Question

Cleavage in the fertilized egg of humans ________
A) Starts in uterus
B) Is meroblastic
C) Starts when egg is in fallopian tube
D) Is discoidal

Answer 1

Hint: Cleavage is the process of rapid division of cells in an early embryo. Cleavage in an embryo leads to formation of a cluster of cells.

Complete answer:
When a sperm enters the ovum (secondary oocyte), it triggers the completion of secondary meiotic division. Then the oocyte divides to produce an ovum and a secondary polar body. The chromosomes of ovum and sperm mix together to restore the normal number of human chromosomes (23 pairs-2n). This process is called fertilization.

Fertilization results in the formation of a diploid zygote nucleus containing both the maternal and paternal chromosomes. The zygote then undergoes repeated cell divisions called cleavage. There are stages: 2 celled stage, 4 celled stage, 8 celled stage and 16 celled stage.

Option A: By the time this cluster of cells reaches the uterus, it has transformed into blastocyst. The early embryo when present in the upper region of the uterus has more than 16 cells. The blastocyst which is formed now gets implanted into the wall of the uterus. Thus cleavage does occur in the uterus. Therefore, this is the incorrect option.

Option B: Meroblastic cleavage occurs in the presence of a large amount of yolk in the fertilized egg cell. This type of cleavage is partial.This type of cleavage is mainly found in birds, amphibians, reptiles, and some fishes. Thus cleavage in human embryos is not meroblastic. Therefore, this is the incorrect option.

Option C: Fertilization occurs in the ampulla region of the fallopian tubes. Cleavage starts in the zygote almost immediately. So the cleavage in the fertilized egg of humans starts in the fallopian tube. Therefore, this is the correct option.

Option D: Discoidal cleavage is a type of meroblastic cleavage. As the type of cleavage occurring in human embryos is not meroblastic, so it's not discoidal as well. Therefore, this is the incorrect option.

Thus, the correct answer is option ‘C’ i.e, Starts when egg is in fallopian tube

Note: The type of cleavage occurring in human embryos is a holoblastic type of cleavage. It is commonly seen in eggs containing moderate to sparse amounts of yolk. This type of cleavage is equal cleavage. It is mainly found in amphibians, mammals, annelids, flatworms, and nematodes.