The key difference between zona pellucida and corona radiata is that zona pellucida is a non-cellular secretory glycoprotein layer that surrounds the plasma membrane of mammalian ovum while corona radiata is a follicle cell layer that surrounds zona pellucida.
In fertilization, a mature ovum fuses with a sperm. Generally, fertilization takes place only between one sperm and an ovum. In order to prevent additional sperms from entering inside the cytoplasm of ovum, there are several protective layers surrounding the ovum. Zona pellucida and corona radiata are two such protective layers that surround and protect the mature ovum. Zona pellucida is a glycoprotein shell which is non-cellular while corona radiata is a cellular layer.
CONTENTS
1. Overview and Key Difference
2. What is Zona Pellucida
3. What is Corona Radiata
4. Similarities Between Zona Pellucida and Corona Radiata
5. Side by Side Comparison – Zona Pellucida vs Corona Radiata in Tabular Form
6. Summary
What is Zona Pellucida?
Zona pellucida is a glycoprotein layer that surrounds the plasma membrane of mammalian ovum. In fact, it is a protective glycoprotein coat or shell of the ovule. It is a non-cellular layer, unlike the corona radiata. It is kind of a thick extracellular matrix secreted from the oocyte and the follicle granulosa cells. There are four types of glycoproteins in zona pellucida as ZP1, ZP2, ZP3 and ZP4. They fulfil different roles in fertilization.
Figure 01: Zona Pellucida
Zona pellucida allows only one sperm to penetrate across it towards the cytoplasm of the ovum (monospermy). Therefore, it prevents the entering of additional sperms inside. In other words, zona pellucida prevents the condition called polyspermy. Once sperms contact the zona pellucida, they bind with the receptors of the zona pellucida. After that, they start the acrosomal reaction in which the enzyme filled caps or acrosomes of sperms initiate the degradation of zona pellucida and create a path for sperm to go near the plasma membrane of the ovum. Then a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane. Finally, that sperm fuses with the oocyte’s plasma membrane and becomes successful in fusing with the ovum nucleus.
What is Corona Radiata?
Corona radiata is a thick follicular cell layer that surrounds the zona pellucida of the ovum. It works as a protective cell barrier for a mature ovum, similar to zona pellucida. Corona radiata releases chemical attractants for sperms. Therefore, corona radiata attracts hundreds of sperms towards the ovum.
Figure 02: Coroma Radiata
Moreover, corona radiata supplies vital proteins to the ovum. It is formed from the follicle cells adhering to the oocyte.
What are the Similarities Between Zona Pellucida and Corona Radiata?
- Zona pellucida and corona radiata are two protective layers that surround a secondary oocyte or mature ovum.
- Both layers protect the ovum.
- Moreover, they prevent entering of additional sperms inside the cytoplasm of the ovum.
- Enzymes that are in the sperm acrosomes degrade both corona radiata and zona pellucida.
- Therefore, both zona pellucida and corona radiata undergo acromosomal reaction.
What is the Difference Between Zona Pellucida and Corona Radiata?
Zona pellucida is a relatively thick extracellular glycoprotein coat that surrounds a mature ovum. In contrast, corona radiata is a thick outer layer of granulosa cells that surrounds zona pellucida. So, this is the key difference between zona pellucida and corona radiata. Functionally, zona pellucida prevents the entry of additional sperms to the ovule while corona radiata protects the ovum and also provides proteins to the ovum. Therefore, this is another difference between zona pellucida and corona radiata.
Moreover, zona pellucida contains glycoproteins, and it is a non-cellular layer while corona radiata is a cellular layer. Thus, we can consider this too as a difference between zona pellucida and corona radiata.
Summary – Zona Pellucida vs Corona Radiata
Zona pellucida and corona radiata are two protective layers located around a mature ovum. Zona pellucida surrounds the plasma membrane of the ovum while corona radiata surrounds the zona pellucida and protects the ovum. Zona pellucida is a transparent, thick glycoprotein shell while corona radiata is a follicular cell layer. Both zona pellucida and corona radiata prevent entering of more than one sperm inside the cytoplasm of the ovum. This summarizes the difference between zona pellucida and corona radiata.