The primordial to primary follicle transition

Citation
Je. Fortune et al., The primordial to primary follicle transition, MOL C ENDOC, 163(1-2), 2000, pp. 53-60
Citations number
28
Categorie Soggetti
Endocrinology, Nutrition & Metabolism
Journal title
MOLECULAR AND CELLULAR ENDOCRINOLOGY
ISSN journal
03037207 → ACNP
Volume
163
Issue
1-2
Year of publication
2000
Pages
53 - 60
Database
ISI
SICI code
0303-7207(20000525)163:1-2<53:TPTPFT>2.0.ZU;2-W
Abstract
The mechanisms that regulate the gradual exit of ovarian follicles from the non-growing, primordial pool are very poorly understood. A better understa nding of the signals that initiate follicular growth in mammals, and of the conditions necessary for sustained growth of early preantral follicles in vitro, could have practical implications for contraception alleviation of i nfertility, and regulation of the rate of follicle depletion (menopause). O ur laboratory has developed two experimental systems that can be used to st udy factors involved in the activation of primordial follicles. In the firs t experimental system, small pieces of ovarian cortex, containing mostly pr imordial follicles, are isolated from fetal ovaries of cattle or baboons an d cultured in serum-free medium. Under these conditions most primordial fol licles become activated between 12 and 24 h of culture; their granulosa cel ls change shape, from flattened to cuboidal and begin to express proliferat ing cell nuclear antigen (PCNA). During 7 days in culture, the newly-formed primary follicles and their oocytes increase significantly in diameter. Th is wholesale 'spontaneous' activation in serum-free medium is quite differe nt from the much more gradual exit of primordial follicles from the resting pool that occurs in vivo and suggests that primordial follicles in vivo ma y be subject to a tonic inhibition of growth initiation or, alternatively, that some aspect(s) of the environment in vitro stimulates growth initiatio n. Recently we developed a second experimental system for studying activati on of primordial follicles. Pieces of ovarian cortex from bovine or baboon fetuses were grafted beneath the developing chorioallantoic membrane (CAM) of 6-day-old chick embryos, a site known to support xenografted tissues. Th e cortical pieces were rapidly vascularized and histological analysis of pi eces recovered after 2, 4, 7, or 10 days 'in ovo' revealed no increase in t he number of primary follicles and maintenance of original numbers of primo rdial follicles. Therefore, grafting ovarian cortical pieces beneath the ch ick CAM provides an experimental system in which follicles remain at the pr imordial stage in a readily accessible environment and which, thus, may be used to study potential regulators of the initiation of follicle growth. Th e results suggest that vascularization of isolated pieces of ovarian cortex provides conditions that maintain follicular quiescence, whereas culture i n vitro allows unrestrained activation of primordial follicles. Future stud ies with and comparisons of the in vitro and in ovo models may provide new insight into the mechanisms that regulate the primordial to primary follicl e transition. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.