Follicular atresia is the breakdown of the ovarian follicles, which consist of an oocyte surrounded by granulosa cells and internal and external theca cells. It occurs continually throughout a woman's life, as she is born with millions of follicles but will only ovulate around 400 times in her lifetime. Typically around 20 follicles mature each month but only a single follicle is ovulated; the follicle from which the oocyte was released becomes the corpus luteum. The rest undergo follicular atresia.
Atresia is a hormonally controlled apoptotic process that depends dominantly on granulosa cell apoptosis. Follicular atresia is inhibited by follicle-stimulating hormone (FSH), which promotes follicle development. Once the follicle has developed, it secretes estrogen, which in high levels decreases secretions of FSH. Granulosa cell apoptosis is considered the underlying mechanism of follicular atresia, and has been associated with five ligand-receptor systems involved in cell death: They are:
- tumor necrosis factor alpha (TNF alpha) and receptors
- Fas ligand and receptors
- TNF-related apoptosis-inducing ligand (TRAIL; also called APO-2) and receptors
- APO-3 ligand and receptors
- PFG-5 ligand and receptors
Fas antigen, a cell surface receptor protein that is expressed on granulosa cells, mediates signals that induce apoptosis by binding Fas ligand and therefore plays an important role in follicular atresia. Lack of a functional Fas ligand / Fas receptor system has been linked to abnormal follicle development, and greater numbers of secondary follicles as a result of the inability to induce apoptosis.
In addition, two intracellular inhibitor proteins, cellular FLICE-like inhibitory protein short form (cFLIPS) and long form (cFLIPL), which were strongly expressed in granulosa cells, may act as anti-apoptotic factors.
Undergoing follicular atresia is necessary in order for women to maintain a healthy reproductive system. The inability to regulate granulosa cell apoptosis and undergo follicular atresia has been linked to the development of some hormone-related cancers and chemo-resistance.
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