Humans are sexually reproducing and viviparous. The Male Reproductive System is located in the pelvic region and includes a pair of testes situated in the scrotum, which helps maintain the low temperature of the testes (2–2.5°C lower than body temperature) necessary for spermatogenesis. Each testis has about 250 testicular lobules. Each lobule contains 1-3 highly coiled seminiferous tubules where sperms are produced. These tubules are lined by male germ cells (spermatogonia) and Sertoli cells (which provide nutrition). The regions outside the tubules, called interstitial spaces, contain Leydig cells, which synthesize and secrete androgens. The male sex accessory ducts include rete testis, vasa efferentia, epididymis, and vas deferens. The vas deferens receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct. The penis is the external genitalia. The male accessory glands include paired seminal vesicles, a prostate, and paired bulbourethral glands. Their secretion constitutes the seminal plasma (rich in fructose, calcium, and certain enzymes).

The Female Reproductive System consists of a pair of ovaries, oviducts, uterus, cervix, vagina, and external genitalia. The ovaries are the primary female sex organs producing the female gamete (ovum) and steroid hormones. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The oviducts (fallopian tubes) consist of the funnel-shaped infundibulum with finger-like fimbriae (help in collection of ovum), the wider ampulla, and the narrow isthmus. The uterus (womb) has a wall with three layers: external thin membranous perimetrium, middle thick smooth muscle myometrium (exhibits strong contraction during delivery), and inner glandular endometrium (undergoes cyclical changes during menstruation). The female external genitalia includes mons pubis, labia majora, labia minora, hymen, and clitoris. The mammary glands consist of glandular tissue divided into mammary lobes containing alveoli (secrete milk), which open into mammary tubules, then ducts, and finally a lactiferous duct.

Gametogenesis is the formation of gametes. In males, Spermatogenesis begins at puberty. Spermatogonia multiply by mitosis; some become primary spermatocytes, which undergo meiosis I to form two haploid secondary spermatocytes. These undergo meiosis II to form four haploid spermatids. Spermatids transform into spermatozoa by Spermiogenesis. Sperm heads become embedded in Sertoli cells and are released by Spermiation. GnRH stimulates the pituitary to release LH (acts on Leydig cells to secrete androgens) and FSH (acts on Sertoli cells to help in spermiogenesis). A sperm has a head (containing the acrosome filled with enzymes), neck, middle piece (containing mitochondria for energy), and tail. In females, Oogenesis starts during embryonic development. Oogonia form primary oocytes (arrested in Prophase I) surrounded by granulosa cells (primary follicle). Many follicles degenerate before puberty (follicular atresia). The primary follicle develops into a secondary and then a tertiary follicle (characterized by a fluid-filled cavity called antrum). Here, the primary oocyte completes meiosis I to form a large haploid secondary oocyte and a tiny polar body. The tertiary follicle matures into a Graafian follicle. The secondary oocyte forms a new membrane called zona pellucida. Rupture of the Graafian follicle releases the ovum (Ovulation).

The Menstrual Cycle in primate females has phases: Menstrual phase (3-5 days, breakdown of endometrium), Follicular phase (primary follicles grow to Graafian follicles, endometrium regenerates; stimulated by LH and FSH), Ovulatory phase (Rapid secretion of LH or LH surge induces ovulation on ~14th day), and Luteal phase (remaining Graafian follicle transforms into Corpus Luteum which secretes large amounts of Progesterone for maintaining endometrium). If fertilization does not occur, the corpus luteum degenerates, leading to menstruation.

Fertilization occurs in the ampullary region of the fallopian tube. The sperm induces changes in the zona pellucida to block entry of additional sperms. The acrosome helps the sperm enter the cytoplasm of the ovum, inducing completion of meiotic division of the secondary oocyte to form a haploid ovum (ootid) and a second polar body. The fusion of gametes forms a Zygote.

The zygote undergoes cleavage to form a Morula (8-16 blastomeres). The morula continues to divide and transforms into a Blastocyst. The outer layer (trophoblast) attaches to the endometrium, and the inner cell mass differentiates into the embryo. This attachment is called Implantation.

Pregnancy and Embryonic Development: The trophoblast forms chorionic villi, which interdigitate with uterine tissue to form the Placenta. The placenta facilitates supply of oxygen/nutrients and removal of waste. It also acts as an endocrine tissue, producing hCG (human Chorionic Gonadotropin), hPL, estrogens, and progestogens. Relaxin is secreted by the ovary. hCG, hPL, and relaxin are produced only during pregnancy. The inner cell mass differentiates into ectoderm, mesoderm, and endoderm. By one month, the heart is formed; by the end of the second month, limbs and digits develop; by 12 weeks (first trimester), major organ systems are formed. The first movements and hair appearance occur in the 5th month.

Parturition (childbirth) is induced by a complex neuroendocrine mechanism. Signals for parturition originate from the fully developed fetus and the placenta, inducing mild uterine contractions (fetal ejection reflex). This triggers the release of Oxytocin from the maternal pituitary, causing stronger uterine contractions. The mammary glands undergo differentiation to produce milk (Lactation). The milk produced during the initial few days is called colostrum (rich in IgA antibodies).