Germ line apoptosis in the mature human ovary

Abstract

BCL2-gene family, which regulates the intrinsic apoptotic pathway, has been implicated in the process of germ cell loss in the human ovary, especially through the balance of expression between anti-apoptotic BCL2 gene and pro-apoptotic BAX. The developing ovary suffers a massive constitutive germ cell loss driven through the overexpression of BAX gene. Almost 85% of the oocytes produced by active proliferation of primordial germ cells colonizing the genital ridges at early fetal life are decimated by the end of gestation leading to a resting primordial follicle reserve at birth of around 400,000 oocytes. The main target of the apoptotic machinery during fetal life is the germ cell proper; after birth, however, apoptosis moves to the somatic granulosa cells which support growth of cyclical activation of dormant primordial follicles. We report here the immunohistochemical analysis of proteins involved in intrinsic apoptosis (BCL2/BAX) as well as in extrinsic apoptosis pathways (FAS/FAS-L), in the mature human ovary. BAX protein was detectable in primordial, primary, secondary, and antral follicles in all samples, whereas BCL2 showed detection restricted to some secondary and antral follicles. FAS/FAS-L proteins were conspicuously expressed both in dormant primordial follicles as well as throughout all follicular stages in the growing pool. TUNEL analysis yield just positive results in some antral follicles and atretic follicles. Apoptosis-related proteins expressed in the mature human ovary in a comparable pattern to that observed in the pre- and pubertal ovary. The level of TUNEL positive follicle detection is not commensurate with the expected magnitude of germ cell loss indicating that concurrent death mechanisms could be acting in the mature ovary.