Male Reproductive System
Sperm Production & Delivery♂️ The Male System — Built for Sperm Production at Scale
The male reproductive system has one primary function: produce hundreds of millions of sperm continuously from puberty and deliver them to the female reproductive tract. Every structure is optimised for this — from the external testes (temperature control) to the multiple glands that produce the seminal fluid carrying sperm.
Primary Organ
Testes — Sperm & Hormone Factory
Location outside the body is not accidental — it is a temperature adaptation.
📍 Structure & Location
- Located in the scrotum — outside the body cavity
- Temperature is 2-3°C lower than core body temperature
- Sperm production (spermatogenesis) requires cooler temperature — cannot occur at 37°C
- Seminiferous tubules — tightly coiled tubes inside testes where sperm are produced by meiosis
- Sertoli cells — nurse cells that support developing sperm
- Leydig (interstitial) cells — produce testosterone
⚙️ Spermatogenesis
- Spermatogonia (2n) → divide by mitosis to maintain stem cell population
- Primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n)
- Secondary spermatocytes → meiosis II → spermatids (n)
- Spermatids → mature into spermatozoa (sperm) with flagellum
- Process takes approximately 74 days
- Produces ~300 million sperm per day from puberty
⚠️ Exam Watch — Sperm Structure
Sperm have 3 main regions: (1) Head — contains nucleus (haploid) with tightly packed DNA; acrosome at tip contains enzymes to penetrate egg. (2) Midpiece — packed with mitochondria for ATP production to power the flagellum. (3) Tail (flagellum) — propels sperm. Every structure links to function — mitochondria = energy for swimming; acrosome = enzymes to penetrate egg.
Sperm Pathway
From Testes to Ejaculation
Seminiferous tubules → epididymis → vas deferens → urethra. Know the route.
1
Seminiferous tubules — where sperm are produced in the testes
2
Epididymis — coiled tube on the back of each testis; sperm mature here and are stored for up to 6 weeks
3
Vas deferens (sperm duct) — muscular tube that carries sperm from epididymis toward the urethra during ejaculation
4
Seminal vesicles add fructose (energy for sperm) and prostaglandins; Prostate gland adds alkaline fluid (neutralises vaginal acidity); Cowpers gland adds lubricating mucus
5
Urethra — carries semen out of the body through the penis during ejaculation (also carries urine — but not simultaneously)
📌 Semen vs Sperm
Sperm are just the cells. Semen is the complete fluid — sperm plus all the secretions from the seminal vesicles, prostate, and Cowpers gland. The average ejaculate contains 2-5ml of semen with 200-500 million sperm. Only one will fertilise the egg.
Female Reproductive System
Egg Production & Uterine Environment♀️ The Female System — Built for Egg Release and Embryo Support
The female reproductive system produces one egg (usually) per month, provides the site for fertilisation, and — if fertilisation occurs — maintains and nourishes the developing embryo for nine months. Every structure is adapted for this. The ovaries, fallopian tubes, uterus, and cervix each play a precisely timed role.
Primary Organ
Ovaries — Egg Production & Hormones
A female is born with all her eggs already present — about 1-2 million. Only 400-500 will ever be released.
🔬 Oogenesis (Egg Production)
- Oogonia → primary oocytes formed before birth — arrested in meiosis I
- At puberty, one primary oocyte completes meiosis I each month → secondary oocyte + first polar body
- Secondary oocyte is released at ovulation — arrested in meiosis II
- Meiosis II is only completed if the egg is fertilised by a sperm
- Produces one large egg + small polar bodies (discarded)
🌕 Follicle Development
- Each egg develops inside a follicle in the ovary
- Follicle = egg + surrounding support cells
- Follicle grows and produces oestrogen as it matures
- At ovulation — Graafian follicle ruptures, releasing egg
- Empty follicle → corpus luteum → produces progesterone
- Corpus luteum degenerates if no pregnancy → menstruation
Structures & Functions
From Ovary to Uterus
Each structure has a specific function — know them all.
