Question

Translucent outer covering of egg is

• A. Zona pellucida
• B. Zona radiata
• C. Corona radiata
• D. None of the above

Answer 1

Answer: (A)

Zona pellucida

In females at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and called as secondary follicles. The secondary follicle soon transforms into a tertiary follicle which is characterized by a fluid filled cavity, called as antrum. The theca layer is organised into an inner theca interna and an outer theca externa. It is at this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte. The tertiary follicle further changes into the mature follicle or Graafian follicle. The secondary oocyte forms a new membrane called as zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called as ovulation.