Human reproductive systems

MDCAT Biology · Biology · Reproduction. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The testes make sperm in the seminiferous tubules and the hormone testosterone in the interstitial cells. Sperm mature and are stored in the epididymis, then travel along the vas deferens to the urethra. The seminal vesicles, prostate gland and Cowper's glands add the fluid of semen.

The ovaries make ova and the hormones oestrogen and progesterone. The egg is swept along the oviduct (Fallopian tube) to the uterus, whose lining is the endometrium.

Spermatogenesis and oogenesis both use mitosis, growth and then meiosis to make haploid gametes. They differ in timing and in the number of gametes made.

Rules to remember

Spermatogonium (2n) → primary spermatocyte (2n) → 2 secondary spermatocytes (n) → 4 spermatids (n) → 4 sperm.
Oogonium (2n) → primary oocyte (2n) → secondary oocyte (n) + first polar body → ovum (n) + second polar body.
One primary spermatocyte gives 4 sperm; one primary oocyte gives 1 ovum.
Primary oocytes are held at prophase I from before birth; meiosis II is completed only after a sperm enters.
Spermatogenesis begins at puberty and is continuous; oogenesis begins before birth and ends at menopause.
In the male, FSH promotes spermatogenesis; LH stimulates the interstitial cells to secrete testosterone.
Seminal vesicles secrete fructose, the energy source of sperm.

Common mistake

The cell released at ovulation is a secondary oocyte, not a finished ovum. It completes meiosis II only if a sperm enters it.

Worked example

Human body cells have 46 chromosomes. How many chromosomes are in an oogonium, a primary oocyte, a secondary oocyte and a first polar body?

An oogonium is formed by mitosis, so it is diploid: 46.
A primary oocyte has grown but has not yet finished meiosis I, so it is still diploid: 46.
Meiosis I halves the number, so the secondary oocyte and the first polar body are both haploid: 23 each.

Answer: Oogonium 46, primary oocyte 46, secondary oocyte 23, first polar body 23.

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