The male reproductive system
A tiny sperm cell has to be made, stored and delivered at the right temperature. The body of a boy even keeps the testes outside the belly for this reason.
The male system has a pair of testes in the scrotum (about 2–3 °C cooler than body temperature), accessory ducts, accessory glands and the penis. Each testis has about 250 lobules, each lobule has 1–3 coiled seminiferous tubules where sperm form. Sertoli cells feed the germ cells and Leydig cells outside the tubules secrete androgens.
In the figure : Sperm leave the testis by the path shown and come out through the urethra and penis.
In the figure : Their secretions make seminal plasma, rich in fructose, calcium and enzymes.
Rule 1: Testis and its cells — Spermatogonia in the tubule wall divide to make sperm. Sertoli cells nourish the germ cells; Leydig cells (in the spaces between tubules) make androgens such as testosterone. The scrotum keeps the temperature lower than body heat, which sperm need.
- Easy : Leydig cells make hormones; Sertoli cells give nourishment.
- Medium : The testes lie in the scrotum because sperm formation needs a temperature lower than that of the abdomen.
- Exam-type : Q: Which cells secrete androgens? A: Leydig (interstitial) cells.
Rule 2: Ducts and glands — Seminal plasma from the seminal vesicles, prostate and bulbourethral glands, along with sperm, forms semen. The bulbourethral glands also lubricate the penis. The penis has erectile tissue; its enlarged tip is the glans, covered by the foreskin.
- Easy : Sperm + seminal plasma = semen.
- Medium : Seminal plasma gives sperm fructose for energy.
- Exam-type : Q: Name the duct through which both sperm and urine pass. A: The urethra.
Testes: about 250 lobules; 1–3 seminiferous tubules per lobule. Sperm path: tubule → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.
Mixing up Sertoli and Leydig cells. Sertoli cells are inside the tubules and nourish; Leydig cells are outside and secrete androgens.
Memory clue: S-in, L-out — Sertoli cells stay inside the tubule; Leydig cells lie outside it.
Spot the difference
| Point | Sertoli cells | Leydig cells |
|---|---|---|
| Position | Inside seminiferous tubules | Between the tubules |
| Function | Nourish germ cells | Secrete androgens |
| Also called | Sustentacular cells | Interstitial cells |
Practice questions
- 1Where are the testes located?
- 2Name the cells that secrete androgens.
- 3What are seminiferous tubules?
- 4Which glands add fluid to semen?
- 5Name the first duct that receives sperm from the seminiferous tubules.
- 6What is the role of the bulbourethral gland?
- 7Why are the testes outside the abdominal cavity?
- 8Trace the route of a sperm from the testis to the outside, naming the last two ducts.
Show answers (try them yourself first)
- 1In the scrotum, outside the abdominal cavity.
- 2Leydig cells.
- 3Coiled tubules inside each lobule of the testis where sperm are made.
- 4Seminal vesicles, prostate and bulbourethral glands.
- 5The rete testis.
- 6It secretes a fluid that lubricates the penis.
- 7Sperm formation needs a temperature about 2–3 °C lower than the body temperature.
- 8Seminiferous tubule → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.
The female reproductive system
A baby grows nine months inside the body of its mother. This needs a safe, stretchable chamber with a well-fed lining, a canal for birth and glands for feeding.
The female system has a pair of ovaries (produce the ovum and steroid hormones), a pair of oviducts (fallopian tubes), the uterus, the vagina, external genitalia and the mammary glands. The uterus wall has three layers: perimetrium (outer), myometrium (muscle) and endometrium (inner lining).
In the figure : The ovum is caught by the funnel-shaped infundibulum; fertilisation happens in the ampulla.
In the figure : The endometrium changes during the menstrual cycle; the myometrium contracts strongly during childbirth.
Rule 1: Ovary, oviduct and uterus — Ovarian follicles at different stages lie in the stroma. The oviduct begins with the infundibulum (finger-like fimbriae), then the ampulla and the narrow isthmus. The pear-shaped uterus opens through the cervix into the vagina; the cervical canal and vagina together form the birth canal.
- Easy : The ovary makes the ovum.
- Medium : Fimbriae collect the ovum after ovulation.
- Exam-type : Q: Where does fertilisation take place? A: In the ampulla of the oviduct.
Rule 2: External genitalia and mammary glands — The external genitalia include mons pubis, labia majora, labia minora, hymen and clitoris. Each breast has 15–20 mammary lobes made of alveoli; the alveoli make milk that collects in the mammary ampulla and comes out through the lactiferous duct.
- Easy : Milk is made in the alveoli.
- Medium : Alveoli → mammary tubules → mammary duct → ampulla → lactiferous duct.
