Lecture 28: hormonal control of male and female reproduction

What is the main role of the HPG axis?

It’s the hormonal feedback system regulating reproduction and sexual function by controlling GnRH, LH, FSH, sex hormones, and gametogenesis.

What triggers FSH and LH release?

Pulsatile GnRH from the hypothalamus stimulates the anterior pituitary to secrete FSH and LH.

What are the roles of FSH and LH at the gonads?

They stimulate gametogenesis (sperm and ova) and steroidogenesis (testosterone, estrogen, progesterone, inhibin).

How do sex hormones regulate the HPG axis?

Rising sex hormones provide negative feedback to the hypothalamus and pituitary, reducing GnRH, LH, and FSH to maintain balance.

Define spermatozoa.

Mature male reproductive cells, motile, with a head (genetic material), midpiece (mitochondria), and tail (flagellum).

Where does spermatogenesis occur and what supports it?

In seminiferous tubules; Sertoli cells support developing sperm and Leydig cells produce testosterone.

What is the role of the epididymis?

Stores sperm and allows them to gain motility before ejaculation.

How does FSH control male reproduction?

FSH acts on Sertoli cells to support spermatogenesis and stimulate inhibin release.

How does LH control male reproduction?

LH stimulates Leydig cells to produce testosterone.

What is the feedback role of inhibin and testosterone?

Inhibin → short negative feedback on FSH; testosterone → long negative feedback on hypothalamus (GnRH) and pituitary (LH).

How do Leydig cells produce testosterone?

LH binds Gₛ receptor → cAMP → PKA → enzymes convert cholesterol → progesterone → testosterone, released to blood and locally acts on Sertoli cells.

How do Sertoli cells interact with testosterone?

FSH binds Gₛ receptor → cAMP → PKA → aromatase converts testosterone → estradiol; supports spermatogenesis and regulates testicular environment.

What is the pathway of androgen receptor activation?

Testosterone (or DHT via 5α-reductase) binds AR → phosphorylation + dimerization → nucleus → binds DNA → male sexual characteristics and spermatogenesis.

How does testosterone secretion vary?

Pulsatile every ~45 min; higher mean levels midnight–noon;

peaks: fetal, neonatal, puberty, high adult, declines in senescence.

Describe follicular development in the ovary.

Primordial → primary → secondary → tertiary/Graafian → ovulation → corpus luteum; supported by granulosa and theca cells; hilum provides structure and hormones.

What does ~450 ovulated dominant follicles mean?

Of ~2 million primordial follicles, only ~450 mature to ovulation over a woman’s reproductive lifespan.

Role of the corpus luteum?

After ovulation, produces progesterone, estrogen, inhibin; degenerates if no pregnancy, triggering menstruation.

Follicular vs luteal phase of ovarian cycle?

Follicular: follicle growth, estradiol ↑, ends with LH surge.
Luteal: corpus luteum active, progesterone ↑, endometrium secretory.

Endometrial cycle phases?

Menstrual (shedding), proliferative (estradiol → regeneration), secretory (progesterone → secretory tissue for implantation).

What is basal body temperature pattern?

Low during follicular, rises after ovulation (~0.3–0.5°C) due to progesterone, falls if no pregnancy.

What is the early follicular phase hormonal control?

GnRH pulses → FSH/LH → FSH stimulates granulosa, LH stimulates theca → granulosa converts androgens → estradiol; inhibin reduces FSH to limit follicle recruitment.

What is the “two-cell, two-gonadotropin” model?

LH → theca cells → androgens

FSH → granulosa → aromatase converts androgens → estrogens.

Describe the preovulatory LH surge.

Rising estradiol → positive feedback on hypothalamus → GnRH surge → anterior pituitary LH surge → ovulation.

What is the estrogen switch?

Estradiol feedback shifts from negative to positive in late follicular phase → triggers GnRH and LH surge → ovulation.

How does oral contraception work?

Synthetic estrogen/progesterone maintain negative feedback → prevent ovulation, thicken cervical mucus, alter endometrium; may increase thromboembolism risk.

Describe early embryonic stages.

Cleavage (day1-4, totipotent), blastocyst (day4-7: trophoblast → placenta, inner cell mass → embryo), implantation (day7 into decidua).

How does the maternal system support the embryo?

Endometrium → decidua, chorionic villi → nutrient/waste exchange, amniotic sac → cushioning, myometrium → structural support and later contractions.

What happens during parturition?

40 weeks gestation, weak contractions from week 30, cervix softens, amniotic sac ruptures, coordinated contractions via gap junctions initiate labor.

Milk synthesis vs ejection hormones?

Prolactin → milk synthesis; oxytocin → milk ejection (let-down).

Prolactin control?

Dopamine → inhibits; suckling/TRH/estrogen → stimulate; short-loop (prolactin → hypothalamus → dopamine ↑), long-loop via spinal reflex.

Milk ejection reflex pathway?

Suckling → nipple mechanoreceptors → spinal cord → hypothalamus → posterior pituitary → oxytocin → myoepithelial contraction → milk ejection; positive feedback sustains flow.