St A - Oogenesis and Follicular Development

Question 1

Describe the first phase of oogensis

Answer 1

• Primordial germ cells (PGC) develop in endodermal endothelium of the embryonic yolk sac. At around week 5 the PGCs migrate to gonadal ridges and colonise the cortex whilst undergoing mitosis. In males medulla develops and cortex regresses, in females the cortex develops and medulla regresses

Question 2

Describe mitosis and meiosis

Answer 2

Mitosis - Process of cell division that results in two genetically identical daughter cells.
Meiosis - Division of germ cell involving two fissions of the nucleus resulting in 4 gametes

Question 3

Describe the second phase of Oogenesis

Answer 3

Occurs at ovulation;

• Luteinizing hormone and follicle stimulating hormone control enlargement and maturation of oocytes, differentiation and proliferation of gransulosa and theca cells, and formation and accumulation of fluid.
• Meiosis resumes, the first division completes.
• Meiosis arrests again and the secondary oocyte is ovulated. 2nd division occurs when fertilised.

Question 4

Describe follicular development

Answer 4

1) Oocyte covered in single cell layer,
2) Development of granulosa cells,
3) Development of theca and stroma cells,
4) Formation of follicular antrum,
5) Follicular antrum filled with follicular fluid
6) Follicle ruptures to release oocyte, formation of corpus haemorrhagicum.
7) Corpus luteum forms,
8) Corpus luteum becomes corpus albicans

Question 5

Describe features of small follicles

Answer 5

• Located in ovarian cortex, in the non-growing resting pool which stockpiles.
• They secrete anti-Mullerian hormone (AMH) and therefore the level of hormone reflects ovarian follicular reserves.

Question 6

Describe features of medium follicles

Answer 6

• Occurs when follicular cells (of the small oocyte) divide forming three layers around the oocyte. It takes 3 cycles to reach 3 layers of follicle cells

Question 7

Describe features of large follicles (secondary, antral, Graafian and preovulatory)

Answer 7

• FSH stimulates the rapid development of medium follicles over 14 days.
• Zona pellucida corms enclosing the oocyte and masking its antigens.
• Rapid mitotic division in follicular cells forms many layers.
• Antrum develops and fills with fluid.

Question 8

Describe the physiological process of ovulation

Answer 8

• LH surge (induces prostaglandin endoperoxide synthase in granulosa cells),
• FSH (stimulates release of plasminogen activator from granulosa cells),
• Prostaglandins E and F (release lysosomal enzymes to digest follicular wall)
• Stigma (forms on surface of follicle, balloons out, forms vesicle and ruptures, oocyte expelled)

Question 9

Describe the formation of the Corpus Luteum

Answer 9

The mature follicle discharges its antral fluid and egg. The granulosa cells and theca cells coalesce and undergo rapid transformation (LH regulated) This forms a gland-like endocrine structure called the corpus luteum

Question 10

What is the function of the corpus luteum.

Answer 10

It secretes;
- Oestrogen, progesterone and inhibin. This secretion leads to inhibition of FSH and LH release. The release of progesterone maintains luteal phase of ovarian cydle.

Question 11

If no egg is fertilised, what occurs to the corpus luteum?

Answer 11

It rapidly degenerates by apoptosis and forms the corpus albicans

Question 12

What are the two phases in the ovarian cycle?

Answer 12

follicular phase and luteal phase