Reproductive Physiology and the HPG Axis

The reproductive system includes: hypothalamus, pituitary, gonads, reproductive tract, accessory sex glands, external genitalia Reproduction depends on the union of male and female gametes to form a new individual. Male and female reproductive systems are designed to enable union of genetic material.

HPG Axis: Hypothalamic: GnRH aka LHRH (10aa peptide cleaved from a longer prohormone) Pituitary: FSH and LH (glycoprotein dimers, share a common alpha subunit and have unique beta subunits) Gonadal: estrogens (17B estradiol), androgens (testosterone), progestins (progesterone) (steroids)

*TSH and GnRH are also glycoproteins and share the same alpha subunit

GnRH and LH/FSH are released in a coordinated, pulsatile fashion- if GnRH is not released in proper pulses, receptors will be down regulated- GnRH agonists are used to suppress the HPG axis in precocious puberty

Primary Sex Organs: Gonads gametogenesis (spermatozoa in males, ova in females) steroidogenesis and secretion (testosterone in males, estrogen/progesterone in females)

Testes (males)- descend into scrotum by 7th month of gestation (cooler enviro for spermatogenesis)- spermatogenesis occurs within the coiled seminiferous tubules- steroidogenesis of testosterone occurs in Leydig (interstitial) cells that lie in connective tissue between seminiferous tubules

Ovaries (females)- oogenesis and ovulation- estrogen: ova maturation and release, establish female secondary sexual characteristics, essential for transport of sperm from vagina to oviduct, breast development for lactation- progesterone: prepares environment for developing embryo, contributes to breast ability to produce milk- androgens

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