Internal and external determinants of the timing of puberty in the female

A working hypothesis is proposed to account for the timing of puberty in female sheep. In the immature female, the frequency of LH pulses is low, and ovarian follicles do not develop to an advanced stage. During the pubertal transition, the frequency of LH pulses increases to drive follicular development and the production of oestradiol which evokes the gonadotrophin surge and ovulation. Central to the hypothesis is the hypothalamic pulse generator for GnRH that directs the pattern and level of LH secretion. Growth-related cues are monitored to regulate the activity of the GnRH pulse generator. When a sufficient body size is attained, the frequency of LH pulses increases both because the sensitivity to oestradiol inhibitory feedback decreases and because the GnRH pulse generator can be accelerated by the steroid. This increase in LH pulse frequency occurs provided the female has experienced the requisite exposure to photoperiod, i.e. the long days of summer followed by the short days of autumn. These photoperiodic cues are transduced by the pineal gland into a humoral signal which is an increased nocturnal production of melatonin. Failure to grow to the appropriate body size or to experience the appropriate exposure to photoperiod leads to a maintenance of the prepubertal anovulatory condition because the GnRH pulse generator operates at low frequency.