Human Reproduction — Notes

Humans reproduce sexually and are viviparous. Events: gametogenesis → insemination → fertilisation → implantation → gestation → parturition. Reproductive events begin at puberty; sperm formation continues into old age, while ovum formation stops at about 50 years.

Testes: a pair, in the scrotum, which keeps them about 2–2.5 °C below body temperature, as needed for spermatogenesis. Each testis has about 250 testicular lobules, each with 1–3 seminiferous tubules lined by spermatogonia (male germ cells) and Sertoli cells (nourish germ cells). Interstitial (Leydig) cells outside the tubules secrete androgens; immunologically competent cells are also present there.

Ducts: seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → (joined by the duct of the seminal vesicle) ejaculatory duct → urethra → urethral meatus.

Accessory glands: paired seminal vesicles, a prostate and paired bulbourethral glands. Their secretions form seminal plasma, rich in fructose, calcium and enzymes; bulbourethral secretions also help lubricate the penis. Semen = sperms + seminal plasma.

External genitalia: the penis, with an enlarged tip, the glans penis, covered by the foreskin.

Ovaries: primary female sex organs (about 2–4 cm long), producing the ovum and ovarian hormones; each is covered by a thin epithelium enclosing the ovarian stroma, with an outer cortex and inner medulla.

Oviduct (fallopian tube), about 10–12 cm: infundibulum (funnel near the ovary, with finger-like fimbriae that collect the ovum after ovulation) → ampulla (wider) → isthmus (narrow, joins the uterus).

Uterus (womb): single, inverted-pear shape, held by ligaments. Wall: perimetrium (outer membrane), myometrium (thick smooth muscle; strong contractions at delivery), endometrium (inner glandular layer; cyclical changes). The cervix opens into the vagina; the cervical canal and vagina form the birth canal.

External genitalia: mons pubis, labia majora, labia minora, hymen and clitoris. The hymen can be torn by many everyday activities, so its presence or absence is not a reliable indicator of anything.

Mammary glands: paired; glandular tissue and fat. Each has 15–20 mammary lobes with clusters of alveoli that secrete milk. Milk path: alveoli → mammary tubules → mammary duct → mammary ampulla → lactiferous duct.

Spermatogenesis begins at puberty. Spermatogonia (2n = 46) multiply by mitosis; some become primary spermatocytes (46), which complete meiosis I to form two secondary spermatocytes (23 each). Meiosis II gives four haploid spermatids, which are transformed into spermatozoa (spermiogenesis). Sperm heads stay embedded in Sertoli cells until they are released from the tubules (spermiation).

Hormonal control: at puberty the hypothalamus increases GnRH → anterior pituitary releases LH and FSH. LH → Leydig cells → androgens, which stimulate spermatogenesis. FSH → Sertoli cells → factors that help spermiogenesis.

Sperm: head (haploid nucleus and cap-like acrosome with enzymes for fertilisation), neck, middle piece (many mitochondria for energy) and tail (motility); covered by a plasma membrane. A human male ejaculates about 200–300 million sperms; for normal fertility at least 60 percent should have normal shape and size and at least 40 percent vigorous motility.

Oogenesis starts during embryonic development: millions of oogonia form in each foetal ovary, and none are added after birth. They begin meiosis and stop at prophase I as primary oocytes, each surrounded by granulosa cells as a primary follicle. Many follicles degenerate from birth to puberty; about 60,000–80,000 remain in each ovary at puberty.

Primary follicle → secondary follicle (more granulosa layers + a new theca) → tertiary follicle (fluid-filled antrum; theca differentiates into theca interna and theca externa). In the tertiary follicle the primary oocyte completes meiosis I by an unequal division: a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle matures into a Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida. The Graafian follicle ruptures to release the secondary oocyte (ovulation). Meiosis II is completed only after a sperm enters, giving the ovum and a second polar body.

The reproductive cycle of female primates (monkeys, apes, humans) is the menstrual cycle. The first menstruation at puberty is menarche; cycles repeat about every 28/29 days and cease at about 50 years (menopause). Usually one ovum is released in the middle of each cycle.

If there is no fertilisation, the corpus luteum degenerates, progesterone falls and menstruation follows. If fertilisation occurs, the corpus luteum is maintained and menstruation does not occur. Lack of menstruation may indicate pregnancy, but it can also be caused by stress or poor health. Menstrual hygiene matters: use clean sanitary products, change them regularly and dispose of them properly.

During insemination, sperms are released into the vagina; motile sperms pass through the cervix and uterus to the oviduct. Fertilisation occurs in the ampullary region of the oviduct, only if the ovum and sperms reach it together. A sperm's contact with the zona pellucida changes the membrane and blocks other sperms. Sperm entry makes the secondary oocyte complete meiosis II (ovum + second polar body); the haploid nuclei fuse to form a diploid zygote.

Sex determination: woman XX, man XY; all ova carry X; half the sperms carry X and half Y. X-sperm → female (XX); Y-sperm → male (XY). The sex of the baby is decided by the father's sperm.

Cleavage: mitotic divisions as the zygote moves through the isthmus → 2, 4, 8, 16 blastomeres; 8–16 blastomeres = morula → blastocyst (outer trophoblast, inner cell mass). The trophoblast attaches to the endometrium; uterine cells cover the blastocyst: implantation. The inner cell mass forms the embryo.

Placenta: chorionic villi from the trophoblast interdigitate with uterine tissue. It supplies oxygen and nutrients, removes carbon dioxide and wastes, and is connected to the embryo by the umbilical cord. It is also an endocrine tissue making hCG, hPL, estrogens and progestogens. Late in pregnancy the ovary secretes relaxin. Estrogens, progestogens, cortisol, prolactin and thyroxine rise in the mother's blood.

Germ layers: the inner cell mass forms ectoderm, mesoderm and endoderm, which give rise to all adult tissues and organs. Inner cell mass cells are stem cells.

Development (about 9 months): heart by the end of month 1; limbs and digits by the end of month 2; most major organ systems by 12 weeks; first movements and head hair in month 5; fine body hair, separated eyelids and eyelashes by 24 weeks; fully developed by 9 months.

Parturition: signals from the fully developed foetus and placenta → mild uterine contractions (foetal ejection reflex) → oxytocin from the maternal pituitary → stronger contractions → more oxytocin (positive feedback) → baby, then placenta, expelled.

Lactation: mammary glands produce milk towards the end of pregnancy. Colostrum, the milk of the first few days, contains antibodies such as IgA, which protect the newborn; breastfeeding is recommended for healthy growth.