Reproductive System
Hypothalamic-pituitary-gonadal axis
GnRH pulses drive LH and FSH. In the ovary, LH stimulates theca cells to make androgens and FSH stimulates granulosa cells to aromatize them into estradiol. Estradiol is initially negative feedback and then positive, producing the mid-cycle LH surge and ovulation. The corpus luteum secretes progesterone, which supports the endometrium and suppresses gonadotropins; the luteal phase is fixed at about 14 days, so cycle length variation comes from the follicular phase. In the male, LH drives Leydig cell testosterone production and FSH drives Sertoli cells, which make inhibin, androgen-binding protein, and the blood-testis barrier.
Differentiation and pregnancy
SRY initiates testis development; testosterone supports Wolffian structures and DHT drives external genitalia and prostate; anti-Mullerian hormone regresses Mullerian structures. In the absence of these, female development proceeds by default. The placenta produces hCG to maintain the corpus luteum, then progesterone, plus human placental lactogen. Fetal circulation shunts blood through the ductus venosus, foramen ovale, and ductus arteriosus.
Clinical correlations
Menopause shows elevated FSH and LH with vasomotor symptoms. Polycystic ovary syndrome shows hyperandrogenism, anovulation, and insulin resistance. Turner syndrome (45,X) shows streak gonads and primary amenorrhea; Kallmann syndrome combines anosmia with hypogonadotropic hypogonadism. Testicular torsion and ectopic pregnancy are surgical emergencies.
Traps
Assuming the follicular phase is fixed. Confusing hCG with LH action on the corpus luteum. Ignoring that progesterone raises basal body temperature.
Worked rule
The LH surge precedes ovulation by roughly 36 hours, and ovulation occurs about 14 days before the next menses regardless of total cycle length.