Unit 8 Human reproduction
Key unit competence: Interpret physical and physiological changes that occur during the sexual maturity of humans.
Key ideas
- Male reproductive system: testes (in the scrotum, about 2–3 °C below body temperature, which suits sperm production) contain seminiferous tubules (sperm production) and interstitial (Leydig) cells (testosterone); Sertoli cells nourish developing sperm. Sperm mature in the epididymis, travel through the vas deferens (sperm duct); the seminal vesicles, prostate gland and Cowper's glands add fluid (fructose for energy, alkaline secretions) to form semen, which passes along the urethra in the penis.
- Female reproductive system: ovaries (produce ova and the hormones oestrogen and progesterone), oviducts (fallopian tubes) with ciliated lining and fimbriae (site of fertilisation), uterus (thick muscular wall — myometrium, and lining — endometrium, where the embryo implants), cervix (ring of muscle at the neck of the uterus), vagina.
- Gametogenesis: spermatogenesis in seminiferous tubules: spermatogonia (2n) divide by mitosis → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n) → meiosis II → spermatids (n) → mature spermatozoa (head with acrosome containing enzymes and haploid nucleus; middle piece with mitochondria; tail/flagellum). Continuous from puberty, millions daily. Oogenesis: oogonia multiply by mitosis before birth; primary oocytes start meiosis I and arrest in prophase I until puberty; each cycle one completes meiosis I forming a secondary oocyte and a polar body; meiosis II is completed only after fertilisation, forming the ovum and a second polar body. One functional gamete per oogonium; stops at menopause.
- Menstrual cycle (about 28 days): FSH (anterior pituitary) stimulates a follicle to develop; the follicle secretes oestrogen, which repairs and thickens the endometrium and, at high levels, triggers an LH surge (positive feedback). Ovulation at about day 14: the Graafian follicle releases the secondary oocyte. The empty follicle becomes the corpus luteum, which secretes progesterone (maintains the endometrium; inhibits FSH and LH). If no fertilisation, the corpus luteum degenerates, progesterone falls and the endometrium breaks down: menstruation (days 1–5).
- Fertilisation: sperm deposited in the vagina swim through the cervix and uterus to the oviduct; capacitation in the female tract; the acrosome reaction releases enzymes that digest the zona pellucida; one sperm fuses with the oocyte membrane; the cortical reaction hardens the zona to prevent polyspermy; meiosis II completes and the nuclei fuse to form a diploid zygote. Cleavage forms a morula then a blastocyst, which implants in the endometrium about 7 days after fertilisation. The embryo is called a fetus from about 8 weeks, when the main organs have formed.
- Placenta: formed from chorionic villi (fetal) and endometrium (maternal); the umbilical cord has two arteries and one vein. Functions: exchange of O₂, glucose, amino acids, antibodies (to fetus) and CO₂, urea (to mother) by diffusion and active transport; barrier (maternal and fetal blood do not mix, protecting against blood pressure differences and many pathogens — but alcohol, nicotine, some drugs and viruses like HIV and rubella can cross); endocrine: hCG (maintains the corpus luteum early in pregnancy, detected by pregnancy tests), progesterone and oestrogen. The amnion and amniotic fluid protect the fetus.
- Pregnancy lasts about 40 weeks. Changes in the mother: menstruation stops, breasts enlarge, weight gain, increased blood volume and heart rate, morning sickness. Good antenatal care: balanced diet (iron, folic acid, calcium), avoiding alcohol and smoking, regular check-ups. Birth: progesterone falls, oxytocin causes uterine contractions (positive feedback), cervix dilates, baby then placenta are delivered. Lactation: prolactin stimulates milk production; oxytocin causes milk ejection when the baby suckles. Colostrum is rich in antibodies.
- Twins: identical (monozygotic) — one zygote splits into two embryos; same genes and sex, usually share a placenta. Non-identical (dizygotic/fraternal) — two ova fertilised by two sperm; different genes, may be different sexes, separate placentas.
- Infertility is failure to conceive after a year of regular unprotected intercourse. Causes: low sperm count or motility, blocked oviducts (e.g. after infections), failure to ovulate, hormonal imbalance, age. Treatments: fertility drugs (FSH, LH), surgery, in vitro fertilisation (IVF) — hormones stimulate many follicles, ova collected, fertilised in a dish, embryos placed in the uterus. Family planning methods: natural (calendar), barrier (condoms also protect against STIs), hormonal (pill, injection, implant — prevent ovulation), intrauterine devices, sterilisation (vasectomy, tubal ligation).
- STIs and HIV: bacterial — gonorrhoea, syphilis, chlamydia (treatable with antibiotics); viral — HIV, herpes, HPV (genital warts, cervical cancer — vaccine available), hepatitis B. HIV infects T helper cells, weakening immunity and leading to AIDS (opportunistic infections such as TB). Transmission: unprotected sex, infected blood, sharing needles, mother to child (pregnancy, birth, breastfeeding). Prevention: abstinence, faithfulness, condoms, testing, safe blood, not sharing sharp objects, PMTCT, antiretroviral therapy (ART) which keeps viral load low.
