Human Reproduction
Senior 6 · Biology
Human Reproduction
Lesson 2 of 5
30 min
From puberty onwards, a healthy man's testes make sperm cells continuously, many millions every day, for the rest of his life. A woman's ovaries work in a completely different way: every egg cell she will ever release began to form before she was born, and only about one is released each month between puberty and the menopause. Both processes use meiosis, which you studied in Senior 3, to halve the chromosome number. This lesson compares how the two kinds of gamete are made and explains why they differ so much.
Definition
Gametogenesis: the production of gametes. Spermatogenesis makes sperm cells in the testes; oogenesis makes egg cells in the ovaries.
Spermatogonia and oogonia: the diploid (2n) germ cells that divide by mitosis to produce cells that will enter meiosis.
Primary spermatocyte or primary oocyte: a diploid cell that undergoes meiosis I. Secondary spermatocyte or secondary oocyte: a haploid (n) cell produced by meiosis I.
Polar body: a small cell, with very little cytoplasm, produced by the unequal divisions of oogenesis; it does not become a gamete.
Sertoli cells: large cells in the wall of the seminiferous tubules that support and nourish developing sperm.
Sperm are made in the walls of the seminiferous tubules, several hundred metres of coiled tubes packed inside each testis. Spermatogonia near the outer wall divide by mitosis; some grow into primary spermatocytes, which undergo meiosis I to give two secondary spermatocytes and then meiosis II to give four spermatids. The spermatids lie embedded in Sertoli cells, which nourish them while they develop a tail and an acrosome and lose most of their cytoplasm, becoming spermatozoa. The whole process moves inwards, towards the centre (lumen) of the tubule, and takes roughly two months. FSH stimulates the Sertoli cells, and LH stimulates the interstitial (Leydig) cells between the tubules to secrete testosterone, which is also needed for spermatogenesis.
The released secondary oocyte is surrounded by a glycoprotein layer, the zona pellucida, and by follicle cells (the corona radiata). Its cytoplasm contains food reserves and cortical granules, which act at fertilisation (lesson 3).
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Where | Seminiferous tubules of the testes | Follicles of the ovaries |
| When it starts | At puberty | Before birth (completed only after puberty) |
| Continuity | Continuous, throughout adult life | Cyclical, about one oocyte a month, until the menopause |
| Gametes from one primary cell | Four sperm | One egg (plus polar bodies) |
| Cytoplasm divided | Equally | Unequally: almost all goes to the oocyte |
| Meiosis completed | Before the gamete is released | Meiosis II only after a sperm enters |
| Size of gamete | Very small (head about 5 µm), able to swim | Very large (about 0.1 mm), cannot move by itself |
Worked example 1: counting gametes
(a) How many sperm can be made from 250 primary spermatocytes? (b) How many ova can be made from 250 primary oocytes, and how many polar bodies would form if every first polar body also divided?
Why the difference? Dividing the cytoplasm unequally gives the single ovum almost all the food reserves, organelles and mRNA, which the zygote needs for its first divisions before it implants. Sperm only need to deliver a nucleus, so making many small cells is the better strategy.
Worked example 2: how fast are sperm made?
Illustrative value: an adult man's testes produce about sperm per day. Calculate the rate per second.
Compare this with oogenesis: about 400 secondary oocytes are released in a whole reproductive life, roughly one a month for about 35 years.
Warning
Common mistakes
Dr Matt & Dr Mike: "Oogenesis and Spermatogenesis". Watch for the cells at each stage and where meiosis pauses in oogenesis.
Practice: gametogenesis
Key points
Sources and credits
Figures 1 and 2, the table, the worked examples (with illustrative values) and the practice questions were written by Ganzaa.
