Reproductive Physiology RP

GnRH drives gonadotropin release; LH supports Leydig-cell testosterone production and FSH acts with intratesticular signals to support spermatogenesis.

Separate endocrine testosterone production from seminiferous-tubule sperm production while linking their feedback.

Hypogonadism and infertility can arise at hypothalamic, pituitary, testicular or outflow levels.

Normal serum testosterone does not alone prove normal spermatogenesis.

Follicular development, estrogen production, ovulation and luteal progesterone secretion emerge from coordinated GnRH, FSH, LH and ovarian feedback.

Follow the cycle by dominant hormone, ovarian event and endometrial response.

Cycle abnormalities can reflect altered ovarian reserve, endocrine signalling, energy balance or systemic disease.

Feedback is not uniformly negative: sustained high estrogen near mid-cycle can participate in the LH surge.

Implantation establishes maternal-fetal exchange and endocrine support. Placental hormones help maintain pregnancy and alter maternal metabolism to support fetal growth.

Think maternal adaptation, placental exchange and fetal needs as an integrated system.

Placental dysfunction can affect fetal growth, maternal blood pressure and pregnancy maintenance.

Maternal and fetal blood normally exchange substances across a barrier rather than freely mixing as one circulation.

Labour reflects increasing uterine excitability and coordinated hormonal/mechanical feedback. Prolactin supports milk production, while oxytocin causes milk ejection in response to suckling.

Separate production from ejection: prolactin versus oxytocin.

Endocrine or mechanical disruption can impair lactation even when breast tissue is structurally present.

Milk production and milk let-down are related but distinct physiological processes.

Pulsatile GnRH from the hypothalamus stimulates pituitary LH and FSH secretion. LH acts mainly on Leydig cells to support testosterone synthesis. FSH acts on Sertoli-cell functions and, together with high local testosterone concentrations, supports spermatogenesis. Testosterone and inhibin participate in feedback regulation.

Endocrine androgen status and sperm production are therefore related but not identical outputs. A serum testosterone concentration cannot by itself establish that spermatogenesis and sperm transport are normal.

Clinical reasoning: infertility or hypogonadism can arise at hypothalamic, pituitary, testicular or outflow levels, and hormone patterns help localize the physiological defect.

Teach it back: explain why normal circulating testosterone does not guarantee normal fertility.

During the follicular phase, gonadotropins support follicular growth and estrogen production. Estrogen promotes proliferative changes in the endometrium. As a dominant follicle matures, sustained high estrogen participates in a switch to positive feedback that generates the mid-cycle LH surge, which triggers ovulatory events and luteinization.

After ovulation, the corpus luteum produces progesterone together with estrogen. Progesterone transforms the endometrium toward a secretory state suitable for implantation and exerts feedback on the hypothalamic-pituitary system. If pregnancy does not occur, luteal hormone support falls and menstruation follows.

Clinical reasoning: cycle timing becomes easier when ovarian events, hormone patterns and endometrial changes are followed together rather than memorized as separate lists.

Teach it back: explain the sequence from follicular estrogen production to LH surge, ovulation and luteal progesterone.

After implantation, endocrine signals support the corpus luteum until placental hormone production becomes sufficient for continued pregnancy support. The placenta functions as an exchange organ and an endocrine organ. Maternal blood supplies the intervillous environment while fetal blood remains within fetal vessels; gases, nutrients and waste products cross the placental barrier by appropriate transport mechanisms.

Pregnancy requires major maternal cardiovascular, respiratory, renal and metabolic adaptations. These changes increase support for the developing fetus while maintaining maternal homeostasis.

Clinical reasoning: placental dysfunction can affect fetal growth and maternal physiology because exchange, vascular regulation and endocrine signalling are interconnected.

Teach it back: explain how maternal and fetal circulations can exchange oxygen and nutrients without normally becoming one freely mixed blood pool.

Near term, uterine excitability and responsiveness increase and mechanical and hormonal signals cooperate to produce coordinated labour. Positive-feedback elements can strengthen contractions as cervical stretch promotes further oxytocin-related activity, but labour ends when the initiating mechanical condition is removed by delivery.

After birth, prolactin supports milk synthesis. Suckling provides neural input that promotes prolactin release and oxytocin release. Oxytocin contracts myoepithelial cells, producing milk ejection. Milk production and milk let-down are therefore physiologically distinct.

Teach it back: explain how suckling can simultaneously support future milk production and immediate milk ejection through different hormonal outputs.