The Reproductive System
Male and female anatomy, meiosis, gametogenesis, hormones and the ovarian and uterine cycles
By the end of this unit you can…
- ✓Describe the structure and function of the testes, duct system, accessory glands and penis
- ✓Compare meiosis with mitosis and describe spermatogenesis and the structure of a sperm
- ✓Explain hormonal regulation of male reproductive function
- ✓Describe the ovaries, uterine tubes, uterus, vagina, external genitalia and mammary glands
- ✓Describe oogenesis and the ovarian and uterine cycles and their hormonal control
- ✓Describe common reproductive disorders and changes across the lifespan
Key terms
Practice →1Male anatomy
The testes (male gonads) make sperm and testosterone. Sperm pass through a duct system — epididymis → ductus (vas) deferens → ejaculatory duct → urethra — and mix with secretions of the accessory glands to form semen.
The scrotum keeps the testes about 3 °C below body temperature, needed for viable sperm. The dartos muscle (wrinkles the skin) and cremaster muscle (elevates the testes) pull them closer to the body in the cold. The pampiniform venous plexus cools arterial blood entering the testes.
- Seminiferous tubules
- Coiled tubules where sperm are made; they converge on the rete testis.
- Sustentocytes (Sertoli cells)
- Support and nourish developing sperm; tight junctions form the blood–testis barrier; secrete inhibin and androgen-binding protein.
- Interstitial (Leydig) cells
- Between the tubules; secrete testosterone.
- Epididymis
- Comma-shaped duct on the testis; sperm mature and gain motility here (~20 days) and are stored.
- Ductus (vas) deferens
- Runs in the spermatic cord through the inguinal canal; propels sperm by peristalsis. Cut in a vasectomy.
| Accessory gland | Contribution to semen |
|---|---|
| Seminal vesicles | ~70% of semen volume: alkaline fluid with fructose (sperm fuel), prostaglandins, clotting proteins |
| Prostate | ~25–30%: milky, slightly acidic fluid with citrate, enzymes and PSA; activates sperm |
| Bulbourethral glands | Clear mucus before ejaculation — neutralizes traces of acidic urine and lubricates |
The penis contains three columns of erectile tissue: two dorsal corpora cavernosa and a ventral corpus spongiosum surrounding the spongy urethra and expanding to form the glans. Erection is a parasympathetic reflex (nitric oxide relaxes arteriole smooth muscle so erectile tissue fills with blood); ejaculation is a sympathetic reflex.
2Meiosis & spermatogenesis
Meiosis is a special cell division in the gonads: two consecutive divisions (meiosis I and II) without DNA replication between them, producing four haploid (n = 23) cells from one diploid (2n = 46) cell. Meiosis I is the reduction division — homologous chromosomes pair up (synapsis, forming tetrads), exchange segments (crossing over) and separate. Meiosis II separates sister chromatids, like mitosis.
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | One | Two |
| Daughter cells | 2 diploid, identical | 4 haploid, genetically different |
| Synapsis & crossing over | No | Yes (prophase I) |
| Role | Growth, repair | Gamete formation |
- 1Spermatogonia (2n stem cells) at the tubule edge divide by mitosis; one daughter becomes a primary spermatocyte.
- 2Primary spermatocyte (2n) completes meiosis I → two secondary spermatocytes (n).
- 3Meiosis II → four spermatids (n).
- 4Spermiogenesis: spermatids lose excess cytoplasm and grow a tail → sperm released into the lumen.
- Head
- Nucleus (DNA) capped by the acrosome — enzymes that let the sperm penetrate the oocyte.
- Midpiece
- Mitochondria spiraled around the core — make ATP for movement.
- Tail
- A flagellum that propels the sperm.
3Male hormones
- 1Hypothalamus releases GnRH.
- 2GnRH stimulates the anterior pituitary to release FSH and LH.
- 3FSH stimulates sustentocytes to secrete androgen-binding protein (keeps testosterone high near sperm) → spermatogenesis.
- 4LH stimulates interstitial cells to secrete testosterone.
- 5Negative feedback: testosterone inhibits GnRH and LH; inhibin (from sustentocytes) inhibits FSH.
Testosterone: needed for spermatogenesis; growth of reproductive organs at puberty; secondary sex characteristics (pubic, axillary and facial hair, deeper voice, thicker skin, increased muscle and bone mass); libido. It also stimulates RBC production (erythropoietin) and metabolism.
4Female anatomy
The ovaries make oocytes and the hormones estrogen and progesterone. The duct system — uterine tubes, uterus and vagina — transports and nurtures gametes and the developing offspring.
- Ovary
- Held by the ovarian ligament (to uterus), suspensory ligament (to pelvic wall) and mesovarium. Cortex contains ovarian follicles — an oocyte surrounded by follicle cells.
- Uterine (fallopian) tube
- Infundibulum with finger-like fimbriae drapes over the ovary; the ampulla is the usual site of fertilization; the isthmus joins the uterus. Cilia and peristalsis move the oocyte toward the uterus — no direct connection to the ovary.