- Exam-type : Q: Name the three layers of the uterine wall. A: Perimetrium, myometrium and endometrium.
Ovary: gamete + hormones. Oviduct: carries the ovum, site of fertilisation (ampulla). Uterus: site of implantation and development. Breasts are secondary sexual characters.
Saying fertilisation happens in the uterus. It occurs in the ampulla of the oviduct; implantation is in the uterus.
Memory clue: PME — Perimetrium, Myometrium, Endometrium — from the outside in.
Spot the difference
| Point | Myometrium | Endometrium |
|---|---|---|
| Position | Middle layer | Inner lining |
| Nature | Thick smooth muscle | Glandular, with blood vessels |
| Role | Contracts at birth | Implantation and menstruation |
Practice questions
- 1Name the female gonads.
- 2What is the inner layer of the uterus called?
- 3Name the funnel-shaped part of the oviduct near the ovary.
- 4Which part of the oviduct is the site of fertilisation?
- 5What forms the birth canal?
- 6Name the milk-producing units of the breast.
- 7Why is the myometrium a thick muscular layer?
- 8Why do doctors say the endometrium is cyclic?
Show answers (try them yourself first)
- 1The ovaries.
- 2The endometrium.
- 3The infundibulum.
- 4The ampulla.
- 5The cervix and the vagina.
- 6The alveoli of the mammary lobes.
- 7It has to contract strongly to push the baby out during birth.
- 8It thickens and sheds during each menstrual cycle under the action of hormones.
Gametogenesis: spermatogenesis and oogenesis
A boy makes millions of sperm every day, but a girl is born with all the egg cells she will ever have, already started. The two processes are quite different in timing.
Spermatogenesis forms sperm in the testes from puberty. Oogenesis forms the ovum in the ovary; it starts before birth, pauses, and continues after puberty. In humans an ovum is released from one follicle at ovulation, one per cycle.
In the figure : One spermatogonium gives four sperm; each division stage is in the seminiferous tubule.
In the figure : Meiosis II of the oocyte is completed only if a sperm enters.
Rule 1: Spermatogenesis — Spermatogonia multiply by mitosis; primary spermatocytes undergo meiosis I to give two secondary spermatocytes and meiosis II to give four spermatids. Spermiogenesis turns spermatids into sperm; spermiation releases them into the tubule lumen. A sperm has a head (nucleus + acrosome), neck, middle piece (mitochondria) and tail. Hormones: GnRH → LH (acts on Leydig cells → androgens) and FSH (acts on Sertoli cells).
- Easy : A sperm has four parts.
- Medium : The acrosome holds enzymes that help the sperm enter the egg.
- Exam-type : Q: What is the role of the mitochondria in the middle piece? A: They give energy for the movement of the tail.
Rule 2: Oogenesis — Primary oocytes are formed in the foetus and are arrested in prophase I; at birth there are millions, at puberty only 60,000–80,000 remain. Each month a follicle grows from primary → secondary → tertiary (Graafian) follicle; the primary oocyte completes meiosis I to form a haploid secondary oocyte and a tiny polar body. The Graafian follicle ruptures to release the secondary oocyte (ovulation).
- Easy : Meiosis in the oocyte is unequal: the oocyte keeps most of the cytoplasm.
- Medium : The secondary oocyte is covered by a membrane called the zona pellucida.
- Exam-type : Q: When is meiosis II of the oocyte completed? A: Only after a sperm enters it.
Spermatogenesis: 1 spermatogonium → 4 sperm (equal division). Oogenesis: 1 oogonium → 1 ovum + polar bodies (unequal division). Both start with mitosis and need meiosis.
Saying a secondary oocyte is released as a complete ovum. It is released at the metaphase II stage and is completed only after fertilisation.
Memory clue: 1 → 4 vs 1 → 1 — One cell makes four sperm but only one ovum.
Spot the difference
| Point | Spermatogenesis | Oogenesis |
|---|---|---|
| Begins | At puberty | In foetal life |
| Products from one cell | 4 sperm | 1 ovum + polar bodies |
| Division of cytoplasm | Equal | Unequal |
Practice questions
- 1Where does spermatogenesis occur?
- 2Name the hormone from the pituitary that acts on Sertoli cells.
- 3What is spermiation?
- 4At what stage is the primary oocyte arrested?
- 5What are the four parts of a human sperm?
- 6What does a Graafian follicle release at ovulation?
- 7Why does the middle piece of a sperm contain many mitochondria?
- 8Why are polar bodies formed during oogenesis?
Show answers (try them yourself first)
- 1In the seminiferous tubules of the testes.
- 2FSH (follicle stimulating hormone).
