Fertilisation and Embryonic Development

Process of Fertilisation

1. Fertilisation occurs when the nucleus of a sperm fuses with the nucleus of an egg to form a diploid zygote.
2. Location**:** Fertilisation takes place in the fallopian tube.
3. Steps in fertilisation:

• The sperm reaches the egg in the fallopian tube.
• Enzymes from the head (acrosome) of the sperm break down the membrane of the egg.
• The head of the sperm enters the egg.

4. After the sperm enters:

• A new membrane forms around the egg to prevent further sperm from entering.
• The egg nucleus fuses with the sperm nucleus, creating a diploid zygote.

Embryonic Development

1. Zygote Formation

• The zygote is the result of the fusion of the egg and sperm nuclei.

2. Morula

• The zygote divides rapidly by mitosis to form a solid ball of cells called a morula.

3. Blastocyst

• Around day 5, the morula becomes a hollow ball of cells called a blastocyst.
• The blastocyst embeds in the endometrium (uterine lining) around day 6–9. This process is called implantation.

4. Placenta Formation

• The outer cells of the blastocyst form the amnion, which produces amniotic fluid to protect the embryo.
• The placenta develops from the embryonic and uterine tissues.
• Functions of the placenta:
• Produces hormones.
• Allows the exchange of materials (e.g., nutrients, oxygen, and waste) between the mother and embryo.
• Prevents mixing of the mother's blood and the embryo's blood.

Embryonic Layers (Day 10 onwards)

The cells of the blastocyst give rise to three layers:

Layer	Develops Into
Ectoderm	Skin, nails, hair, and the nervous system.
Mesoderm	Muscles, skeleton, excretory, circulatory, and respiratory systems.
Endoderm	Liver, pancreas, and the inner lining of the digestive system.

Gestation

• Gestation period**:** The time the baby spends in the uterus.
• In humans, gestation lasts approximately 9 months from fertilisation to birth.

Birth and Hormonal Control

During pregnancy:

• The corpus luteum produces hormones (oestrogen and progesterone) for the first 10–12 weeks.
• After week 12, the placenta produces these hormones.
• Progesterone:
• Prevents uterine contractions.
• Inhibits FSH and LH, preventing new eggs from forming.

Before birth:

• The placenta stops producing progesterone, causing uterine contractions.
• The pituitary gland produces oxytocin, which increases the strength of uterine contractions, aiding birth.