Hypothalamic-Pituitary Axis (Female)

Overview

• Overview 
  • hypothalamic-pituitary axis controls 2 functions of ovaries
    • oogenesis
    • synthesis and secretion of female sex steroid hormones
      • progesterone and 17β-estradiol (estrogen)
• Hypothalamic-pituitary axis
  • hypothalamus
    • GnRH
      • aka gonadotropin-releasing hormone
      • pulsatile GnRH secretion via preoptic nucleus of hypothalamus
        • GnRH travels in hypothalamic-hypophyseal portal blood system
        • GnRH acts on gonadotrophs in anterior lobe of pituitary
          • GnRH → ↑ FSH and ↑ LH (pulsatile)
  • pituitary
    • follicular phase of menstrual cycle
      • LH
        • aka luteinizing hormone
        • LH → (+) theca cells → ↑ progesterone and androstenedione
          • LH acts on theca cells
            • increases activity of cholesterol desmolase 
              • increases synthesis/secretion of progesterone
              • increases synthesis/secretion of androstenedione
                • androstenedione diffuses from theca cells to granulosa cells
      • FSH
        • aka follicle-stimulating hormone
        • FSH → (+) granulosa cells → ↑ 17β-estradiol
          • FSH acts on granulosa cells 
            • increases activity of aromatase
              • increases synthesis/secretion of 17β-estradiol
            • androstenedione diffuses from theca cells to granulosa cells
              • granulosa cells express 17β-hydroxysteroid dehydrogenase
                • catalyzes conversion of androstenedione to testosterone
              • granulosa cells express aromatase
                • catalyzes conversion of testosterone to 17β-estradiol
      • negative feedback
        • 17β-estradiol (via granulosa cells)
          • ↑ 17β-estradiol → (-) hypothalamus → ↓ GnRH
            • 17β-estradiol acts on hypothalamus
              • downregulates GnRH secretion
          • ↑ 17β-estradiol → (-) anterior pituitary → ↓ FSH and ↓ LH
            • 17β-estradiol acts on anterior lobe of pituitary
              • downregulates LH and FSH secretion
    • midpoint of menstrual cycle (ovulation)
      • positive feedback
        • 17β-estradiol (via granulosa cells)
          • ↑↑ 17β-estradiol → (+) hypothalamus → ↑ GnRH
            • 17β-estradiol acts on hypothalamus
              • upregulates GnRH secretion
          • ↑↑ 17β-estradiol → (+) anterior pituitary → ↑↑ FSH, ↑↑ LH
            • 17β-estradiol acts on anterior lobe of pituitary
              • upregulates LH, FSH secretion
    • luteal phase of menstrual cycle
      • LH surge
        • ↑↑ LH → (+) corpus luteum → ↑ progesterone
          • LH acts on corpus luteum
            • increases synthesis/secretion of progesterone
      • negative feedback
        • progesterone (via granulosa lutein cells and theca lutein cells)
          • ↑ progesterone → (-) hypothalamus → ↓ GnRH
            • progesterone acts on hypothalamus
              • downregulates GnRH secretion
          • ↑ progesterone → (-) anterior pituitary → ↓ FSH and ↓ LH
            • progesterone acts on anterior lobe of pituitary
              • downregulates LH and FSH secretion