Reproductive System — Notes

Male reproductive system

• Testes (primary organs): make sperms in the seminiferous tubules and secrete testosterone from the interstitial cells. They lie in the scrotum, outside the abdomen, where the temperature is about 2–3 °C lower, as sperm formation needs.
• Epididymis: coiled tube on each testis; stores sperms while they mature.
• Vas deferens: carries sperms from the epididymis over the urinary bladder to the urethra.
• Urethra: common passage for urine and semen; runs through the penis.
• Accessory glands: seminal vesicles, prostate gland and Cowper's glands; their secretions nourish and activate the sperms and, with the sperms, form semen.

Female reproductive system

• Ovaries: produce eggs (ova) and secrete oestrogen and progesterone.
• Oviducts (fallopian tubes): each has a fringed funnel (fimbriae) near the ovary; carry the egg; fertilisation takes place here.
• Uterus: hollow, pear-shaped, muscular organ where the embryo implants and develops.
• Cervix: the narrow neck of the uterus. Vagina: the passage to the outside; the birth canal.

Secondary sexual characters appear at puberty; they distinguish the sexes but do not make gametes.

• Boys (testosterone): deeper voice, facial and body hair, broader shoulders, more muscular body.
• Girls (oestrogen): development of breasts, broader hips, start of menstruation.

Sperm (tiny, motile):

• Head: acrosome at the tip (enzymes to pass through the egg coverings) and a haploid nucleus (23 chromosomes in humans).
• Middle piece: spirally arranged mitochondria release energy for swimming.
• Tail: lashes to drive the sperm forward.

Egg (ovum) (large, round, non-motile):

• Haploid nucleus (23 chromosomes) and a large amount of cytoplasm for the early development of the zygote.
• Surrounded by a clear zona pellucida and a layer of follicle cells.
• Moved along the oviduct by cilia and muscle contractions.

• Ovulation: release of a mature egg from the ovary (about mid-cycle).
• Fertilisation: fusion of the nuclei of a sperm and an egg in the oviduct, forming a diploid zygote (23 + 23 = 46 chromosomes).
• Implantation: the young embryo, formed by repeated divisions of the zygote, sinks into the thick lining of the uterus about a week after fertilisation.
• Gestation: the period of development in the uterus; about 280 days (40 weeks) counted from the first day of the last menstruation. The embryo is called a foetus once its organs have formed.
• Parturition: childbirth. Oxytocin causes strong contractions of the uterus, the cervix widens and the baby is pushed out through the vagina; the placenta follows.

The placenta is a disc-shaped organ formed partly from the embryo and partly from the uterine wall. It is joined to the foetus by the umbilical cord. The bloods of mother and foetus do not mix; materials diffuse across a thin barrier.

Harmful substances (nicotine, alcohol, some drugs and germs) can also cross the placenta.

Foetal membranes: the amnion (innermost) encloses the amniotic fluid; the chorion lies outside it and helps form the placenta. Amniotic fluid: a shock absorber; keeps the foetus moist and at an even temperature; allows free movement.

The menstrual cycle lasts about 28 days (cycles of about 26–32 days are normal).

• Days 1–5, menstruation: the thick uterine lining breaks down and is shed with some blood.
• Days 6–13, repair: a follicle grows in the ovary and secretes oestrogen, which rebuilds the lining.
• About day 14, ovulation: the follicle releases the egg (about 14 days before the next menstruation).
• Days 15–28: the empty follicle becomes the corpus luteum, which secretes progesterone; the lining stays thick and ready for implantation. If the egg is not fertilised, the corpus luteum breaks down, progesterone falls and menstruation follows. If it is fertilised, the lining is kept and menstruation stops during pregnancy.