- 3Release of sperm from Sertoli cells into the lumen of the seminiferous tubule.
- 4Prophase I of meiosis I.
- 5Head, neck, middle piece and tail.
- 6The secondary oocyte.
- 7Sperm swim long distances and mitochondria supply the energy for the tail movement.
- 8Unequal divisions keep most cytoplasm in one cell and the other cells, which are small, get no cytoplasm and degenerate.
The menstrual cycle
The cycle repeats about every four weeks, like a monthly rehearsal of the body for a possible pregnancy.
The menstrual cycle is the cyclic change in the ovary and uterus of female primates, repeated every 28/29 days on average. It begins at puberty (menarche) and ends at menopause (about 50 years of age). Ovulation takes place near the middle of the cycle (about day 14).
In the figure : Menstruation days 1–5, follicular phase, ovulation at about day 14, luteal phase till day 28.
Rule 1: Hormones control the cycle — FSH makes the follicle grow, and the growing follicle makes estrogen; estrogen repairs the endometrium. A sharp surge of LH near day 14 causes ovulation. The remains of the follicle turn into the corpus luteum which secretes progesterone to keep the endometrium ready.
- Easy : LH surge causes ovulation.
- Medium : Progesterone from the corpus luteum keeps the endometrium ready for implantation.
- Exam-type : Q: Which structure forms from the Graafian follicle after ovulation? A: The corpus luteum.
Rule 2: If there is no fertilisation — The corpus luteum degenerates, progesterone falls, and the endometrium breaks down; blood and tissue flow out as menstruation. If fertilisation occurs, the corpus luteum persists and menstruation does not occur. Cyclic menstruation is a sign of the normal reproductive phase, and hygiene during menstruation is important.
- Easy : No fertilisation means menstruation.
- Medium : The first day of bleeding is counted as day 1.
- Exam-type : Q: Why does menstruation stop in pregnancy? A: The corpus luteum keeps making hormones, so the endometrium is not shed.
LH surge on about day 14 → ovulation. Corpus luteum → progesterone. Menstrual flow lasts about 3–5 days.
Thinking ovulation happens on the first day of bleeding. It happens about the middle of the cycle.
Memory clue: M-F-O-L — Menstrual → Follicular → Ovulation → Luteal.
Spot the difference
| Point | Follicular phase | Luteal phase |
|---|---|---|
| Main hormone | Estrogen (FSH rising) | Progesterone |
| Ovary | Follicle grows | Corpus luteum forms |
| Endometrium | Repairs and thickens | Ready for implantation |
Practice questions
- 1What is menarche?
- 2On about which day does ovulation take place in a 28-day cycle?
- 3Which hormone surge triggers ovulation?
- 4Name the hormone secreted by the corpus luteum.
- 5What is menopause?
- 6What happens to the corpus luteum if there is no fertilisation?
- 7Why does the endometrium shed if the ovum is not fertilised?
- 8Why is hygiene stressed during menstruation?
Show answers (try them yourself first)
- 1The first menstruation at puberty.
- 2About the 14th day.
- 3LH (luteinising hormone).
- 4Progesterone.
- 5Permanent stopping of menstruation at around 50 years of age.
- 6It degenerates.
- 7The corpus luteum degenerates, progesterone level falls, and the lining cannot be maintained.
- 8Poor hygiene can cause infections of the reproductive tract.
Fertilisation, cleavage and implantation
Out of hundreds of millions of sperm only one meets the ovum. The race ends at a spot just inside the oviduct.
Fertilisation is the fusion of a sperm and an ovum to form a diploid zygote, in the ampulla of the oviduct. The zygote divides by cleavage into a morula and then a blastocyst, which gets attached to the endometrium (implantation).
In the figure : Cleavage is mitotic; the embryo reaches the uterus as a morula and becomes a blastocyst.
In the figure : The inner cell mass differentiates into the embryo.
Rule 1: Fertilisation — The sperm contacts the zona pellucida and the acrosomal enzymes digest a path through it. The ovum then blocks entry of other sperm. The oocyte completes meiosis II, the sperm and egg nuclei fuse. The sex of the child depends on the sperm: X gives a girl (XX), Y gives a boy (XY).
- Easy : Only one sperm enters an ovum.
- Medium : The sex of the baby is decided by the sperm.
- Exam-type : Q: What helps the sperm enter the zona pellucida? A: Acrosomal enzymes.
Rule 2: Cleavage and implantation — Mitotic divisions called cleavage make cells called blastomeres. A 8–16-celled morula forms, then a blastocyst with an outer trophoblast and an inner cell mass. The trophoblast attaches to the endometrium; the uterine cells cover the blastocyst and it embeds.
- Easy : Cleavage divides the zygote without growth.
