CHAPTER 39 – Puberty in the Sheep

This chapter discusses an integrated model for puberty in the lamb by focusing on the gonadotropin-releasing hormone (GnRH) neurosecretory system and what type of information is integrated to increase the frequency of GnRH pulses that drives the transition into adulthood. This model accounts for both the sex difference in when this transition occurs and the different signals that time it in males and females. The core pattern of GnRH secretion in the sheep, like in other long-lived species, is an increase in activity beginning before birth and then a decline of varying durations followed by the pubertal rise. The response to the early increase in GnRH secretion occurring before birth programs the type of information that is used to produce the sex-specific pattern of GnRH secretion after birth. If no testes are present in utero, the multiple neuroendocrine feedback controls remain operative, and the (female) lamb becomes photoperiodic with respect to the timing of puberty using heightened sensitivity to steroid negative feedback. Sexual maturity is masked, the ovary remains quiescent, and she immediately enters seasonal anestrus. Once she has experienced the requisite photoperiod, the long days of summer followed by the short days of autumn, the neuroendocrine sensitivity to steroid negative feedback decreases and high frequency GnRH pulses are expressed. The other feedback controls remain operative, and ovulatory cycles are initiated to begin the breeding season. This strategy allows young females to synchronize the beginning of reproductive activity with those of mature seasonally breeding females. However, the male lamb tracks photoperiod in utero much like the female lamb but does not use photoperiod to time the expression of high frequency GnRH pulses.