Gametogenesis, puberty and the menstrual cycle

Puberty

Puberty is the stage at which the reproductive organs mature and a person becomes able to reproduce. It is triggered by hormones from the hypothalamus and pituitary gland, which stimulate the testes to secrete testosterone and the ovaries to secrete oestrogen.

Males — sperm production starts; voice deepens; facial and body hair grow; muscles develop.
Females — ovulation and menstruation start; breasts develop; hips widen; body hair grows.

Spermatogenesis

Sperm are made in the seminiferous tubules of the testes, from puberty onwards, under the influence of testosterone and FSH.

Diploid cells of the germinal epithelium divide by mitosis.
Some undergo meiosis to form four haploid cells.
These mature into sperm cells.

Structure of a sperm: the acrosome contains enzymes that digest a path into the ovum; the head contains the haploid nucleus; the middle piece has many mitochondria that release energy; the tail propels the sperm.

Oogenesis

Before birth, diploid cells in the ovary divide to form many potential egg cells, which start meiosis and pause.
From puberty, each month FSH stimulates one follicle (a Graafian follicle) to develop.
Meiosis continues to form a haploid secondary oocyte, which is released at ovulation.
Meiosis is completed only if a sperm fertilises the oocyte.

	Spermatogenesis	Oogenesis
Where	Seminiferous tubules (testes)	Ovaries
When	Continuously from puberty	Starts before birth; one ovum a month from puberty to menopause
Gametes from one cell	Four sperm	One ovum (other cells are small polar bodies)

Hormonal control of the menstrual cycle

Hormone	Made by	Effect
FSH	Pituitary gland	Stimulates a follicle to develop; follicle secretes oestrogen
Oestrogen	Developing follicle	Thickens the endometrium; high levels inhibit FSH and trigger the LH surge
LH	Pituitary gland	Surge causes ovulation (about day 14) and turns the empty follicle into the corpus luteum
Progesterone	Corpus luteum	Maintains the thick endometrium; inhibits FSH and LH

If the ovum is not fertilised, the corpus luteum degenerates, progesterone falls, the endometrium breaks down (menstruation) and, with the inhibition removed, FSH rises again to start a new cycle. If fertilisation occurs, the corpus luteum (and later the placenta) keeps progesterone high, so menstruation stops.

Worked example: reading the hormone graph

✏️ Interpreting a graph of hormone levels over 28 days

1A sharp peak of LH just before day 14 shows when ovulation happens.

2Oestrogen rises in the first half of the cycle as the follicle grows, peaking before the LH surge.

3Progesterone is highest in the second half (days 15–25) while the corpus luteum is active.

4If progesterone stays high after day 28, the woman is probably pregnant.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (4 marks)

Describe the process of spermatogenesis.

Show memo

Takes place in the seminiferous tubules of the testes ✓, stimulated by testosterone (and FSH) ✓. Diploid cells of the germinal epithelium undergo meiosis ✓ to form haploid cells that mature into sperm ✓.

Question 2 · (4 marks)

Label and give the function of FOUR parts of a sperm cell.

Show memo

Acrosome — enzymes that digest a path into the ovum ✓
Head/nucleus — carries the haploid set of chromosomes ✓
Middle piece — mitochondria release energy for movement ✓
Tail — propels the sperm ✓

Question 3 · (3 marks)

State THREE differences between spermatogenesis and oogenesis.

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Any THREE: site (testes vs ovaries) ✓; timing (continuous from puberty vs begins before birth, one a month) ✓; number of gametes per cell (four sperm vs one ovum) ✓; size of gametes (small, motile vs large, non-motile).

Question 4 · (5 marks)

Explain how the levels of progesterone and FSH change if fertilisation does NOT take place, and the effect on the endometrium.

Show memo

The corpus luteum degenerates ✓, so progesterone levels fall ✓. The endometrium is no longer maintained and breaks down (menstruation) ✓. Low progesterone no longer inhibits the pituitary ✓, so FSH rises and a new follicle starts to develop ✓.