Question

In human female reproductive system state the changes that take place once fertilisation has taken place. Write the role of placenta in this process. What happens when the egg is not fertilised ?

Answer 1

Changes After Fertilization in the Human Female Reproductive System

1. Zygote Formation

Fertilization (fusion of sperm and egg) usually occurs in the fallopian tube, resulting in a single-celled zygote.

2. Cleavage and Morula Formation

The zygote undergoes rapid mitotic divisions (cleavage) and becomes a solid ball of cells called a morula as it moves toward the uterus.

3. Blastocyst Formation

The morula develops into a blastocyst — a hollow ball with an inner cell mass (which becomes the embryo) and an outer layer called the trophoblast.

4. Implantation

The blastocyst embeds itself into the uterine wall (endometrium) about 6–12 days after fertilization.

5. Embryonic Development and Placenta Formation

The inner cell mass forms the embryo. The trophoblast and uterine tissue form the placenta.

6. Hormonal Changes

• The embryo and placenta produce hCG (human chorionic gonadotropin).
• hCG maintains the corpus luteum in the ovary.
• The corpus luteum secretes progesterone to prevent menstruation and maintain the pregnancy.

Role of the Placenta

• Nutrition: Transfers nutrients from mother to fetus.
• Respiration: Allows oxygen exchange to the fetus and removes CO₂.
• Excretion: Removes fetal waste products like urea to the mother’s blood.
• Endocrine Function: Secretes hCG, progesterone, estrogen, and hPL.
• Barrier Function: Protects the fetus from some pathogens and harmful substances.

What Happens When the Egg is Not Fertilized?

• The egg disintegrates within 24–48 hours.
• The corpus luteum degenerates.
• Progesterone and estrogen levels fall.
• The uterine lining (endometrium) is not maintained.
• The endometrium breaks down and is shed — menstruation.
• A new menstrual cycle begins.

Summary

Embryonic Stages

Zygote → Morula → Blastocyst → Implanted Embryo

Placenta Roles

• Nutrient and waste exchange
• Secretion of hCG and progesterone
• Acts as a maternal-fetal barrier

Without Fertilization

• Egg degenerates
• Corpus luteum regresses → Progesterone decreases
• Endometrial shedding (menstruation)