Common misconceptions
- "Fertilisation happens in the uterus." No: it normally happens in the oviduct.
- "Mother's and baby's blood mix in the placenta." No: they are separated by a thin barrier; substances pass by diffusion and active transport.
- "Identical twins can be a boy and a girl." No: they come from one zygote and have the same sex and genes.
- "HIV attacks red blood cells." No: it attacks T helper lymphocytes.
Worked examples
Example 1. A woman has a regular 32-day cycle. On which day would she most likely ovulate? Explain.
Solution
The luteal phase (from ovulation to the next menstruation) is fairly constant at about 14 days. So ovulation occurs around day 32 − 14 = 18. Her follicular phase is longer than in a 28-day cycle.
Example 2. Explain why a pregnancy test detects hCG.
Solution
The implanted embryo (chorion) secretes hCG, which maintains the corpus luteum so that progesterone continues and the endometrium is not shed. hCG appears in the urine; monoclonal antibodies on the test strip bind it, producing a coloured line.
Practice questions
Describe the functions of five parts of the male reproductive system.
Show answer
Testes: sperm and testosterone; scrotum: keeps testes cool; epididymis: stores and matures sperm; vas deferens: carries sperm; seminal vesicles/prostate: secrete fluid; urethra: carries semen and urine.
Common mistake: Saying the prostate makes sperm.
Compare spermatogenesis and oogenesis.
Show answer
Both: mitosis then meiosis, haploid gametes. Spermatogenesis: continuous from puberty, four sperm per cell, in testes. Oogenesis: starts before birth, arrested stages, one ovum and polar bodies, cyclical, stops at menopause.
Common mistake: Saying oogenesis produces four eggs.
Explain the roles of hormones in the menstrual cycle.
Show answer
FSH: follicle growth; oestrogen: repairs endometrium, triggers LH surge; LH: ovulation and corpus luteum; progesterone: maintains endometrium, inhibits FSH/LH; fall in progesterone causes menstruation.
Common mistake: Saying FSH causes ovulation.
Describe fertilisation.
Show answer
Sperm reach oviduct; capacitation; acrosome reaction digests zona pellucida; sperm and oocyte membranes fuse; cortical reaction prevents polyspermy; meiosis II completes; nuclei fuse forming diploid zygote.
Common mistake: Ignoring the acrosome.
State five functions of the placenta.
Show answer
Supplies O₂ and nutrients; removes CO₂ and urea; passes antibodies; barrier to some pathogens and pressure; secretes hCG, oestrogen and progesterone.
Common mistake: Saying blood mixes.
Distinguish identical from non-identical twins.
Show answer
Identical: one ovum, one sperm, zygote splits, same genes and sex, often share placenta. Non-identical: two ova, two sperm, different genes, may differ in sex, separate placentas.
Common mistake: Mixing up monozygotic and dizygotic.
Describe four methods of family planning.
Show answer
Barrier (condoms, diaphragm) block sperm; hormonal (pill, injection, implant) stop ovulation; IUD prevents implantation; sterilisation (vasectomy, tubal ligation); natural (calendar).
Common mistake: Listing abortion as contraception.
Explain how HIV is transmitted and prevented.
Show answer
Transmitted by unprotected sex, infected blood, shared needles, mother to child. Prevented by abstinence, faithfulness, condoms, testing, safe blood, not sharing sharps, PMTCT and ART.
Common mistake: Saying HIV spreads by mosquitoes or handshakes.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Primary oocytes are arrested in: a. prophase I 2. The embryo implants in the: b. T helper cells 3. HIV mainly infects: c. endometrium 4. Testosterone is made by: d. interstitial cells Show answer
1 → a (prophase I) · 2 → c (endometrium) · 3 → b (T helper cells) · 4 → d (interstitial cells)
- Until puberty.
- Uterine lining.
- Weakens immunity.
- Leydig cells.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
LH levels rise · prolactin · hCG · progesterone levels fall- Milk production is stimulated by .
- Pregnancy tests detect .
- Menstruation occurs when .
Show answer
1: prolactin · 2: hCG · 3: progesterone levels fall
- After birth.
- Made by the embryo.
- Endometrium breaks down.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Twins from one zygote are: a. acrosome 2. The part of the sperm containing enzymes is the: b. condoms 3. The cortical reaction prevents: c. polyspermy 4. Which method also protects against STIs? d. identical twins Show answer
1 → d (identical twins) · 2 → a (acrosome) · 3 → c (polyspermy) · 4 → b (condoms)
- Monozygotic.
- Digests the zona pellucida.
- Only one sperm enters.
- Barrier method.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
FSH · epididymis · seminal vesicle · progesterone- The hormone that stimulates follicle development is .
- Sperm mature and are stored in the .
- The corpus luteum secretes .
Show answer
1: FSH · 2: epididymis · 3: progesterone
- From anterior pituitary.
- Coiled tube on the testis.
- Maintains endometrium.