- Medium : The blastocyst, not the morula, implants.
- Exam-type : Q: Which part of the blastocyst forms the embryo? A: The inner cell mass.
Fertilisation in the ampulla; implantation in the uterus. The zygote is the first cell of the new human, with 46 chromosomes.
Writing that cleavage increases the size of the embryo. The cells become smaller; the total size stays nearly the same.
Memory clue: Z-M-B-I — Zygote → Morula → Blastocyst → Implantation.
Spot the difference
| Point | Morula | Blastocyst |
|---|---|---|
| Cells | 8–16, solid ball | Hollow, with trophoblast and inner cell mass |
| Stage | Before reaching the uterus | Ready to implant |
| Can implant? | No | Yes |
Practice questions
- 1Where does fertilisation occur?
- 2What is the diploid cell formed after fertilisation called?
- 3What is the number of chromosomes in a human zygote?
- 4Name the outer layer of the blastocyst.
- 5What is the ball of 8–16 cells called?
- 6Which sperm gives a female child?
- 7Why is the sperm's acrosome important for fertilisation?
- 8Why is the sex of the baby decided by the father?
Show answers (try them yourself first)
- 1In the ampulla of the oviduct.
- 2The zygote.
- 346 (23 pairs).
- 4The trophoblast.
- 5The morula.
- 6A sperm carrying the X chromosome.
- 7The enzymes in it digest the zona pellucida so that the sperm can reach the egg.
- 8The ovum always carries X, while a sperm carries X or Y.
Pregnancy, placenta, parturition and lactation
From the day the blastocyst sticks to the uterus wall, a nine-month building project begins: a new organ, the placenta, links the baby to the mother.
After implantation, the finger-like chorionic villi of the trophoblast and the uterine tissue together form the placenta, which supplies oxygen and food and removes CO₂ and wastes; it is joined to the embryo by the umbilical cord. The placenta also makes hCG, hPL, estrogen, progesterone and relaxin. Parturition is childbirth, triggered by a neuroendocrine mechanism.
In the figure : The heart begins to form in the first month, and limbs and digits in the second month.
In the figure : Oxytocin and cortisol work with estrogen to bring on strong contractions of the uterus.
Rule 1: Embryonic development — The inner cell mass gives rise to the three germ layers that form all tissues and organs. The heart is formed after one month; limbs and digits appear by the end of the second month; hair on the head is seen in the fifth month; by about 24 weeks the body has fine hair. After nine months the foetus is ready for birth.
- Easy : The heart is the first organ to form.
- Medium : Limbs and digits are formed by the end of the second month.
- Exam-type : Q: What does the placenta do? A: It supplies oxygen and nutrients and removes wastes and CO₂ from the foetus.
Rule 2: Parturition and lactation — Strong uterine contractions (the foetal ejection reflex) push the baby out and the umbilical cord is cut. Mammary glands develop during pregnancy and begin producing milk after birth. The first milk, colostrum, has antibodies that protect the new-born; breast-feeding for the initial months is recommended for the baby's growth.
- Easy : Colostrum is the first milk.
- Medium : Colostrum carries antibodies to resist infections.
- Exam-type : Q: Name the hormone that increases uterine contractions at birth. A: Oxytocin.
Placenta = chorionic villi + uterine tissue. Colostrum has antibodies. Relaxin helps in childbirth by softening the pelvis ligaments (read it with hPL and hCG).
Thinking the placenta is only a food supplier. It also removes wastes and works as an endocrine organ.
Memory clue: H-H-R — hCG, hPL, Relaxin — the three extra hormones of the placenta, besides estrogen and progesterone.
Spot the difference
| Point | Placenta | Umbilical cord |
|---|---|---|
| Meaning | Organ joining foetus to uterus | Tube that joins foetus to placenta |
| Made of | Villi and uterine tissue | Blood vessels in a jelly-like tissue |
| Hormones | Makes hCG, hPL, relaxin | No hormone formed |
Practice questions
- 1What is parturition?
- 2Name the structure that links the foetus to the placenta.
- 3What is the first milk of the mother called?
- 4Name two hormones made by the placenta.
- 5By which month does the heart form?
- 6When do limbs and digits form?
- 7Why is colostrum important for a new-born?
- 8Why is the placenta called an endocrine tissue?
Show answers (try them yourself first)
- 1The process of childbirth.
- 2The umbilical cord.
- 3Colostrum.
- 4hCG and hPL (also estrogen, progesterone, relaxin).
- 5By the end of the first month.
- 6By the end of the second month.
- 7It contains antibodies that give the baby protection against infection.
- 8It secretes hormones such as hCG, hPL, estrogen, progesterone and relaxin.