What is true about cleavage in fertilized egg in humans?

Options

A. It is meroblastic
B. It is identical to normal mitosis
C. It starts when the egg reaches uterus
D. It starts while the egg is in fallopian tube

Answer

### Step-by-Step Solution:

1. **Understanding Cleavage**: Cleavage refers to the series of rapid cell divisions that occur after fertilization, transforming the zygote into a multicellular structure.

2. **Formation of Zygote**: The process begins with the fusion of a male gamete (sperm) and a female gamete (ovum), resulting in the formation of a diploid zygote.

3. **Type of Cleavage in Humans**: In humans, the type of cleavage that occurs is called "holoblastic cleavage." This means that the entire zygote divides completely, which is typical for eggs with little yolk.

4. **Yolk Content**: Human eggs are classified as "alecithal," meaning they contain very little or negligible yolk. This is important because the type of cleavage is influenced by the amount of yolk present in the egg.

5. **Location of Cleavage**: Cleavage does not begin when the zygote reaches the uterus. Instead, it starts while the fertilized egg is still in the fallopian tube. This is crucial because the zygote must undergo division before it can implant into the uterine wall.

6. **Mitosis Comparison**: While cleavage is a form of cell division, it is not identical to normal mitosis. In mitosis, there is a maintenance of the nucleocytoplasmic ratio, which is not the case during cleavage. Cleavage involves rapid divisions without this constant ratio.

7. **Conclusion**: The correct statement regarding cleavage in the fertilized egg in humans is that it begins while the egg is in the fallopian tube.