HUMAN REPRODUCTION

Human reproduction includes the formation of male and female gametes, insemination, fertilisation, implantation, embryonic and foetal development, parturition and lactation.

The male reproductive system is located in the pelvis. It consists of a pair of testes, accessory ducts, glands and external genitalia.

Testes and ducts

• Each testis lies outside the abdominal cavity in a scrotum, which maintains a temperature about 2-2.5°C lower than body temperature for spermatogenesis.
• Each testis has about 250 testicular lobules; each lobule contains one to three highly coiled seminiferous tubules where sperm are formed.
• Sperm pass through rete testis, vasa efferentia, epididymis and vas deferens. The vas deferens joins the duct of seminal vesicle to form the ejaculatory duct, opening into the urethra.

Cells and glands

• Sertoli cells provide nourishment to developing germ cells. Leydig (interstitial) cells secrete androgens, chiefly testosterone.
• Seminal vesicles, prostate gland and bulbourethral glands contribute secretions that form seminal plasma, rich in fructose, calcium and enzymes.

The female reproductive system includes paired ovaries, paired oviducts, uterus, cervix, vagina and external genitalia, along with mammary glands.

• Each ovary produces ova and ovarian hormones. It is covered by cuboidal germinal epithelium and contains outer cortex and inner medulla.
• Each oviduct has infundibulum with fimbriae, ampulla and isthmus. Fimbriae collect the ovum after ovulation.
• The uterus has a thick muscular myometrium and an inner glandular endometrium. The cervix and vagina form the birth canal.
• Mammary glands contain 15-20 mammary lobes. Alveoli secrete milk, which passes through mammary tubules, ducts and ampulla to the lactiferous duct.

Spermatogenesis begins at puberty in seminiferous tubules. Spermatogonia divide mitotically, primary spermatocytes undergo meiosis I to form secondary spermatocytes, and meiosis II gives spermatids. Spermiogenesis transforms spermatids into spermatozoa.

Oogenesis begins during embryonic development. Primary oocytes enter prophase I and remain arrested until puberty. In each menstrual cycle, a primary oocyte completes meiosis I to form a secondary oocyte and first polar body; meiosis II completes only after fertilisation.

The menstrual cycle starts at puberty (menarche), repeats on average every 28-29 days and ceases around 50 years (menopause). It is regulated by hypothalamic GnRH, pituitary FSH and LH, and ovarian oestrogens and progesterone.

• Menstrual phase: if fertilisation does not occur, the endometrium breaks down and is shed for about 3-5 days.
• Follicular / proliferative phase: FSH promotes follicular development; growing follicles secrete oestrogen, causing endometrial proliferation.
• Ovulation: the mid-cycle LH surge causes release of the secondary oocyte from the Graafian follicle, about day 14.
• Luteal / secretory phase: corpus luteum secretes progesterone, maintaining the endometrium for implantation. In absence of pregnancy it degenerates, causing hormone levels to fall and menstruation to begin.

During coitus, semen is inseminated into the vagina. Sperm travel through the cervix and uterus to the ampullary-isthmic junction, where fertilisation can occur if a secondary oocyte is present.

• Acrosomal enzymes help a sperm penetrate the corona radiata and zona pellucida. Entry of one sperm induces changes that prevent polyspermy.
• Fusion of sperm and secondary oocyte forms a diploid zygote and triggers completion of meiosis II.
• Cleavage produces 2, 4, 8 and then 8-16 blastomeres; this solid mass is the morula. It becomes a blastocyst with outer trophoblast and inner cell mass.
• The blastocyst implants in the endometrium, beginning pregnancy.

After implantation, the trophoblast forms chorionic villi that interdigitate with the uterine tissue to form the placenta. It allows exchange of nutrients, oxygen and carbon dioxide, and removal of wastes between mother and foetus.

• Placenta also acts as an endocrine tissue, producing hCG, hPL, oestrogens, progesterone and later relaxin. hCG is detected in pregnancy tests.
• After one month, the embryo's heart forms. By the end of the second month, limbs and digits develop; by the end of 12 weeks, major organ systems form.
• After five months, the first movement of the foetus and head hair are usually observed. A fully developed foetus is formed by the end of nine months.
• Amnion surrounds the embryo and secretes amniotic fluid, cushioning it from shocks.

Parturition is childbirth. Signals from the fully developed foetus and placenta induce mild uterine contractions, initiating the foetal ejection reflex. This causes oxytocin release, which strengthens contractions in a positive feedback loop until delivery.

After childbirth, mammary glands undergo lactation. Prolactin supports milk production and oxytocin causes milk ejection. The first milk, colostrum, is rich in antibodies (particularly IgA) and provides important passive immunity to the newborn.

• Testes make sperm and androgens; ovaries make ova and ovarian hormones.
• Spermatogenesis starts at puberty; oogenesis begins before birth and has long meiotic arrests.
• Ovulation is triggered by the LH surge, while progesterone prepares and maintains endometrium.
• Fertilisation occurs at the ampullary-isthmic junction and is followed by cleavage, blastocyst formation and implantation.
• Placenta supports exchange and endocrine functions during pregnancy.
• Parturition is driven by the foetal ejection reflex and oxytocin; colostrum protects the newborn.