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.