Animal Reproduction

Male reproductive system.

• Testes — produce sperm in the seminiferous tubules and testosterone in the surrounding Leydig (interstitial) cells; held outside the body in the scrotum for a lower temperature.
• Epididymis — sperm mature and are stored here.
• Sperm duct (vas deferens) — carries sperm during ejaculation.
• Seminal vesicles + prostate gland — add fluid (fructose for energy, alkaline secretions) to form semen.
• Urethra (in the penis) — delivers semen.

Female reproductive system.

• Ovaries — produce eggs and secrete oestrogen and progesterone.
• Oviduct (Fallopian tube) — cilia waft the egg toward the uterus; this is the site of fertilisation.
• Uterus — its lining (endometrium) thickens to receive and support the embryo.
• Cervix + vagina — the cervix is the neck of the uterus; the vagina receives sperm.

Spermatogenesis vs oogenesis. Both are forms of gametogenesis (meiosis producing haploid gametes), but they differ greatly:

Feature	Spermatogenesis	Oogenesis
Location	Seminiferous tubules of testes	Ovaries
Starts	At puberty	Before birth (in the foetus)
Pattern	Continuous, lifelong	Cyclical; arrested then resumed
Products per division	4 functional sperm	1 egg + (2–3) polar bodies
Number	Hundreds of millions per day	~1 egg per menstrual cycle
Cytoplasm	Little (small cells)	Large (egg keeps the cytoplasm)

Timing of meiosis in oogenesis (important). The primary oocyte begins meiosis I before birth but arrests in prophase I. At each cycle one oocyte completes meiosis I to give a secondary oocyte, which begins meiosis II but arrests in metaphase II. Meiosis II is only completed if a sperm fertilises the egg. So the cell released at ovulation is technically a secondary oocyte.

Gamete structure related to function.

• Sperm — tiny and motile: a head with haploid nucleus and an acrosome (enzyme cap to penetrate the egg), a midpiece packed with mitochondria (ATP for swimming), and a flagellum (tail) for propulsion.
• Egg (secondary oocyte) — large and non-motile: large cytoplasm rich in food reserves and organelles for the early embryo; surrounded by the zona pellucida (glycoprotein layer) and follicle/corona cells, with cortical granules ready to block polyspermy.

Four hormones drive the ~28-day cycle. Two come from the pituitary gland (FSH and LH) and two from the ovary (oestrogen and progesterone).

FSH (follicle-stimulating hormone). Released by the pituitary; stimulates a follicle to grow and develop, and stimulates the follicle to secrete oestrogen.

Oestrogen. Secreted by the growing follicle. It repairs and thickens the endometrium after menstruation. As the follicle matures, oestrogen levels rise sharply.

The LH surge (ovulation trigger). When oestrogen reaches a high level, it switches to positive feedback on the pituitary, causing a sharp LH surge (and a smaller FSH peak) around day 14. The LH surge triggers ovulation — release of the secondary oocyte from the ovary.

Progesterone. After ovulation, the empty follicle becomes the corpus luteum, which secretes progesterone (and some oestrogen). Progesterone maintains the thick endometrium, ready for implantation.

Negative feedback. High progesterone (with oestrogen) gives negative feedback on the pituitary, inhibiting FSH and LH — this stops a new follicle developing during the luteal phase.

If no fertilisation: the corpus luteum degenerates → progesterone (and oestrogen) fall → the endometrium can no longer be maintained → menstruation. Falling progesterone removes the inhibition, so FSH rises again and a new cycle begins.

Memory aid — order of dominance: FSH → oestrogen → LH surge → progesterone, repeating each cycle.

Fertilisation (in the oviduct).

1. Many sperm reach the egg; one sperm binds the zona pellucida.
2. Acrosome reaction — the acrosome releases digestive enzymes that break down the zona pellucida and follicle cells, letting the sperm head reach the egg membrane.
3. The sperm and egg membranes fuse; the sperm nucleus enters.
4. Cortical reaction — cortical granules release their contents, hardening the zona pellucida into a fertilisation membrane. This blocks polyspermy, ensuring only one sperm fertilises the egg (maintaining the diploid number).
5. The egg now completes meiosis II; the haploid sperm and egg nuclei fuse → diploid zygote.

The placenta. After implantation, the placenta forms from foetal and maternal tissue.

As an exchange organ (materno-foetal exchange):

• A large surface area (villi) and short diffusion distance allow exchange between maternal and foetal blood without the bloods mixing.
• To the foetus: oxygen, glucose, amino acids, water, antibodies.
• From the foetus: carbon dioxide, urea and other wastes.

As an endocrine organ:

• The early embryo and placenta secrete hCG (human chorionic gonadotropin), which maintains the corpus luteum so it keeps producing progesterone in early pregnancy (hCG is what pregnancy tests detect).
• Later the placenta itself secretes progesterone (and oestrogen), which maintains the endometrium and prevents menstruation and further ovulation throughout pregnancy.

Sexual differentiation. All early embryos are anatomically similar. The SRY gene on the Y chromosome triggers the gonads to develop into testes; the testes secrete testosterone, which drives development of male structures. Without SRY (XX), ovaries develop and female structures form. At puberty, rising sex hormones (testosterone in males; oestrogen in females) drive the secondary sexual characteristics.

IVF (in vitro fertilisation). A reproductive technology used when natural conception fails:

1. Down-regulate / suspend the woman's normal cycle with a drug.
2. Superovulation — inject FSH (and LH) so several follicles mature at once.
3. Collect eggs from the ovaries and collect sperm.
4. Fertilise in vitro (in glassware) and culture the embryos for a few days.
5. Transfer one or more healthy embryos into the uterus; progesterone supports the endometrium.