Question

(b) Explain the changes that the primary oocyte undergoes while in different follicular stages before ovulation.

Hint:

The primary oocyte stays arrested in prophase I, completes meiosis I in the tertiary follicle and is released as a secondary oocyte.

Answer 1

Step 1: Before birth:
During foetal life the oogonia divide and enter meiosis I as primary oocytes.
They stop at prophase I and stay arrested there.
Each primary oocyte is surrounded by a single layer of granulosa cells. This is a primary follicle.

Step 2: From puberty: primary to secondary follicle:
At puberty only about 60,000 to 80,000 primary follicles remain in each ovary.
The primary follicles are surrounded by more layers of granulosa cells and a new theca layer. They become secondary follicles.
The primary oocyte grows in size during this period but is still arrested in prophase I.

Step 3: Tertiary follicle:
The secondary follicle changes into a tertiary follicle, which has a fluid-filled cavity called the antrum.
The theca layer forms an inner theca interna and an outer theca externa.
Just before ovulation, the primary oocyte inside completes meiosis I. It divides unequally.
This gives a large secondary oocyte and a tiny first polar body.

Step 4: Graafian follicle and ovulation:
The tertiary follicle matures into the Graafian follicle.
The secondary oocyte forms a membrane called zona pellucida around itself.
Under LH surge, the Graafian follicle ruptures and the secondary oocyte is released. This is ovulation.
The secondary oocyte is now stopped at metaphase II until a sperm enters.

Final Answer:
Primary oocyte: arrested at prophase I (primary follicle), grows in secondary follicle, completes meiosis I in the tertiary follicle to give a secondary oocyte and a polar body, then is released from the Graafian follicle.