The male and female reproductive systems
Humans are viviparous: the baby grows inside the mother's body. Both systems have primary sex organs (gonads: testes or ovaries) that make gametes and hormones, plus ducts and glands.
Male system
- Testes (pair) sit in the scrotum, outside the belly, because sperm need a temperature 2–2.5 °C lower than body temperature.
- Duct path: seminiferous tubules → rete testis → vasa efferentia → epididymis (sperm mature and are stored) → vas deferens → ejaculatory duct → urethra, which opens out through the penis.
- Accessory glands: seminal vesicles (fructose for energy), prostate and bulbourethral glands (fluid and lubrication). Sperm + these fluids = semen.
Female system
- Ovaries (pair) in the lower belly make ova and hormones.
- Oviduct (fallopian tube): a funnel with finger-like fimbriae that catch the ovum → wide ampulla (where fertilisation happens) → narrow isthmus → uterus.
- Uterus (womb): wall with three layers; the inner lining endometrium changes every cycle; the thick muscle layer myometrium contracts during birth. The cervix opens into the vagina.
- Mammary glands (a pair) have milk-making tissue in lobes; milk flows through ducts to the nipple.
Structure of the testis and the ovary
Testis
Each testis has about 250 compartments (lobules). Each lobule holds 1–3 tightly coiled seminiferous tubules. Their wall has two kinds of cells:
- Spermatogonia: germ cells that will make sperm.
- Sertoli cells: big 'nurse' cells that feed the developing sperm.
Between the tubules, Leydig (interstitial) cells make the male hormone testosterone.
Ovary
The ovary has an outer thin layer and a stroma inside. The outer part of the stroma (cortex) holds follicles at different stages; each follicle is an oocyte wrapped in helper cells. A mature follicle with a big fluid space is the Graafian follicle.
Gametogenesis: spermatogenesis and oogenesis
Spermatogenesis (starts at puberty, continues lifelong)
- Spermatogonium (2n) grows → primary spermatocyte (2n).
- Meiosis I → 2 secondary spermatocytes (n).
- Meiosis II → 4 spermatids (n).
- Spermatids change shape into sperm (spermiogenesis) and are released into the tubule (spermiation).
Hormones: GnRH from the hypothalamus → the pituitary releases LH (acts on Leydig cells → testosterone) and FSH (acts on Sertoli cells).
A sperm has a head (nucleus + acrosome with enzymes to enter the egg), a neck, a middle piece full of mitochondria (energy) and a tail (swimming).
Oogenesis (starts before birth)
- Oogonia form in the fetal ovary. Many become primary oocytes (2n) that start meiosis I and pause for years.
- At puberty, each cycle one primary oocyte finishes meiosis I → a large secondary oocyte (n) + a tiny first polar body.
- The secondary oocyte is released at ovulation. Meiosis II finishes only if a sperm enters → ovum + second polar body.
Follicle stages: primary → secondary → tertiary (fluid space, antrum) → Graafian follicle.
Key difference
One spermatogonium → 4 sperm. One oogonium → 1 ovum (+ 2 or 3 polar bodies). The unequal split gives the ovum almost all the food and cytoplasm.
The menstrual cycle
The menstrual cycle is a repeating change in the ovary and uterus, about 28 days long, from puberty (menarche) to about age 50 (menopause).
- Menstrual phase (days 1–5): if no pregnancy, the endometrium breaks down and flows out with blood.
- Follicular (proliferative) phase (days 6–13): FSH and LH make a follicle grow into a Graafian follicle. The follicle makes oestrogen, which rebuilds the endometrium.
- Ovulation (about day 14): a sudden big rise of LH (LH surge) bursts the Graafian follicle and releases the secondary oocyte.
- Luteal (secretory) phase (days 15–28): the empty follicle turns into the yellow corpus luteum, which makes progesterone. Progesterone keeps the endometrium thick and ready. If there is no fertilisation, the corpus luteum shrinks, progesterone falls and menstruation starts again.
During pregnancy, cycles stop because the corpus luteum keeps working.
Fertilisation to blastocyst
- Sperm swim up to the ampulla of the oviduct. Only one sperm enters the secondary oocyte. The egg surface (zona pellucida) then changes to block other sperm.
- Sperm entry makes the oocyte finish meiosis II. The sperm and ovum nuclei fuse → zygote (2n). The sex of the baby is fixed here: an X-carrying sperm gives a girl (XX), a Y-carrying sperm gives a boy (XY). So the father's sperm decides the sex.
