Fertilisation, cleavage and implantation

Two cells meet

A girl is born with all her primary oocytes, and a man makes sperms all his life. Yet a baby begins only when one sperm meets one ovum. This is fertilisation.

Before the meeting

During sexual intercourse the semen is placed in the vagina. This is insemination. The sperms swim through the cervix into the uterus and on to the ampulla of the oviduct. The ovum released at ovulation also moves to the ampulla.

Fertilisation can take place only if the ovum and the sperms reach the junction of the ampulla and the isthmus at about the same time. This is why not every sexual intercourse leads to pregnancy. A sperm can live in the female body for some days, but an ovum can be fertilised for only about a day after ovulation.

The steps of fertilisation

1. A sperm touches the zona pellucida of the secondary oocyte.
2. The acrosome of the sperm releases its enzymes. They help the sperm to move through the zona pellucida and the plasma membrane into the cytoplasm of the oocyte.
3. The entry of the sperm causes a change that blocks the entry of other sperms. So only one sperm can fertilise an ovum.
4. The sperm also makes the secondary oocyte finish meiosis II. This division is unequal. It makes the second polar body and a haploid ovum.
5. The haploid nucleus of the sperm and the haploid nucleus of the ovum fuse. The result is a diploid cell, the zygote, with 46 chromosomes.

The sex of the baby

The sex is decided at fertilisation. The ovum always carries an X chromosome. Half of the sperms carry X and half carry Y. If an X sperm fertilises the ovum, the zygote is XX and the baby is a girl. If a Y sperm fertilises it, the zygote is XY and the baby is a boy. So the sex depends on the father's sperm.

Cleavage

The zygote does not stay one cell. It begins to divide by mitosis while it is still in the oviduct. These divisions are called cleavage.

• The zygote moves along the isthmus towards the uterus. It divides into 2, 4, 8 and 16 cells. These cells are the blastomeres.
• Cleavage does not make the embryo larger. It only cuts the same volume into more and smaller cells.
• The embryo with 8 to 16 blastomeres is the morula, a solid ball of cells.
• As the morula moves into the uterus it continues to divide and forms the blastocyst.

The blastocyst has a fluid filled cavity. Its cells are arranged in two groups.

• An outer layer, the trophoblast.
• A small group of cells inside, attached to the trophoblast, the inner cell mass. It will develop into the embryo.

Many textbooks give about 5 days from fertilisation to the blastocyst. NCERT does not print these days.

Implantation

The trophoblast attaches to the endometrium, while the inner cell mass differentiates into the embryo. The cells of the uterus divide quickly and cover the blastocyst. The blastocyst becomes embedded in the endometrium. This is implantation. Implantation leads to pregnancy. Many textbooks say that it begins about 6 to 7 days after fertilisation.

You should find that the volume of all the cells stays the same, while the volume of one cell falls by half at each division.

Try it in Python

The program counts the cells and finds the volume of one cell after each cleavage.

The next program lists every way in which a sperm and an ovum can join, to find the chance of a girl.

Practise with new numbers

Common mistakes

• Saying that the embryo gets larger during cleavage. The cells become smaller, and the total volume stays the same.
• Saying that fertilisation takes place in the uterus. It takes place in the ampulla. The embryo reaches the uterus later.
• Saying that the sex depends on the ovum. The ovum always carries X. The sperm carries X or Y.
• Calling the morula a hollow ball. The morula is solid. The blastocyst is hollow.
• Mixing trophoblast and inner cell mass. The trophoblast attaches to the uterus. The inner cell mass forms the embryo.

Remember these

• Insemination, then the sperm reaches the ampulla. One sperm enters, other sperms are blocked, meiosis II is completed, and the nuclei fuse to give a zygote (46).
• Cleavage: 2, 4, 8, 16 blastomeres. Morula: 8 to 16 cells. Blastocyst: trophoblast and inner cell mass.
• Implantation: the trophoblast attaches to the endometrium and the blastocyst becomes embedded. This starts pregnancy.
• Sex: ovum X, sperm X or Y.

Check yourself

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

1. Where does fertilisation take place, and what does the acrosome do?

2. How is the entry of more than one sperm prevented?

3. What is the number of chromosomes in the zygote? How is it made?

4. What is a morula, and what is a blastocyst?

5. What is implantation?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 28, Development and Inheritance.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 82.