Question

What is the difference between Zona Pellucida and Corona Radiata?

Answer 1

Hint: Ovum or ootid (Latin. Ovum- egg) is the non-motile female gamete. The process of formation of functional haploid ova from the diploid germinal cells in the ovary or female gonad is called oogenesis. Early stages of oogenesis occur by the time the female foetus is only 25 weeks old. At this time all the oogonia have been formed and entered into the primary oocyte stage. Human ovum is rounded alecithal (without yolk) and non-cleidoic (without shell). Ovum is about 100 micrometers in diameter.

Complete answer:
Ovum is covered by its own membrane called plasmalemma or oolemma. Cytoplasm of ovum is called ooplasm. In the outer region ooplasm contains a number of mucopolysaccharide cortical granules. Cytoplasm contain mitochondria, microtubules, ribosomes, RNA, proteins, lipid globules, and glycogen particles. Nucleus is large. Nucleus is also called a germinal vesicle. Nucleus is generally in the metaphase II stage prior to fertilization. Typical nucleus or pronucleus appears after the entry of sperm.
Eggs have a polarity. In an egg, there is an animal pole and a vegetal pole. Outside the plasma membrane one or two small polar bodies occur near the area of the animal pole. The space where the polar bodies occur is known as perivitelline space.
Ovum has two coverings, zona pellucida and Corona Radiata.

Zona Pellucida	Corona Radiata
1) Zona Pellucida is non-cellular.	Corona Radiata is multicellular.
2) It is a thick glycoprotein covering which is secreted by joint activity of secondary oocytes and inner follicular cells.	It is a part of granulosa cells which remains around the ovum at the time of fertilization.
3) It surrounds the plasma membrane of the ovum.	It surrounds the zona Pellucida.
4) Zona Pellucida bear ingrowths of follicular cells for transfer of nutrients and biochemicals to the egg.	Corona Radiata is attached to the zona Pellucida and their main purpose is to supply vital proteins to the cell.
5) It carries three receptor proteins ZP1, ZP2, and ZP3 for determining attachment or non-attachment of sperm. It prevents polyspermy.	It protects the ovum.

Note:
Types of egg:
1. Alecithal: Egg without yolk, eg: human egg.
2. Microlecithal: Egg with small amount of yolk.
3. Mesolecithal: Egg with moderate amount of yolk.
4. Macrolecithal: Egg with large amount of yolk, e.g., bird and reptile eggs.
5. Isolecithal: Egg with uniform distribution of yolk.
6. Heterolecithal: Egg with uneven distribution of yolk.
7. Telolecithal: Egg with more yolk on one half.
8. Centrolecithal: Egg with yolk concentrated towards centre, eg insects.
9. Discoidal: yolk is present all around except for a protoplasmic disc, e.g., bird egg, reptilian egg.