Sexual differentiation of reproductive neuroendocrine function in sheep.

The transition from a sexually immature state to full reproductive activity is timed by signals that act in the brain to increase activity of the hypothalamo-pituitary-gonadal axis, ultimately resulting in the production of mature gametes. Profound sex differences exist in the timing of puberty in many species, including the sheep, yet the pituitary hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), that underlie gonadal maturation in the male are the same as those in the female. Evidence is accumulating to support the view that it is the neuroendocrine control of pituitary function which is sexually differentiated to bring about the male-female difference in the timing of puberty. The purpose of my dissertation research was to determine factors responsible for the sex difference in the timing of neuroendocrine sexual maturation using the sheep as an experimental model. To this end, I have examined both prenatal organization and postnatal activation of the reproductive neuroendocrine system. First, the role of prenatal gonadal steroids in the postnatal control of gonadotropin secretion was determined to understand how sex differences in neuroendocrine function arise during development (Chapter 2). Next, to probe how prenatal androgens alter the control of LH secretion, GnRH neurons of males and females during the prenatal critical period for sexual differentiation were compared (Chapter 3). Finally, I determined if sex differences exist in the response to internal and external signals activating the reproductive neuroendocrine system postnatally (growth: Chapter 4, and photoperiod: Chapter 5). The results of these studies have revealed that the earlier onset of puberty in the male lamb is determined in utero by the actions of gonadal steroids on the developing brain. However, androgens do not produce a sexual dimorphism in GnRH neurons visible at the light microscopic level. Postnatally, prenatal steroid exposure reduces sensitivity to photoperiod. The difference in the photoperiod requirements for puberty in males and females is likely to play a major role in the sex difference in the timing of puberty in the lamb.PhDPhysiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105474/1/9124141.pdfDescription of 9124141.pdf : Restricted to UM users only