Corresponding to the expression of Fas in the ovarian oocytes as previously reported (Guo et al., Biochem Biophys Res Commun 1994; 203:1438-1446; Mori et al., JSIR 1995; 9:49-50), the expression of Fas ligand (FasL) in the ovarian follicle was found to be restricted in the area of granulosa cells by the indirect immunofluorescence (IIF) test. Reverse transcriptase/polymerase chain reaction (RT/PCR) technique coupled with Southern blot hybridization analysis showed that the highest level of FasL mRNA was demonstrated in murine ovaries and granulosa cells 1 day after the administration of pregnant mare's serum gonadotropin (PMSG), while the level of FasL mRNA became very weak on the day 5, respectively. The observed gradual decrease in FasL mRNA could not be attributed to a generalized degradation of cellular RNA during atresia, as evidenced by the presence of constitutive expression of elongation factor 1 alpha (EF-1 alpha) mRNA in murine ovaries and granulosa cells treated with PMSG. Furthermore, in situ hybridization analysis with a FasL-specific probe confirmed that FasL was specifically localized in the granulosa cells of most follicles and its expression was regulated by PMSG administration. FasL localized in granulosa cells might possibly play an important role in the formation of the ovarian atretic follicles, most likely depending on PMSG administration.

We examined the contribution of Fas and its ligand (FasL) in the process of follicular atresia using murine intraovarian follicles and pregnant mare’s serum gonadotropin (PMSG)-hyperovulated eggs. Reverse transcriptase/polymerase chain reaction-Southern blot hybridization demonstrated positive expression of Fas in both intraovarian oocytes and hyperovulated eggs. In contrast, expression of FasL was only detected in granulosa cells. These findings were histologically confirmed by in situ hybridization using Fas- and FasL-specific probes. A time-course study showed that Fas mRNA was positive in atretic follicles through day 0 and day 2 of PMSG stimulation and negative thereafter. Levels of FasL mRNA were the highest on day 1 and tapered off toward day 5 of PMSG stimulation. Levels of elongation factor 1 α mRNA, a constitutive element, were constantly maintained throughout the experimental period. Coculture of ovulated eggs, intact and zona-free, and granulosa cells demonstrated positive TUNEL staining only in zona-free eggs. These findings indicate that follicular atresia is caused by apoptosis, and the apoptosis associated with internucleosomal DNA fragmentation is directly regulated by the Fas/FasL system.

Fas-FasL system seemed to play an apoptotic role in spermatogenesis by the molecular interaction between FasL on Sertoli cells and Fas on germ cells.