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

Post-Fertilization Changes in the Human Female Reproductive System

1. Zygote Formation

Fertilization (sperm and egg fusion) generally occurs in the fallopian tube, forming a single-celled zygote.

2. Cleavage and Morula Formation

The zygote quickly divides (cleavage) into a solid ball of cells called a morula as it moves towards the uterus.

3. Blastocyst Formation

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

4. Implantation

The blastocyst implants in the uterine wall (endometrium) roughly 6–12 days after fertilization.

5. Embryonic Development and Placenta Formation

The inner cell mass forms the embryo. The trophoblast and uterine tissue combine to 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 sustain the pregnancy.

Placenta Function

• Nutrition: Provides nutrients from mother to fetus.
• Respiration: Facilitates oxygen exchange to the fetus and removes CO₂.
• Excretion: Removes fetal waste (e.g., urea) to the mother’s blood.
• Endocrine Function: Secretes hCG, progesterone, estrogen, and hPL.
• Barrier Function: Shields the fetus from certain pathogens and harmful substances.

What Happens if the Egg is Not Fertilized?

• The egg degrades within 24–48 hours.
• The corpus luteum degenerates.
• Progesterone and estrogen levels decrease.
• 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
• hCG and progesterone secretion
• Acts as a maternal-fetal barrier

Without Fertilization

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