Hypothalamus Differences in the Male and Female - Anatomy & Physiology

()Map REPRODUCTIVE SYSTEM (Map) PUBERTY

Introduction

• The hypothalamus is inherently female.
• Testosterone 'defeminizes' the brain during embryogenesis and eliminates the GnRH surge centre in males.
• The female foetus has no testes to produce testosterone, thus developes a hypothalamic GnRH surge centre.

Defeminizing the Hypothalamus in the Male

• Testosterone crosses the blood-brain barrier and is converted into oestradiol.
• In the brain, oestradiol 'defeminizes' the hypothalamus.
• Surge centre function is minimized.
• Complete 'defeminization' of the brain requires postnatal exposure to androgens.
• Continued exposure to androgens is required to render the surge centre inoperative.
• As a result, LH does not surge, it maintains a relatively consistent episodic pattern of secretion every day.
  • Episodes occur every 2-6 hours in the postpubertal male.
• Steady, pulsatile GnRH and LH release results in a steady, pulsatile release of testosterone.

The Female Hypothalamus

• Foetal ovaries produce oestradiol, but this does NOT defeminize the hypothalamus.
• A protein called alpha-fetoprotein binds oestradiol to prevent it from crossing the blood brain barrier, therefore oestradiol cannot effect the hypothalamus.
• Alpha-fetoprotein is synthesized firstly by the embryonic yolk sac and then by the foetal liver.
• Alpha fetoprotein functions as:
  • Foetal blood osmotic regulator
  • Carrier of fatty acids
• As GnRH surges, LH and Oestradiol surge ~ every 20 days in the female.
  • Surge frequency varies between species, dependent on the length of their oestrous cycles.
• Between surges, there are low amplitude LH pulses.