Control of Primordial Follicle Recruitment by Anti-Müllerian Hormone in the Mouse Ovary*

The dimeric glycoprotein anti-Müllerian hormone (AMH) is a member of the transforming growth factor-b 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-dayand 4-month-old AMH null females, compared to those of wild-type females, contain more preantral and small antral follicles. In addition, in 4and 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-monthold 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. (Endocrinology 140: 5789–5796, 1999) A HORMONE (AMH), also called Müllerian-inhibiting substance, is a member of the transforming growth factor-b superfamily of peptide growth and differentiation factors (1). During male fetal sex differentiation, AMH is synthesized by testicular Sertoli cells and induces degeneration of the Müllerian ducts, which form the anlagen of the uterus, the oviducts, and the upper part of the vagina (2). During female fetal development, no ovarian AMH production occurs (3). However, AMH messenger RNA (mRNA) expression is detected in ovarian granulosa cells from postnatal day 3 onward (3). Immunohistochemical (4, 5) and mRNA in situ hybridization (3, 6) studies in rats revealed specific expression of AMH and its type II receptor (AMHRII) in granulosa cells of mainly nonatretic preantral and small antral follicles, whereas the signal was lost in nonatretic large antral follicles and atretic follicles of all size classes (6). During the estrous cycle, no marked changes were detected in the patterns of AMH and AMHRII mRNA expression, except at estrus, when a heterogeneous decrease in the expression of both mRNAs was found in nonatretic preantral follicles compared with the more homogeneous expression pattern on other days of the cycle (6). In cultured rat granulosa cells, exogenous AMH inhibits biosynthesis of aromatase and decreases LH receptor number (7). Furthermore, AMH opposes the proliferation of cultured granulosaluteal cells (8, 9). The marked changes in AMH and AMHRII mRNA expression and the reported effects of AMH in in vitro culture systems indicate that AMH may play a role during follicle development in the postnatal ovary. However, little is known about the exact function of AMH in the ovary. In the present study the function of AMH in the postnatal ovary has been studied with the help of an AMH null mouse model generated by Behringer et al. (10). As in female animals, AMH and its sole receptor (AMHRII) (11, 12) are predominantly found in the postnatal ovary, nonovarian effects of the deletion of the amh gene are unlikely to occur, and therefore, the AMH null mouse provides an excellent model for examining the function of AMH in the mouse ovary. AMH null males develop Müllerian duct derivatives, including oviducts, a uterus, and a vagina, in addition to a complete male reproductive system. AMH null females have macroscopically normal uteri, oviducts, and ovaries. Furthermore, these females are fertile and have litters of normal size (10). Yet, we hypothesized that AMH might be involved in subtle and Received May 11, 1999. Address all correspondence and requests for reprints to: Dr. A. L. L. Durlinger, Department of Endocrinology and Reproduction, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. E-mail: [email protected]. * This work was supported by the Foundation Vereniging Trustfonds Erasmus Universiteit Rotterdam, The Netherlands. 0013-7227/99/$03.00/0 Vol. 140, No. 12 Endocrinology Printed in U.S.A. Copyright © 1999 by The Endocrine Society