (n = 4), 10 to 19 y (n = 4), and 20 to 27 y (n = 4). Only the number of primordial follicles differed significantly by age (overall P = 0.02), with significantly greater numbers of primordial follicles in the youngest group compared with the oldest group (a compared with b, P = 0.03). Data are presented as mean ± SE of follicles counted per ovary. Monkey 1245 is excluded from this data set. (B) Association of mean primordial follicle number per monkey with monkey age (years; n = 13). Data are presented as square root (SQRT) of mean primordial follicle number correlated with age (years).

Figure 2.

Correlation between serum antiMüllerian hormone (AMH) level and age (5 to 26 y) in female vervet monkeys (n = 68). Correlation between AMH (square-root-transformed) and age: r = −0.48, P < 0.0001. Data presented are AMH levels back-transformed for graphic depiction.

Figure 3.

Ovarian histologic sections illustrating decreasing primordial follicle populations with age. (A) Neonatal (6 d) vervet monkey shows a myriad of primordial follicles characterized by a flattened single granulosa cell layer and occasional apoptotic oocytes and mitotic oocytes. (B) Reproductive-age (9 y) animal demonstrates abundant primordial follicles (thin arrow), primary follicles (arrowhead), secondary follicles (thick arrow), and antral follicles (not shown). (C) Aged (23 y) vervet monkey has paucity of all follicle types in the ovarian cortex. Bar, 50 μm.

Figure 4.

(A) Right and left ovaries (bar, 1 mm) and (B) ovarian cortex (bar, 50 μm) from a reproductive-age monkey (9.1 y) with suspected premature ovarian failure. Primordial, primary, and secondary follicles are rare.

Figure 5.

Menstrual cycle lengths (days) of an aged vervet monkey (ID = 979) transitioning to menopause. Menses was recorded between age 22.5 and 26.8 y by using daily vaginal swabbing. Cycle length (days; mean ± SE) for all complete cycles is 64.5 ± 13.5, with a SD of 60.6 d (minimum, 9; maximum, 280). The dashed line represents the upper range of normal menstrual cycle length for vervet monkeys (approximately 35 d). *, This bar represents the number of days (313) since the monkey's last recorded vaginal bleed until the time she died at age 26.8 y.

Figure 6.

(A) Percentage of female vervets with offspring according to age category. (B) Correlation between age and percentage of female monkeys with offspring. Data are presented for a single breeding season (2008) by using female monkeys with equivalent male exposure time.