Oogenesis

Question 1

At what stage of meiosis are oocytes arrested at before birth? When does this last until?

Answer 1

The diplotene stage of Prophase 1. Lasts until puberty

Question 2

Is oestrogen dominant in the first or second half of the menstrual cycle?

Answer 2

First

Question 3

Why is there not a need for such a large mitotic proliferation in the ovary compared to the testis?

Answer 3

Only one/two oocytes are shed each cycle compared to the many millions of sperm.

Question 4

What are the 5 stages of oocyte development?

Answer 4

1. Primordial follicle
2. Primary Follicle
3. Secondary follicle
4. Preantral follicle
5. Antral/Graffian follicle

Question 5

How long does the process of oocyte development take in humans?

Answer 5

6 months

Question 6

List three ways in which the egg is helped to move down the fallopian tube

Answer 6

1. Oviductal fluid for the ciliated secretory epithelium resulting in the flow of fluid towards the uterus. These secretions also nourish the developing embryo.
2. The cells lining the oviduct have cilia that beat in a single direction ensuring the flow of fluid is unidirectional towards the uterus.
3. The wall of the oviduct is muscular and contractions propel the egg towards the uterus.

Question 7

When is the zona pellucida formed?

Answer 7

During follicular development it is produced by the oocyte/

Question 8

What is the zona pellucida made up of?

Answer 8

4 different glycoproteins

Question 9

What are the cellular connections between the zona pellucida and the oocyte called?

Answer 9

Transzonal processes

Question 10

What are the two main functions of the zona pellucida?

Answer 10

Protect the oocyte during and after ovulation (mice without a zona are unable to ovulate fertile eggs)
Prevent polyspermy

Question 11

What hormone is secreted by the granulosa cells?

Answer 11

Estradiol

Question 12

What happens to the stromal cells in the ovary as the follicles develop into secondary follicles?

Answer 12

They align themselves around each follicle and differentiate into theca cells which synthesize and secrete androgens.

Question 13

What marks the change in status of the follicle from gonadotrophin independant to gonadotrophin dependant?

Answer 13

The formation of the antrum (development to an antral follicle) consisting of follicular fluid.

Question 14

Is the granulosa layer vascular or avascular?

Answer 14

Avascular

Question 15

When do FSH and LH receptors first appear on follicles?

Answer 15

Late preantral/ early antral follicle stage

Question 16

What cells bind LH?

Answer 16

Only cells in the theca interna are able to bind LH

Question 17

What cells bind FSH?

Answer 17

Only granulosa cells are able to bind FSH

Question 18

Describe the production of steroid hormones by antral follicles?

Answer 18

They release increasing amounts of steroids as they grow under the influence of gonadotrophins. The main oestrogens produced are oestradiol and oestrone as well as androgens (antral follicles account for 30 - 70% of total circulating androgens in women)

Question 19

What happens to the antral follicle if there is no LH surge?

Answer 19

It will die

Question 20

Describe what happens within 3 - 12 hours after the LH surge

Answer 20

1. The nuclear membrance surrounding the chromosomes breaks down and the arrested meiotic prophase is ended.
2. Chromosomes pass through meiosis but almost all cytoplasm goes to one cell.
3. The remaining chromosomes are disgarded in a small bag of cytoplasm called the first polar body. The polar body is expelled to sit just outside of the oocyte.
4. The chromosomes in the secondary oocyte immediately enter the second meiotic division and come to lie on the second metaphase spindle.
5. Then meiosis arrests again and the oocyte is ovulated in this arrested metaphase state. The significance of this arrest is not fully understood but it is causes by a protein called cytostatic factor.
6. At this stage there is also cytoplasmic maturation of the oocyte and the intimate connection between the oocyte and granulosa cells is lost by withdrawal of the cytoplasmic processes which connect them. Centrioles are lost and only pericentriolar material organises the microtubules. This is different from spermiogenesis where OCM is lost and reduced centrioles remain. Remember LH does not bind to the oocyte itself! Its effect is mediated by the cumulus cells of the follicle
7. Within 2 hours of the LH surge there is a transient rise in the output of follicular oestrogens and androgens followed by a decline in these. This rise also coincides with distinctive changes in the thecal layer which stops dividing and converting oestrogens to androgens and also starts to synthesize progesterone. LH stimulates the synthesis of progesterone via the newly acquired LH receptors. Granulosa cells lose their capacity to bind oestrogen and FSH but gain the ability to bind progesterones.