- While moving down the oviduct the zygote divides by mitosis (cleavage): 2, 4, 8, 16 cells. The 8–16 cell ball is the morula.
- The morula becomes a blastocyst: an outer layer (trophoblast) and an inner group of cells (inner cell mass) that will become the baby.
Implantation
The trophoblast sticks to the endometrium. The uterine cells grow over the blastocyst and it becomes buried in the wall. This is implantation, and it marks the start of pregnancy. The inner cell mass forms three germ layers (ectoderm, mesoderm, endoderm); from them every organ is made. These inner cells are stem cells that can become any tissue.
Pregnancy, placenta, birth and lactation
Placenta
Finger-like chorionic villi from the embryo lock into the uterus wall. Together they make the placenta. It joins the fetus through the umbilical cord. The placenta passes food and oxygen in, takes carbon dioxide and waste out, and also makes hormones (hCG, hPL, oestrogen, progesterone). Mother's and baby's blood do not mix directly.
Growth in months (approximate)
- Month 1: heart forms. Month 2: limbs and fingers. End of month 3: most major organs formed.
- Month 5: first movements, hair on head. End of month 6: eyelashes, body covered with fine hair. About 9 months: ready for birth.
Birth (parturition)
The fully grown fetus and placenta send signals that start mild contractions. These make the pituitary release oxytocin, which makes the uterus muscle contract harder, which releases more oxytocin: a positive feedback loop that pushes the baby out.
Lactation
The mammary glands start making milk near the end of pregnancy. The milk of the first few days is colostrum, rich in antibodies that protect the newborn.
Try it: map a 28-day cycle
- Draw a line with 28 boxes. Colour days 1–5 red, days 6–13 yellow, day 14 blue, days 15–28 green.
- In the 3D, slide to day 10, 14 and 21. Write which hormone bar is the tallest each time.
- Predict, then check: if the cycle is 32 days long, ovulation still happens about 14 days before the next period. Which day is that?
(Answer: day 18. The luteal phase stays about 14 days.)
Key formulas and definitions
- Spermatogonium (2n) → primary spermatocyte (2n) → 2 secondary spermatocytes (n) → 4 spermatids (n) → 4 sperm
- Oogonium (2n) → primary oocyte (2n) → secondary oocyte (n) + 1st polar body → ovum (n) + 2nd polar body
- Cycle: menstrual (1–5) → follicular (6–13) → ovulation (~14, LH surge) → luteal (15–28, progesterone)
- Zygote → cleavage → morula (8–16 cells) → blastocyst (trophoblast + inner cell mass) → implantation
- Placenta hormones: hCG, hPL, oestrogen, progesterone; birth: oxytocin (positive feedback)
- Human cells: 2n = 46; sperm and ovum: n = 23
Worked examples
1. How many sperm are made from 25 primary spermatocytes?
Each primary spermatocyte gives 4 sperm. 25 × 4 = 100 sperm.
2. How many ova are made from 25 primary oocytes (all completing meiosis)?
Each primary oocyte gives only 1 ovum. 25 × 1 = 25 ova (the rest of the cells are polar bodies).
3. Give the chromosome number of: spermatogonium, secondary spermatocyte, spermatid, zygote in humans.
Spermatogonium 46 (2n); secondary spermatocyte 23 (n, after meiosis I); spermatid 23; zygote 46.
4. A woman has a regular 30-day cycle. On which day does ovulation most likely happen?
The luteal phase is about 14 days. Ovulation ≈ 30 − 14 = day 16.
5. Count the cells after 4 rounds of cleavage of a zygote.
Each round doubles: 1 → 2 → 4 → 8 → 16. After 4 rounds there are 2⁴ = 16 cells, the morula stage.
6. Why does a woman not menstruate during pregnancy? Explain with hormones.
After implantation hCG from the embryo keeps the corpus luteum alive. It keeps making progesterone (later the placenta also makes it). High progesterone keeps the endometrium thick, so it does not break down and no period comes.
Common mistakes
- Saying the ovum is released at ovulation. A secondary oocyte is released; it becomes an ovum only after a sperm enters and meiosis II is completed.
- Thinking fertilisation happens in the uterus. It happens in the ampulla of the oviduct; the embryo then travels to the uterus.
- Saying Leydig cells make sperm. Leydig cells make testosterone; sperm form from spermatogonia inside the tubules, fed by Sertoli cells.
- Mixing up morula and blastocyst. Morula is a solid ball of cells; blastocyst is hollow with an inner cell mass.