Question

Cortical granules are associated with
A. Oogenesis
B. Spermatogenesis
C. Cleavage
D. Fertilization

Answer 1

The cortical granules are a type of secretory organelles that regulate the changes in the zona pellucida. These granules prevent polyspermy and are present in almost all mammals, vertebrates, and invertebrates. These are activated by a signalling pathway induced before zygote formation.

Complete answer: During the early stages of oocyte development, the formation of the cortical granules occurs once the ovarian follicle becomes multilayered. The Golgi complex enlarges and produces small vesicles which turn into cortical granules on reaching the periphery of the oocyte. A high level of carbohydrates and glycosylated molecules are enclosed in the cortical granules of mammals.
The cortical granules are a type of regulatory secretory type organelles that are present at the boundary or the cortical region of the oocytes. The oocyte is the final stage of female gamete development which undergoes fertilization. When fertilization occurs a signalling pathway induces the cortical granules to fuse with the cell membrane of the fertilized egg. The fertilized egg consists of perivitelline space, zona pellucida, and cumulus cells outwardly. Once the cortical granules are fused with the cell membrane, the exocytosis of granules occurs. Exocytosis of cortical granules releases their secretion into the outer three layers of the fertilized egg. This induces a change in the zona pellucida layer. This modification process of zona pellucida is known as zona reaction. The modifications in zona pellucida block other sperms to enter the egg. Thus, it acts as a block to polyspermy.
Therefore, option D is the correct answer.

Note: Another functional aspect of cortical granules is the protection and stability of the fertilized egg until it gets implanted in the uterus wall. Thus, it provides protection to developing embryos. Once the function of cortical granules is completed, the egg does not replenish them and hence no new cortical granules are made after fertilization.