| Structure | Function |
|---|---|
| Ovaries | Produce eggs (oogenesis) and hormones (oestrogen, progesterone) |
| Fallopian tubes (oviducts) | Carry egg from ovary to uterus; site of fertilisation; cilia and peristalsis move egg/zygote |
| Fimbriae | Finger-like projections at the end of fallopian tube — sweep egg from ovary surface into tube |
| Uterus | Muscular organ where embryo implants and develops; endometrium = inner lining that thickens each cycle |
| Endometrium | Inner lining of uterus; thickens under oestrogen and progesterone; shed during menstruation if no implantation |
| Cervix | Lower narrow part of uterus; mucus plug protects uterus from infection; dilates during labour |
| Vagina | Birth canal; receives penis during copulation; slightly acidic environment (harmful to sperm — neutralised by seminal fluid) |
The Menstrual Cycle
28-Day Hormonal Blueprint🔄 A Monthly Preparation for Possible Pregnancy
The menstrual cycle is a precisely orchestrated sequence of hormonal events that prepares the uterus for possible pregnancy every month. Four hormones — FSH, LH, oestrogen, and progesterone — interact in a precise sequence through negative and positive feedback loops. The cycle averages 28 days but varies between individuals.
The Four Phases
1
Menstruation (Days 1–5) — Progesterone and oestrogen levels fall. Endometrium breaks down and is shed (menstrual flow). FSH begins to rise, stimulating new follicle development.
2
Follicular Phase (Days 1–13) — FSH from pituitary stimulates follicle growth in ovary. Growing follicle produces increasing amounts of oestrogen. Oestrogen causes endometrium to thicken and repair. Rising oestrogen eventually triggers LH surge.
3
Ovulation (Day 14) — LH surge (positive feedback from high oestrogen) triggers Graafian follicle to rupture. Egg is released into fallopian tube. Fertilisation window: approximately 12–24 hours.
4
Luteal Phase (Days 15–28) — Empty follicle becomes corpus luteum. Corpus luteum secretes progesterone and some oestrogen. Progesterone maintains and thickens endometrium ready for implantation. If no fertilisation: corpus luteum degenerates → progesterone falls → menstruation begins again.
| Hormone | Source | Main Role in Cycle |
|---|---|---|
| FSH (Follicle Stimulating Hormone) | Anterior pituitary | Stimulates follicle growth in ovary; stimulates oestrogen production |
| LH (Luteinising Hormone) | Anterior pituitary | Surge triggers ovulation; stimulates corpus luteum formation and progesterone production |
| Oestrogen | Developing follicle | Thickens and repairs endometrium; inhibits FSH (negative feedback); triggers LH surge (positive feedback) at high levels |
| Progesterone | Corpus luteum (then placenta) | Maintains thickened endometrium; inhibits FSH and LH (prevents new follicle development); essential for maintaining pregnancy |
⚠️ Exam Watch — Feedback Loops
Negative feedback: High oestrogen (early) inhibits FSH — prevents multiple follicles maturing simultaneously. High progesterone inhibits FSH and LH — prevents ovulation during pregnancy.Positive feedback: Very high oestrogen (just before ovulation) triggers the LH surge — the one exception to negative feedback in this cycle. This is critical to understand — it is the trigger for ovulation.
📌 If Pregnancy Occurs
The embryo implants and begins producing HCG (human chorionic gonadotropin) — this is the hormone detected by pregnancy tests. HCG signals the corpus luteum to keep producing progesterone, preventing menstruation. Eventually the placenta takes over progesterone production and the corpus luteum is no longer needed.
Fertilisation & Pregnancy
From Zygote to Birth🤱 Nine Months of Extraordinary Development
After fertilisation in the fallopian tube, the single-celled zygote undergoes a remarkable journey — dividing, implanting, forming the placenta, and developing over 38 weeks into a fully formed human being capable of independent life. Each trimester brings distinct developmental milestones and physiological challenges.
The Moment
Fertilisation
One sperm, one egg, one window of about 24 hours. Here is exactly what happens.
