Why the Testes Sit Outside the Body

Humans reproduce sexually, with separate male and female individuals. Each reproductive system has primary sex organs that make gametes and hormones, ducts that carry the gametes, and accessory glands whose secretions support them.

This part covers the male reproductive system, the microscopic structure of the testis, the female reproductive system, and the ovary and mammary gland.

The male reproductive system consists of a pair of testes that make sperm, a duct system that carries them — epididymis, vas deferens, ejaculatory duct and urethra — and accessory glands — seminal vesicles, prostate and bulbourethral glands — whose secretions form the seminal plasma.

Testes:

- Lie outside the abdominal cavity in a pouch called the scrotum, which keeps them about $2$ to $2.5^{\circ }$C below body temperature, as sperm production requires

Duct system:

- Rete testis and vasa efferentia carry sperm out of the testis
- Epididymis — a coiled tube along the back of each testis, where sperm are stored and mature
- Vas deferens — rises into the abdomen, loops over the urinary bladder and joins the duct of the seminal vesicle
- Ejaculatory duct — opens into the urethra, which carries both urine and semen out through the penis

Accessory glands:

- Seminal vesicles — a paired gland
- Prostate — a single gland
- Bulbourethral glands — a paired gland whose secretion also helps lubrication
- Their secretions form the seminal plasma, rich in fructose, calcium and certain enzymes

An everyday example. Just as a refrigerator keeps milk fresh by holding it below room temperature, the scrotum keeps the testes slightly cooler than the rest of the body so sperm can form.

The substance. The male urethra is a shared passage — it carries both urine and semen.

Each testis is divided into about 250 compartments called testicular lobules, each holding one to three highly coiled seminiferous tubules lined by male germ cells that form sperm and Sertoli cells that nourish them, while interstitial Leydig cells between the tubules secrete androgens such as testosterone.

Inside a seminiferous tubule:

- Male germ cells (spermatogonia) — divide by mitosis and then meiosis, eventually forming sperm
- Sertoli cells — large supporting cells that nourish the developing germ cells

Between the tubules — the interstitial spaces:

- Leydig cells — synthesise and secrete androgens, mainly testosterone
- Small blood vessels and immunologically competent cells

What testosterone does. Androgens drive sperm production and control male secondary sexual characters, such as a deeper voice, growth of facial and body hair, and increased muscle mass.

An everyday example. A pomegranate divided into chambers packed with seeds is a useful picture of a testis divided into lobules packed with coiled tubules.

The substance. Sertoli cells feed developing sperm, but Leydig cells make the hormone — two cell types that are easy to swap in an answer.

The female reproductive system consists of a pair of ovaries that produce ova and hormones, a pair of oviducts that carry the ovum, a muscular uterus with a three-layered wall in which the foetus develops, and the cervix and vagina, which form the birth canal.

Ovaries — the primary female sex organs, each about $2$ to $4$ cm long, lying one on each side of the lower abdomen and held in place by ligaments.

Oviducts (fallopian tubes), each about $10$ to $12$ cm long:

- Infundibulum — the funnel-shaped part nearest the ovary, edged with finger-like fimbriae that collect the ovum at ovulation
- Ampulla — the wider middle part
- Isthmus — the narrow part joining the uterus

Uterus — shaped like an inverted pear, with three wall layers:

- Perimetrium — the thin outer membrane
- Myometrium — the thick middle layer of smooth muscle, which contracts strongly during childbirth
- Endometrium — the inner glandular lining, which changes during the menstrual cycle

Cervix and vagina. The uterus opens into the vagina through the narrow cervix; the cervical canal and vagina together form the birth canal.

An everyday example. Labour pains during childbirth are powerful contractions of the myometrium pushing the baby out.

The substance. Fertilisation happens in the oviduct, not the uterus — the uterus receives the embryo only afterwards.

The ovary is covered by a thin epithelium enclosing the ovarian stroma, which has an outer cortex holding follicles at different stages and an inner medulla; the mammary gland contains glandular tissue divided into lobes of milk-secreting alveoli, whose tubules and ducts join into lactiferous ducts that carry milk out.

Ovary:

- A thin epithelium covers the ovarian stroma
- Cortex — the outer zone, containing ovarian follicles at various stages of development
- Medulla — the inner zone, with blood vessels and connective tissue

Mammary gland — a pair of glands containing glandular tissue and a variable amount of fat:

- The glandular tissue is divided into 15 to 20 mammary lobes
- Each lobe contains clusters of cells called alveoli, which secrete milk into their cavities
- Alveoli open into mammary tubules, and the tubules of each lobe join into a mammary duct
- Several mammary ducts join into a wider mammary ampulla, connected to a lactiferous duct through which milk flows out

An everyday example. A bunch of grapes on branching stalks resembles a mammary lobe — the grapes are the alveoli and the stalks the tubules and ducts that carry milk.

The substance. Follicles lie in the cortex, not the medulla — the medulla carries the ovary's blood supply.