Summary

Human reproduction begins at puberty, when the hypothalamus-pituitary-gonad axis activates the testes and ovaries and the sex hormones bring about the secondary sexual characters. The male reproductive system (testes, epididymis, vas deferens, seminal vesicles, prostate, bulbourethral glands, penis) produces sperm continuously by spermatogenesis within the seminiferous tubules, nourished by Sertoli cells and regulated by GnRH, LH, FSH and testosterone secreted by the Leydig cells; the female reproductive system (ovaries, fallopian tubes, uterus, cervix, vagina) produces a single mature ovum roughly every 28 days by oogenesis within a growing ovarian follicle, culminating in ovulation at metaphase II of meiosis.

The menstrual cycle coordinates ovarian and uterine events across four phases — menstrual, follicular, ovulatory (triggered by the LH surge) and luteal (dominated by progesterone from the corpus luteum) — and ends either in menstruation or, if fertilisation occurs, in the establishment of pregnancy. Fertilisation, normally in the ampullary-isthmic junction of the fallopian tube, requires sperm capacitation and the acrosomal reaction, and is followed by the cortical reaction, which blocks polyspermy. The resulting zygote undergoes cleavage through the morula stage to form a blastocyst, differentiated into an outer trophoblast and an inner cell mass, which implants into the endometrium about a week after fertilisation. …