1
Sperm arrive at egg — hundreds of millions deposited in vagina; ~200 survive to reach the egg in the fallopian tube (most are destroyed by acidic vaginal environment or fail to navigate cervical mucus).
2
Acrosome reaction — sperm bind to the zona pellucida (glycoprotein coat around egg); acrosomal enzymes digest through the zona pellucida.
3
Sperm enters egg — one sperm penetrates the egg membrane. Immediately, a cortical reaction changes the zona pellucida (zona hardening/cortical reaction) — preventing any further sperm entry (polyspermy block).
4
Meiosis II completes — the secondary oocyte completes meiosis II (arrested since ovulation), producing the mature egg nucleus and a second polar body.
5
Pronuclei fuse — sperm nucleus (n) and egg nucleus (n) fuse to form the zygote (2n). Fertilisation is complete. Location: fallopian tube.
Days 1–14 Post-Fertilisation
Cleavage, Morula, Blastocyst & Implantation
From a single cell to implantation in 6-10 days.
1
Cleavage — zygote divides by mitosis repeatedly (without growing). Cells become smaller with each division (blastomeres). No growth — just division.
2
Morula — solid ball of 16+ cells; travels down fallopian tube toward uterus over 3-4 days.
3
Blastocyst — fluid-filled hollow ball; has inner cell mass (embryoblast — will become the embryo) and outer layer (trophoblast — will become the placenta).
4
Implantation (Day 6-10) — blastocyst attaches to and burrows into the endometrium. Trophoblast cells invade the endometrium, establishing the placenta.
5
HCG production begins — trophoblast secretes HCG, maintaining corpus luteum and progesterone production, preventing menstruation.
Weeks 1–38
Three Trimesters of Development
The key milestones, organ formation, and foetal development by trimester.
🌱 First Trimester (Weeks 1–12)
- Embryo stage: weeks 1–8; foetus from week 9
- All major organs and body systems begin forming
- Heart begins beating around week 6
- Neural tube (brain and spinal cord) forms weeks 3–4
- Limb buds appear by week 5
- Most critical period — highest risk from teratogens (drugs, alcohol, infections)
- Placenta fully formed by week 10
🌿 Second Trimester (Weeks 13–26)
- Rapid growth — foetus grows from ~9cm to ~35cm
- Movements felt by mother (quickening) around week 18-20
- Sex organs visible by week 14
- Hair, fingernails, eyebrows develop
- Hearing develops — foetus responds to sound
- Risk of miscarriage drops significantly
🌳 Third Trimester (Weeks 27–38)
- Rapid weight gain — foetus triples in weight
- Lungs mature (surfactant production — allows lung inflation after birth)
- Brain development accelerates
- Foetus moves into head-down position for birth (normally)
- Passive immunity — maternal antibodies cross placenta, providing protection in first months of life
⚠️ Exam Watch — Birth (Parturition)
Labour is triggered by a shift in hormone balance: progesterone (which inhibits uterine contractions) decreases; oestrogen and oxytocin increase. Oxytocin stimulates uterine contractions — a positive feedback loop (contractions stimulate more oxytocin → stronger contractions → more oxytocin) until the baby is delivered. After delivery, the placenta (afterbirth) is expelled.
🎯 Blueprint Assessment
Eight questions covering the full human reproduction system.
Question 1 of 8
Why are the testes located in the scrotum outside the body cavity rather than inside the abdomen?
Question 2 of 8
A sperm cell has a large number of mitochondria in its midpiece. Why is this an important structural adaptation?
Question 3 of 8
On Day 14 of the menstrual cycle, there is a sudden surge in LH. What causes this surge and what does it trigger?
Question 4 of 8
Why does the endometrium NOT break down (menstruation does not occur) if a fertilised egg successfully implants?
Question 5 of 8
Where in the female reproductive system does fertilisation normally occur?
Question 6 of 8
What is the role of the corpus luteum in the menstrual cycle and in early pregnancy?
Question 7 of 8
A developing embryo at the blastocyst stage has two distinct cell populations. What are they and what does each become?
Question 8 of 8
What hormone triggers labour (uterine contractions), and what type of feedback loop does this involve?