Hypothalamic-Pituitary Axis (Female) - Reproductive

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