Reproduction in Humans
Senior 3 · Biology
Reproduction in Humans
Lesson 2 of 5
25 min
The numbers in human reproduction are startling. Estimates suggest that a healthy adult man makes more than a thousand sperm every second, every day of his adult life. A woman, in contrast, is born with all the immature eggs she will ever have, typically one to two million, and over her whole life only about 400 or 500 of them will be released. Both kinds of gamete are made by the same type of cell division, meiosis (Unit 5), so why are the numbers so different? The answer lies in how each gamete is made, and in the hormones that start the whole process at puberty.
Definition
Gametogenesis: the making of gametes. Spermatogenesis makes sperm in the testes; oogenesis makes eggs (ova) in the ovaries.
Haploid (n): having one set of chromosomes, 23 in humans, as in gametes. Diploid (2n): having two sets, 46 in humans, as in body cells.
Polar body: a tiny cell, with very little cytoplasm, made by the unequal divisions of oogenesis; it breaks down.
Secondary sexual characteristics: the features that develop at puberty under the control of sex hormones, other than the reproductive organs themselves, such as a deeper voice in boys or the growth of breasts in girls.
Figure 1: simplified; 2n = diploid (46 chromosomes in humans), n = haploid (23). The polar bodies are tiny and break down.
Inside each testis are hundreds of long, coiled seminiferous tubules. Germ cells (2n) in their walls divide by mitosis, and some grow into primary spermatocytes (2n). Each undergoes meiosis: the first division gives two cells (n), and the second gives four spermatids (n), which develop a tail and mature into four sperm. Spermatogenesis begins at puberty and continues throughout adult life.
Before a girl is born, germ cells in her ovaries multiply by mitosis and grow into primary oocytes (2n), which begin meiosis and then pause. From puberty, about once a month, one primary oocyte (usually) completes the first division of meiosis. The division is unequal: one large secondary oocyte (n) gets almost all the cytoplasm, and a tiny first polar body gets almost none. The secondary oocyte is what is released at ovulation. It completes the second division only if a sperm enters it, giving one large ovum and a second polar body.
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Where | Seminiferous tubules of the testes | Ovaries |
| When | From puberty, continuously, throughout life | Begins before birth; one cell (usually) each month from puberty to menopause |
| Gametes from one cell that enters meiosis | Four sperm | One ovum (and polar bodies) |
| Division of cytoplasm | Equal | Unequal: the ovum keeps most of it as a food store |
| Size of gamete | Tiny and mobile | Large, with stored food; cannot move by itself |
At puberty, the hypothalamus makes the pituitary gland release more FSH (follicle-stimulating hormone) and LH (luteinising hormone). These act on the gonads: the testes start making sperm and testosterone; the ovaries start releasing eggs and making oestrogen and progesterone. Puberty typically begins between about 8 and 13 years in girls and 9 and 14 in boys; every body has its own timing, and earlier or later is usually normal.
| Boys (testosterone) | Girls (oestrogen) |
|---|---|
| Voice deepens as the larynx grows | Breasts develop |
| Hair grows on the face, chest, under the arms and in the pubic area | Hair grows under the arms and in the pubic area |
| Muscles develop; shoulders broaden | Hips widen |
| Testes and penis grow; sperm production starts | Menstruation (periods) starts |
| Growth spurt | Growth spurt, usually earlier than in boys |
Worked example 1: counting gametes and chromosomes
Worked example 2: how many eggs in a lifetime?
A woman's periods start at 13 and stop at 50. Assume a typical 28-day cycle with one egg released each cycle, and ignore pregnancies.
Warning
Common mistakes. (1) Saying gametes are made by mitosis: the final divisions are meiosis, which halves the chromosome number. (2) Writing that oogenesis gives four eggs: it gives one ovum; the other products are polar bodies. (3) Thinking puberty must happen at one exact age: the range is wide, and a health worker can advise anyone who is worried.
Spermatogenesis and Oogenesis in 2 minutes!, Dr Matt and Dr Mike. Watch for the four sperm and the one ovum, and compare them with Figure 1. The video uses a few extra terms that you do not need for the examination.
Tip
Memory hook: "sperm share, eggs hoard". Spermatogenesis shares the cytoplasm equally among four cells; oogenesis hoards it in one large ovum.
Practice: gametogenesis and puberty
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the tables, the worked examples (typical figures) and the practice questions.
