Question

Zona pellucida disintegrates just
(a) Before fertilization
(b) After fertilization
(c) Midway during cleavage
(d) After completion of cleavage

Answer 1

The ovum released from the ovary is not fully mature and is called a secondary oocyte. The secondary oocyte is covered by Zona pellucida. It is secreted by the oocyte, and appears between the follicular cells and oocyte. It's a homogeneous membrane.

Complete answer:
Zona pellucida disintegrates just after the completion of cleavage. This remains intact throughout the cleavage divisions which means the development of blastocyst creates pressure on growing blastocyst and leads to the formation of the slit in zona pellucida. The zona pellucida is dissolved by the enzyme acrosin.
-When sperm comes in contact with the oocyte, the plasma membrane of the two cells fuse. The head of the spermatozoon enters the cytoplasm of the ovum.
-Alterations take place in the plasma membrane of the oocyte and zona pellucida ensures that no other spermatozoon can enter the oocyte.
The fate of the oocyte is followed by the fusion of sperm is described as below-
-The fertilized ovum undergoes cleavage.
-The two daughter cells are surrounded by Zona pellucida.
-Two days after fertilization the embryo is at the two-celled stage.
-Three days after fertilization the embryo becomes Morula.
- On the 4th day, Blastocyst forms.
-On the 7th-day implantation of the blastocyst in the endometrium occurs.

So, the correct option is “After completion of cleavage.”

Note:
-The zona pellucida, during oogenesis supports communication between oocytes and follicle cells.
-The main function of zona pellucida is to prevent implantation of the blastocyst at an abnormal site.
-Therefore, Zona pellucida disappears soon after the blastocyst reaches the uterine lining.
-Each daughter cell is smaller than the size of the ovum.
-The duration of development of various organs in the fetus differs.
-The fetus’s heart is formed after the 1st month of pregnancy (4th week) while limbs and digits are formed in the 8th week.
-Most of the fetus major organs and systems are developed by the end of the 12th week (1st trimester) of pregnancy.