Question

Which is the correct sequence of egg layers from outside to inside?
A. Radiata, Proliferous and Pellucida
B. Proliferous, radiata and Pellucida
C. Proliferous, Yolk Sac and Radiata
D. None of the above

Answer 1

Hint: After ovulation the mammalian ovum three-layer forms around the ovum that is corona radiata, zona pellucida, proliferous, and plasma membrane. These layers give protection to the ovum and other numerous favorable conditions to the ovum.

Complete Answer:
- In this specific question, we can discuss the 'Proliferous, Radiata and Pellucida' which is the correct sequence of egg layers from outside to the inside. The ovum is non-motile and rounded cells which are enclosed by two egg coats one is inner zona pellucida and another one is outer thick corona radiata.
- Corona radiata is the site where sperms penetrate the eggs. It is made up of cumulus oophorus as well as radially extended follicular cells which are a secondary egg membrane. It is formed when the granulosa cells extend and become cuboidal.
- Note that the zona pellucida is concealed by the ovum itself, and forms a primary egg membrane. It is made up of three different glycoproteins and proteins. It is the extracellular membrane that surrounds the oocyte. This layer induces several morphological and physiological alterations in the membrane by which it blocks the penetration of additional sperms.
- It also covers the plasma membrane which is an innermost layer of the ovum. Remember that perivitelline space is the space found between the zona pellucida and the cell membrane of the ovum. The secretion of acrosome takes place in the cytoplasm through the zona pellucida and plasma membrane. This process induces the completion of the meiotic division of the secondary oocyte.
- The formation of a second polar body and a haploid ovum (ootid) is a result of the second meiotic division that is unequal. The haploid nucleus of the sperm and ovum fuses together to give rise to a diploid zygote.
- According to this explanation, we can say that the correct sequence of our egg layers from outside to inside is ‘Proliferous, corona radiata, zona pellucida and then plasma membrane’.

So that the correct answer is option ‘B’.

Note: Corona radiata, zona pellucida, and plasma membrane are the correct sequence of the mammalian egg layers. They all have different functions accordingly and facilitate the fusion of sperm and ovum or the process of fertilization.