Oogenesis Flashcards
At what stage of meiosis are oocytes arrested at before birth? When does this last until?
The diplotene stage of Prophase 1. Lasts until puberty
Is oestrogen dominant in the first or second half of the menstrual cycle?
First
Why is there not a need for such a large mitotic proliferation in the ovary compared to the testis?
Only one/two oocytes are shed each cycle compared to the many millions of sperm.
What are the 5 stages of oocyte development?
- Primordial follicle
- Primary Follicle
- Secondary follicle
- Preantral follicle
- Antral/Graffian follicle
How long does the process of oocyte development take in humans?
6 months
List three ways in which the egg is helped to move down the fallopian tube
- Oviductal fluid for the ciliated secretory epithelium resulting in the flow of fluid towards the uterus. These secretions also nourish the developing embryo.
- The cells lining the oviduct have cilia that beat in a single direction ensuring the flow of fluid is unidirectional towards the uterus.
- The wall of the oviduct is muscular and contractions propel the egg towards the uterus.
When is the zona pellucida formed?
During follicular development it is produced by the oocyte/
What is the zona pellucida made up of?
4 different glycoproteins
What are the cellular connections between the zona pellucida and the oocyte called?
Transzonal processes
What are the two main functions of the zona pellucida?
Protect the oocyte during and after ovulation (mice without a zona are unable to ovulate fertile eggs)
Prevent polyspermy
What hormone is secreted by the granulosa cells?
Estradiol
What happens to the stromal cells in the ovary as the follicles develop into secondary follicles?
They align themselves around each follicle and differentiate into theca cells which synthesize and secrete androgens.
What marks the change in status of the follicle from gonadotrophin independant to gonadotrophin dependant?
The formation of the antrum (development to an antral follicle) consisting of follicular fluid.
Is the granulosa layer vascular or avascular?
Avascular
When do FSH and LH receptors first appear on follicles?
Late preantral/ early antral follicle stage
What cells bind LH?
Only cells in the theca interna are able to bind LH
What cells bind FSH?
Only granulosa cells are able to bind FSH
Describe the production of steroid hormones by antral follicles?
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)
What happens to the antral follicle if there is no LH surge?
It will die
Describe what happens within 3 - 12 hours after the LH surge
- The nuclear membrance surrounding the chromosomes breaks down and the arrested meiotic prophase is ended.
- Chromosomes pass through meiosis but almost all cytoplasm goes to one cell.
- 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.
- The chromosomes in the secondary oocyte immediately enter the second meiotic division and come to lie on the second metaphase spindle.
- 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.
- 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
- 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.
