Search

FSH and androgen act to stimulate and maintain spermatogenesis. FSH acts directly on the Sertoli cells to stimulate germ cell number and acts indirectly to increase androgen production by the Leydig cells. In order to differentiate between the direct effects of FSH on spermatogenesis and those mediated indirectly through androgen action, we have crossed hypogonadal (hpg) mice, which lack gonadotrophins, with mice lacking androgen receptors (AR) either ubiquitously (ARKO) or specifically on the Sertoli cells (SCARKO). These hpg.ARKO and hpg.SCARKO mice were treated with recombinant FSH for 7 days and testicular morphology and cell numbers were assessed. In untreated hpg and hpg.SCARKO mice, germ cell development was limited and did not progress beyond the pachytene stage. In hpg.ARKO mice, testes were smaller with fewer Sertoli cells and germ cells compared to hpg mice. Treatment with FSH had no effect on Sertoli cell number but significantly increased germ cell numbers in all groups. In hpg mice, FSH increased the numbers of spermatogonia and spermatocytes, and induced round spermatid formation. In hpg.SCARKO and hpg.ARKO mice, in contrast, only spermatogonial and spermatocyte numbers were increased with no formation of spermatids. Leydig cell numbers were increased by FSH in hpg and hpg.SCARKO mice but not in hpg.ARKO mice. Results show that in rodents 1) FSH acts to stimulate spermatogenesis through an increase in spermatogonial number and subsequent entry of these cells into meiosis, 2) FSH has no direct effect on the completion of meiosis and 3) FSH effects on Leydig cell number are mediated through interstitial ARs.

Measurement of inhibins A and B in the serum of normal cyclic rodents has implicated FSH in the regulation of these peptides within the ovary. To extend these observations we have used a panel of mutant mice carrying mutations which affect either the production of, or the ability to respond to, FSH and LH. As a consequence, the females are infertile and show different degrees of follicular development. The aim of this study was to measure inhibin gene transcription in the ovaries of these mutant females together with inhibin protein levels in ovaries and serum and to relate these to follicular development within the ovary. Comparison was made with a pool of normal/heterozygous females. In hpg females where lack of GnRH production results in the absence of gonadotropin synthesis, in FSHbeta knockout (FSHbetaKO) females where disruption of the gene encoding FSHbeta results in the absence of FSH production, and in FSH receptor knockout (FSHRKO) females which are unable to respond to circulating FSH, follicular development remains at the pre-antral stage in these three mutants. Only in the hpg females were common inhibin alpha subunit mRNA levels significantly lower than normal. In these three mutants, however, mRNA levels for both the betaA and betaB subunits were extremely low compared with normal mice. At the protein level, neither inhibin A nor B was detected in the serum of these three mutants; however inhibin B, albeit at very low levels, was detectable within the ovaries. These observations confirm a major role for FSH in the control of transcription of the betaA and betaB genes but suggest that the constitutive transcription of the alpha subunit is less dependent on FSH. In contrast, in LH receptor knockout (LuRKO) female mice inhibin betaA subunit mRNA levels were similar to those measured in normal/heterozygous females but levels of inhibin alpha and betaB subunit mRNAs were significantly higher than in the normal group. This was reflected in significantly higher inhibin B protein levels in ovaries and serum. An inability to respond to LH combined with high circulating levels of FSH leads to a high proportion of antral follicles in LuRKO females, with granulosa cells constituting the major cell type within the ovary. The high percentage of antral granulosa cells is likely to account for the significantly higher levels of inhibin B production in these ovaries.

Female mice in which the gene encoding the follicle-stimulating hormone FSH receptor (FSHR) knockout (KO) or its ligand (FSHbetaKO) have been disrupted were infertile. Ovaries of these mice were significantly smaller than those of heterozygous littermates but significantly larger than those of hypogonadal mice of the same age. Uterine masses in all three mutants were <6 mg, significantly reduced compared with heterozygous mice. At 1 year of age uterine mass had increased to >12 mg in 63% of FSHRKO females and 88% of FSHbetaKO females. Despite the increase in uterine size there was no evidence of contractility: uteri were flaccid and unresponsive to electrical or pharmacological stimulation. In most females in which uterine growth had occurred there was evidence of ovarian growth with hypertrophy of the interstitial tissue, occurrence of ovarian cysts and epithelial and tubular inclusions. There was no evidence of uterine or ovarian hypertrophy in hypogonadal (hpg) mice at any age or in 1 year old females in which the FSH mutations were bred onto the hpg background. There was an inverse correlation of plasma LH concentrations and uterine mass in 1 year old mutant females with uterine hypertrophy. Ovariectomy of both FSHRKO and FSHbetaKO females with large uteri resulted in decreased uterine mass and increased plasma concentration of LH. The number of mice with ovarian pathology, reminiscent of the serous ovarian adenocarcinomas found in humans, was significantly greater in the FSHbetaKO mice, indicating that the presence of an intact FSH receptor on ovarian cells of FSHbetaKO females may allow constitutive basal stimulation of the ovary, which is absent in mice lacking FSH receptors.