Preantral follicular atresia occurs mainly through autophagy, while antral follicles degenerate mostly through apoptosis

Abstract

There is a general agreement that granulosa cell apoptosis is the cause of antral follicle attrition. Less clear is whether this pathway is also activated in case of preantral follicle degeneration, as several reports mention that the incidence of granulosa cell apoptosis in preantral follicles is negligible. Our objective is therefore to determine which cell-death pathways are involved in preantral and antral follicular degeneration.Atretic preantal and antral follicles were investigated using immunohistochemistry and laser-capture microdissection followed by quantitative real-time reverse transcription polymerase chain reaction. Microtubule-associated light-chain protein 3 (LC3), sequestosome 1 (SQSTM1/P62), Beclin1, autophagy-related protein 7 (ATG7), and cleaved caspase 3 (cCASP3) were used as markers for autophagy and apoptosis, respectively. P62 immunostaining was far less intense in granulosa cells of atretic compared to healthy preantral follicles, while no difference in LC3 and BECLIN1 immunostaining intensity was observed. This difference in P62 immunostaining was not observed in atretic antral follicles. mRNA levels of LC3 and P62 were not different between healthy and atretic (pre)antral follicles. ATG7 immunostaining was observed in granulosa cells of preantral atretic follicles, not in granulosa cells of degenerating antral follicles. The number of cCASP3-positive cells was negligible in preantral atretic follicles, while numerous in atretic antral follicles. Taken together, we conclude that preantral and antral follicular atresia is the result of activation of different cell-death pathways as antral follicular degeneration is initiated by massive granulosa cell apoptosis, while preantral follicular atresia occurs mainly via enhanced granulosa cell autophagy.

Autophagy and Cathepsin D mediated apoptosis contributing to ovarian follicular atresia in the Nile tilapia

“…During follicular atresia, a drastic tissue remodeling occurs in the ovaries, with morphological changes of the somatic and germ cells followed by cell death by apoptosis (Hsueh, Billig, & Tsafriri, 1994;Santos et al, 2008). However, nonapoptotic processes, such as autophagy, can contribute to the efficient elimination of granulosa cells in mammals (Repnik Meng et al, 2018). In teleost fish, recent studies indicate that autophagy can be involved in follicular atresia (Cassel, Camargo, Jesus, & Borella, 2017;Morais, Thomé, Lemos, Bazzoli, & Rizzo, 2012;Morais, Thomé, Santos, Bazzoli, & Rizzo, 2016;Thomé et al, 2009), but the interactions between these signaling pathways are not fully elucidated.…”

Ovarian ageing and the impact on female fertility

“…Follicular loss occurs via ovulation; however, the vast majority of follicles will undergo atresia before reaching ovulation. While preantral and pre-ovulatory follicles rarely undergo atresia, most follicular degeneration occurs at the follicle-stimulating hormone (FSH)-dependent, early antral follicle stage 5 . The point at which menopause occurs corresponds to a decline in the primordial follicle pool below a critical threshold of about 750 to 1000 remaining follicles 6 – 9 .…”

Ultrastructure of Granulosa Cells of Bovine Ovarian Antral Follicles in Relation to Atresia

“…in granulosa cells may occur much earlier than the morphological changes in follicular atresia [11,12]. M e n g et al [13] studied the ovarian atresia, besides immuno- Among the external factors, this may be, for example, the body condition of cows and level of milk productivity.…”

“…In mammalian ovaries, more than 90 % of the follicles undergo a degenerative process known as atresia [9,13]. Follicular atresia in mammals is a process that destroys ovarian follicles and significantly eliminates from the ovary the oocytes prepared for selection, ovulation and potential fertility [19].…”