Bcl-2 and Bax regulation of apoptosis in germ cells during prenatal oogenesis in the mouse embryo

Abstract

Apoptosis is the main cause of primordial germ cell and oocyte degeneration in the developing fetal ovary. In this study we examined by immunohistochemistry and immunoblotting the expression of the anti-and pro-apoptotic proteins Bcl-2 and Bax in primordial germ cells and fetal oocytes during pre natal oogenesis in the mouse embryo. While Bcl-2 and Bax were not detectable in primordial germ cells in vivo, both proteins were upregulated when they undergo apoptosis in culture. Treatment with the stem cell factor (SCF), a growth factor known to partially reduce primordial germ cell apoptosis, resulted in decreased Bax expression. Bcl-2 was barely detectable in oocytes entering into meiosis and its expression did not change during the stage of meiotic prophase I examined. On the contrary, high levels of Bax was expressed in degenerating oocytes while low levels of the protein was present in many apparently healthy oocytes between 15.5 days post coitum (d.p.c.) and birth, when Bax was downregulated. Oocytes isolated from 15.5 days post coitum (d.p.c.) ovaries that progress through prophase I and undergo a wave of apoptosis at the stage of pachytene/diplotene in vitro, showed a pattern of Bax expression similar to the in vivo condition. Although the addition of SCF to the culture medium reduced significantly apoptosis in oocytes at the pachytene/ diplotene stages, it was not possible to directly correlate this effect with the downregulation of Bax in the surviving oocytes. These findings indicate that whereas a balance between Bcl-2 and Bax might regulate apoptosis of proliferating primordial germ cells under a partial control by SCF, Bax-mediated apoptosis in meiotic oocytes may be due to intrinsic meiotic checkpoints which act to monitor aberrant DNA recombination rather than to a growth factor-dependent process. Elimination of supernumerary oocytes might be a subsequent apoptotic phenomenon controlled by the availability of growth factors such as SCF within the ovary.

Follicular assembly: mechanisms of action

“…Oocyte loss and cyst breakdown begin after birth in the cortical region of the ovary, but in the medullary region, these processes begin as early as 17.5 dpc (Pepling et al 2010). This is similar to other studies on the mouse and also on humans where oocyte loss has been observed during fetal development (De Pol et al 1997, De Felici et al 1999, McClellan et al 2003, Ghafari et al 2007). In the rat, follicles are first observed in the core of the ovary and formation gradually shifts toward the surface (Rajah et al 1992).…”

Apoptosis in the germ line

“…Deletion of the pro-apoptotic protein BAX yields increased oocyte numbers as primordial follicles in neonatal ovaries when compared with WT (Greenfeld et al 2007). The role of BAX in primordial germ cell, oogonial, and oocyte death is also supported by the observation that BAX expression is upregulated in ovaries during follicular endowment and in cultured fetal oocytes undergoing apoptosis (De Felici et al 1999). BAX expression in oocytes of the human fetal ovary has been detected from the 14th week until term (Vaskivuo et al 2001).…”

Analysis of programmed cell death in mouse fetal oocytes

“…It has been previously shown that KL and other growth factors (see above) are able to partly prevent apoptosis of primordial germ cells and of isolated and/or ovaryenclosed fetal oocytes as detected by cell morphology and histochemical markers (Pesce et al 1993, Pesce & De Felici 1994, De Felici et al 1999, 2001). Moreover, reported that the rate of oocyte degeneration in cultured embryonic ovaries was not affected by the presence of serum.…”