Recruitment of primordial follicles is essential for female fertility; however, the exact mechanisms regulating this process are largely unknown. Earlier studies using anti-Müllerian hormone (AMH)-deficient mice suggested that AMH is involved in the regulation of primordial follicle recruitment. We tested this hypothesis in a neonatal ovary culture system, in which ovaries from 2-d-old C57Bl/6J mice were cultured for 2 or 4 d in the absence or presence of AMH. Ovaries from 2-d-old mice contain multiple primordial follicles, some naked oocytes, and no follicles at later stages of development. We observed that in the cultured ovaries, either nontreated or AMH-treated, follicular development progressed to the same extent as in in vivo ovaries of comparable age, confirming the validity of our culture system. However, in the presence of AMH, cultured ovaries contained 40% fewer growing follicles compared with control ovaries. A similar reduction was found after 4 d of culture. Consistent with these findings, we noted lower inhibin alpha-subunit expression in AMH-treated ovaries compared with untreated ovaries. In contrast, expression of AMH ligand type II receptor and the expression of oocyte markers growth and differentiation factor 9 and zona pellucida protein 3 were not influenced by AMH. Based on the results, we suggest that AMH inhibits initiation of primordial follicle growth and therefore functions as an inhibitory growth factor in the ovary during these early stages of folliculogenesis.

Although ovarian follicle growth is under the influence of many growth factors and hormones of which FSH remains one of the most prominent regulators. Therefore, factors affecting the sensitivity of ovarian follicles to FSH are also important for follicle growth. The aim of the present study was to investigate whether anti-Müllerian hormone (AMH) has an inhibitory effect on follicle growth by decreasing the sensitivity of ovarian follicles to FSH. Furthermore, the combined action of AMH and FSH on ovarian follicle development was examined. Three different experiments were performed. Using an in vitro follicle culture system it was shown that FSH-stimulated preantral follicle growth is attenuated in the presence of AMH. This observation was confirmed by an in vivo experiment showing that in immature AMH-deficient females, more follicles start to grow under the influence of exogenous FSH than in their wild-type littermates. In a third experiment, examination of the follicle population of 4-month-old wild-type, FSH beta-, AMH-, and AMH-/FSH beta-deficient females revealed that loss of FSH expression has no impact on the number of primordial and preantral follicles, but the loss of inhibitory action of AMH on the recruitment of primordial follicles in AMH-deficient mice is increased in the absence of FSH. In conclusion, these studies show that AMH inhibits FSH-stimulated follicle growth in the mouse, suggesting that AMH is one of the factors determining the sensitivity of ovarian follicles for FSH and that AMH is a dominant regulator of early follicle growth.

The dimeric glycoprotein anti-Müllerian hormone (AMH) is a member of the transforming growth factor-beta superfamily of growth and differentiation factors. During male fetal sex differentiation, AMH is produced by Sertoli cells and induces degeneration of the Müllerian ducts, which form the anlagen of part of the internal female genital system. In females, AMH is produced by the ovary, but only postnatally. The function of AMH in the ovary is, however, still unknown. Female AMH null mice were reported to be fertile, with normal litter size, but this does not exclude a more subtle function for ovarian AMH. To investigate the function of AMH in the ovary, the complete follicle population was determined in AMH null mice, in mice heterozygous for the AMH null mutation, and in wild-type mice of different ages: 25 days, 4 months, and 13 months. In the present study we found that ovaries of 25-day- and 4-month-old AMH null females, compared to those of wild-type females, contain more preantral and small antral follicles. In addition, in 4- and 13-month-old AMH null females, smaller numbers of primordial follicles were found. Actually, in 13-month-old AMH null females, almost no primordial follicles could be detected, coinciding with a reduced number of preantral and small antral follicles in these females. In almost all females heterozygous for the AMH null mutation the number of follicles fell in between the numbers found in wild-type and AMH null females. In 4-month-old AMH null females serum inhibin levels were higher and FSH levels were lower compared to those in wild-type females. In contrast, inhibin levels were lower in 13-month-old AMH null females, and FSH levels were unchanged compared to those in wild-type females. Furthermore, the weight of the ovaries was twice as high in the 4-month-old AMH null females as in age-matched wild-type females. We conclude that AMH plays an important role in primordial follicle recruitment, such that more primordial follicles are recruited in AMH null mice than in wild-type mice; the mice heterozygous for the AMH null mutation take an in-between position. Consequently, the ovaries of AMH null females and those of females heterozygous for the AMH null mutation will show a relatively early depletion of their stock of primordial follicles. The female AMH null mouse may thus provide a useful model to study regulation of primordial follicle recruitment and the relation between follicular dynamics and ovarian aging.

Polycystic ovary syndrome (PCOS), the most common endocrine disorder in women in their reproductive age, is characterized by both reproductive and metabolic features. Recent studies in human, nonhuman primates, and sheep suggest that hyperandrogenism plays an important role in the development of PCOS. We investigated whether chronic dihydrotestosterone (DHT) exposure in mice reproduces both features of PCOS. Such a model would allow us to study the mechanism of association between the reproductive and metabolic features in transgenic mice. In this study, prepubertal female mice received a 90 d continuous release pellet containing the nonaromatizable androgen DHT or vehicle. At the end of the treatment period, DHT-treated mice were in continuous anestrous, their ovaries contained an increased number of atretic follicles, with the majority of atretic antral follicles having a cyst-like structure. Chronic DHT-exposed mice had significantly higher body weights (21%) than vehicle-treated mice. In addition, fat depots of DHT-treated mice displayed an increased number of enlarged adipocytes (P < 0.003). Leptin levels were elevated (P < 0.013), adiponectin levels were diminished (P < 0.001), and DHT-treated mice were glucose intolerant (P < 0.001). In conclusion, a mouse model of PCOS has been developed showing reproductive and metabolic characteristics associated with PCOS in women.