Spermatogenesis and oogenesis

Why gametes need meiosis

A body cell of a human has 46 chromosomes. If a sperm and an ovum also had 46 each, the first cell of a baby would have 92. That cannot go on. So the gametes are made by meiosis, a special division that halves the number. A gamete has 23 chromosomes, and fertilisation brings the number back to 46.

The making of gametes is called gametogenesis. In the male it is spermatogenesis, and in the female it is oogenesis. Both happen in the gonads, the testes and the ovaries.

Spermatogenesis

Spermatogenesis begins at puberty and goes on through the life of a man. It happens in the wall of the seminiferous tubules.

1. Spermatogonia are the male germ cells in the wall. Each has 46 chromosomes. They divide by mitosis, and so their number grows.
2. Some spermatogonia grow and become primary spermatocytes, which still have 46 chromosomes.
3. A primary spermatocyte completes meiosis I, the reduction division. It makes two equal secondary spermatocytes, each with 23 chromosomes.
4. Each secondary spermatocyte completes meiosis II. Together they make four equal spermatids, each with 23 chromosomes.
5. In spermiogenesis, each spermatid changes into a sperm. It grows a tail and forms the acrosome.
6. The sperm heads are for a while embedded in the Sertoli cells. Then they are set free into the space of the tubule. This release is spermiation.

So one primary spermatocyte gives four sperms.

The hormones of the male

At puberty the hypothalamus secretes more GnRH (gonadotropin releasing hormone). GnRH acts on the anterior pituitary, which then secretes two gonadotropins, LH and FSH.

• LH acts on the Leydig cells. They make androgens, which stimulate spermatogenesis and the male features.
• FSH acts on the Sertoli cells. They secrete factors that help spermiogenesis.

This part is not in the NEET syllabus, but A-Level and IB ask it. The androgens lower the secretion of GnRH, LH and FSH. A hormone of the Sertoli cells, inhibin, lowers the secretion of FSH. This is negative feedback, and it keeps the level of the hormones steady.

Oogenesis

Oogenesis begins before the girl is born. It is a slow process with long pauses.

1. In the ovary of the foetus, millions of oogonia are formed. After birth no more are added.
2. The oogonia start meiosis I and stop in prophase I. They are now primary oocytes.
3. Each primary oocyte is surrounded by a layer of granulosa cells. It is now a primary follicle.
4. Many follicles degenerate between birth and puberty. NCERT gives 60000 to 80000 primary follicles left in each ovary at puberty. Other books give a higher number, about 300000 to 400000, but all agree that only a few hundred will ever release an ovum.
5. A primary follicle grows into a secondary follicle and then a tertiary follicle with the antrum.
6. The primary oocyte of a tertiary follicle completes meiosis I. The division is unequal. It forms a large secondary oocyte with 23 chromosomes and a tiny first polar body. The secondary oocyte keeps almost all of the food and the cytoplasm.
7. The follicle matures into the Graafian follicle. The secondary oocyte gets its zona pellucida. At ovulation the follicle bursts and the secondary oocyte leaves the ovary.
8. Meiosis II is finished only if a sperm enters. The oocyte divides again, unequally, into a large ovum and a second polar body.

Many textbooks add that the released secondary oocyte has stopped at metaphase of meiosis II, and that it waits there for a sperm. The polar bodies have no further use. They are small cells that carry away the extra chromosomes. The first polar body may divide too, so a primary oocyte gives one ovum and up to three polar bodies.

Comparing the two

• Begins. Spermatogenesis at puberty. Oogenesis before birth.
• Place. Spermatogenesis in the seminiferous tubules. Oogenesis in the follicles of the ovary.
• Divisions. Spermatogenesis has equal divisions. Oogenesis has unequal divisions.
• Cells made from one primary cell. Four sperms. One ovum and up to three polar bodies.
• Pauses. None in spermatogenesis. Oogenesis pauses at prophase I, and again at meiosis II until fertilisation.
• Time of making. Sperms are made through life. One ovum is released each month until menopause.

You should find that the same number of starting cells gives four times as many sperms as ova. A primary oocyte gives one ovum, and its other products are polar bodies.

Try it in Python

The program counts the cells made from five primary spermatocytes and five primary oocytes.

Practise with new numbers

Common mistakes

• Saying that a primary spermatocyte makes two sperms. Two divisions give four spermatids, and each becomes a sperm.
• Saying that a primary oocyte makes four ova. It makes one ovum. The other cells are polar bodies.
• Saying that the primary oocyte is haploid. It has 46 chromosomes. The secondary oocyte has 23.
• Mixing spermiogenesis and spermiation. Spermiogenesis changes a spermatid into a sperm. Spermiation sets the sperm free.
• Saying that LH acts on the Sertoli cells. LH acts on the Leydig cells. FSH acts on the Sertoli cells.

Remember these

• Spermatogonia (46) divide by mitosis. A primary spermatocyte (46) gives two secondary spermatocytes (23) and then four spermatids (23). Spermiogenesis makes the sperms. Spermiation sets them free.
• LH acts on the Leydig cells for androgens. FSH acts on the Sertoli cells.
• Oogenesis starts in the foetus. The primary oocyte stops in prophase I. A tertiary follicle completes meiosis I and gives a secondary oocyte and a first polar body.
• Meiosis II is completed only at fertilisation. It gives the ovum and the second polar body.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many sperms are formed from one primary spermatocyte? How many chromosomes does each have?

2. Which hormone acts on the Leydig cells, and which acts on the Sertoli cells?

3. At what stage does a primary oocyte stop before birth?

4. What two cells are made when a primary oocyte of a tertiary follicle completes meiosis I? How do they differ in size?

5. When is meiosis II of the oocyte completed?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 27, The Reproductive System.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapters 80 and 81.