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The results of the current study, however, were unable to support these latter findings. The spontaneous activation of primordial follicles in cultured ovarian tissue has been observed for a number of species, including cattle, primates, and, to a much lesser extent, rats. It is unknown why primordial follicles are triggered to spontaneously activate when maintained in vitro, although clues as to the mechanism are beginning to emerge. In vivo, primordial follicles exit the resting pool in a gradual manner. It has therefore been hypothesized that an inhibitor of activation that is absent in vitro regulates follicle recruitment. It is noteworthy that factors secreted by later-stage follicles, such as anti-Mullerian hormone and activin A, are inhibitory to primordial follicle activation. The suggestion that growing follicles negatively regulate the earliest stages of development also is corroborated by studies demonstrating that primordial follicle activation is accelerated during the initial waves of folliculogenesis, when growing follicles are yet to form in the ovary. Therefore, a reduced number of later-stage growing follicles in rabbit ovarian cortical explants and in neonatal mouse ovaries may be responsible for at least some of the observed spontaneous activation in vitro.