- Uterus
- Hollow organ for implantation and fetal development: fundus, body, cervix (narrow neck projecting into the vagina).
- Uterine wall
- Perimetrium (serous), myometrium (smooth muscle — contracts in labor), endometrium (mucosa). The endometrium's stratum functionalis is shed in menstruation; the stratum basalis regenerates it.
- Vagina
- Thin-walled muscular tube — birth canal and copulatory organ; acidic environment protects against infection.
The vulva includes the mons pubis, labia majora (homologous to the scrotum), labia minora, the vestibule (containing the urethral and vaginal openings and the greater vestibular glands, which secrete lubricating mucus) and the clitoris (erectile tissue homologous to the penis).
Mammary glands are modified sweat glands. Each breast has 15–25 lobes, each made of lobules of milk-secreting alveoli. Milk flows through lactiferous ducts into lactiferous sinuses and out through the nipple, surrounded by the pigmented areola. Suspensory ligaments support the breast.
5Oogenesis & the ovarian cycle
- 1Before birth: oogonia divide by mitosis, then become primary oocytes, which begin meiosis I and arrest in prophase I — a girl is born with her lifetime supply (~1 million at birth, only a few hundred thousand left by puberty).
- 2From puberty, each month: one primary oocyte completes meiosis I → a large secondary oocyte + a small first polar body.
- 3The secondary oocyte arrests in metaphase II and is ovulated.
- 4Only if a sperm penetrates it does meiosis II finish → ovum + second polar body. One oocyte → one functional ovum (vs four sperm).
| Follicle stage | Features |
|---|---|
| Primordial follicle | Primary oocyte + single layer of squamous follicle cells |
| Primary follicle | Cuboidal/columnar granulosa cells |
| Secondary follicle | Multiple granulosa layers; fluid-filled antrum begins; zona pellucida around oocyte |
| Vesicular (antral, Graafian) follicle | Mature follicle bulging from the ovary; ready to ovulate |
| Corpus luteum | Ruptured follicle after ovulation; secretes progesterone and estrogen; degenerates into corpus albicans if no pregnancy |
The ovarian cycle (~28 days) has a follicular phase (days 1–14, follicle growth), ovulation (~day 14) and a luteal phase (days 14–28, corpus luteum) — the luteal phase is the more constant ~14 days.
6Hormones & the uterine cycle
- 1GnRH → anterior pituitary releases FSH and LH.
- 2FSH (and LH) stimulate follicle growth; granulosa/thecal cells secrete estrogen.
- 3Rising estrogen first inhibits FSH/LH (negative feedback), then — once high enough — triggers a positive-feedback LH surge.
- 4The LH surge triggers ovulation (~day 14) and forms the corpus luteum.
- 5Corpus luteum secretes progesterone and estrogen → inhibit GnRH, FSH and LH. Without pregnancy (no hCG), it degenerates after ~10 days → hormones fall → menstruation and a new cycle.
| Uterine (menstrual) phase | Days | Events |
|---|---|---|
| Menstrual phase | 1–5 | Falling progesterone → stratum functionalis shed with blood |
| Proliferative phase | 6–14 | Rising estrogen rebuilds the functionalis; cervical mucus thins to let sperm through |
| Secretory phase | 15–28 | Progesterone from the corpus luteum makes the endometrium glandular and nutrient-rich for implantation; cervical mucus thickens |
Estrogens promote oogenesis, growth of reproductive organs at puberty and female secondary sex characteristics (breast development, wider pelvis, fat deposition in hips and breasts), and help maintain bone density and favorable cholesterol levels. Menopause — no menstrual periods for a full year (usually ~ages 46–54) — follows depletion of ovarian follicles, with declining estrogen.
7Reproductive disorders
| Disorder | Key facts |
|---|---|
| Benign prostatic hyperplasia (BPH) | Prostate enlargement with age → compresses the urethra → difficulty urinating, retention, UTIs |
| Prostate cancer | Common cancer in men; screened with PSA blood tests and digital rectal exam |
| Testicular cancer | Most common in young men (15–35); usually a painless lump; cure rate high |
| Cryptorchidism | Failure of a testis to descend — sterility and cancer risk if not corrected |
| Erectile dysfunction | Inability to attain erection; vascular disease, diabetes, drugs; treated with PDE5 inhibitors (enhance nitric oxide effect) |
| Ectopic pregnancy | Implantation outside the uterus, usually in a uterine tube — emergency |
| Endometriosis | Endometrial tissue grows outside the uterus → pain, infertility |
| Pelvic inflammatory disease (PID) | Infection (often gonorrhea or chlamydia) spreading to the uterine tubes — scarring, infertility |
| Cervical cancer | Linked to HPV; screened with Pap test; HPV vaccine prevents most cases |
| Breast cancer | Usually begins in the lactiferous duct epithelium; detected by self-exam and mammography; BRCA1/2 mutations raise risk |
Sexually transmitted infections: bacterial (gonorrhea, chlamydia, syphilis — curable with antibiotics) and viral (HPV, genital herpes, HIV — manageable but not curable). Chlamydia is the most common bacterial STI and often symptomless.