Analysis of the Testicular Phenotype of the Follicle-Stimulating Hormone β-Subunit Knockout and the Activin Type II Receptor Knockout Mice by Stereological Analysis*

Abstract

This study evaluated the role of FSH and activin A on testicular function using quantitative stereological analysis of testicular cell types in mice with targeted disruption of genes encoding the FSH ␤-subunit and the activin type IIA receptor (ActRIIA). Using the optical dissector technique, the numbers of Sertoli cells and germ cells per testis were determined. Testis weights in homozygous males lacking the FSH␤ gene or the ActRIIA gene were decreased approximately 60% compared with wild-type or respective heterozygotes. Sertoli cell numbers decreased in both homozygous mice by 30 -39%, and there was a comparable decline in germ cell numbers in both models. The degree of germ cell attrition increased in the later stages of spermatogenesis from a 46% reduction of spermatogonia to a 60% decrease in round spermatids. As the FSH levels are decreased in both models, the cellular lesion in both is most likely due to the FSH deficiency. Although the decrease in the Sertoli cell complement represents one cause of lower germ cell numbers, the ability of Sertoli cells to nurture germ cells is compromised by the lower FSH levels, as shown by a decrease in the round spermatid to Sertoli cell ratios in both homozygous models. We conclude that the defects in FSH ␤-subunit gene knockout and ActRIIA knockout mice are related to diminished FSH action on both Sertoli cell proliferation and the capacity of Sertoli cells to nurture germ cells. (Endocrinology 142: 2916 -2920, 2001) T HE RELATIVE ROLES of FSH and testosterone (T) in the control of mammalian spermatogenesis have been the subject of some debate for years (1-9). In particular, the requirement for FSH has been questioned on several grounds. First, several studies have shown that T alone could maintain spermatogenesis after hypophysectomy (1, 2) or the withdrawal of gonadotropic support using antisera to GnRH (5-7) or GnRH antagonists (10). However, some of the conclusions of these studies have been questioned by the demonstration that high levels of T replacement cause a stimulation of FSH secretion by the pituitary in contrast to the suppression of this hormone observed when lower doses of T were used (9, 10).Several recent studies have again challenged the requirement for FSH for the maintenance of spermatogenesis by removing the ability of the pituitary to produce FSH in certain murine models. Using hpg mice, which, due to the failure of GnRH secretion by the hypothalamus, have undetectable levels of FSH and LH, Singh et al. (11) showed that spermatogenesis could be initiated and maintained by the use of high doses of exogenous T. Although the mice treated in this manner were fertile, their testes were smaller than those of normal controls, and the numbers of testicular spermatids were decreased. However, other studies using passive immunization against FSH (5) in the rat have supported a role for FSH during spermatogenesis.In the human, several studies have emphasized the importance of FSH in the stimulation of spermatogenesis in some men with hypogonad...

Role of Follicle-Stimulating Hormone in Spermatogenesis

“…During peri-puberty, the rising FSH concentration triggers the second phase of Sertoli cell proliferation (5, 21), and the concentration of circulating FSH correlates strongly with Sertoli cell number and testis size in adulthood (22, 23). In the absence of FSH or FSHR, the Sertoli cell number is considerably decreased, by 30–45%, in comparison to normal testicular development [Table 1,(5, 24, 32)]. This is of high importance, as the Sertoli cells number determines the quantity of sperm produced; a Sertoli cell is able to support a certain maximum number of germ cells (3, 5, 24, 26, 33, 34).…”

“…In the absence of FSH or FSHR, the Sertoli cell number is considerably decreased, by 30–45%, in comparison to normal testicular development [Table 1,(5, 24, 32)]. This is of high importance, as the Sertoli cells number determines the quantity of sperm produced; a Sertoli cell is able to support a certain maximum number of germ cells (3, 5, 24, 26, 33, 34).…”

“…The lack of FSH (25) or FSHR (37, 38) in mice does not lead to sterility, albeit it decreased testis size (Figure 1), reflecting reduced Sertoli cell number and capacity to support and nurture germ cells (24, 26). Fshb - and Fshr -knockout (-KO) mice present with complete spermatogenesis, but the amount of germ cells remained lower than in control animals.…”

“…Fshb - and Fshr -knockout (-KO) mice present with complete spermatogenesis, but the amount of germ cells remained lower than in control animals. In more detail, the KO mice have <50% of spermatogonial cells and ~50% of spermatocytes in comparison to wild-type animals, and the number of germ cells is reduced further to <40% of wild-type mice at postmeiotic stages [Table 1, (24, 26)]. While FSH influences solely the proliferation of Sertoli cells, T and FSH impact additively on the germ cells' entry into meiosis and stimulate synergistically its completion and entry into spermiogenesis (26).…”

Spermatogenesis

“…317 In the hpg mouse, a model of GnRH deficiency, 318 testosterone can initiate spermatogenesis, albeit with a lower than normal sperm output, thus calling into question the need for FSH in the initiation of spermatogenesis. 321 Experimental evidence to support the need for a fully functional hypothalamic-hypophyseal unit emerges from the testis regression seen in animals immunized against GnRH. Furthermore, in several men with inactivating mutations of the FSH receptor, spermatogenesis proceeded to completion, but in most cases, testicular volume and sperm count were impaired.…”