Early Follicle Development Flashcards
Describe early follicle development.
1) . Pod of primordial follicles is essentially quiescent. There is very little atresia.
2) . Follicles become committed and there is no turning back. There is a very low rate of atresia.
3) . The Antrum forms. The follicle becomes gonadotrophin-responsive. Development may continue in the absence of FSH and LH but gonadotrophins can influence development. Some atresia.
4) . Gonadotrophin-dependent follicles. Will become atretic if FSH falls below 1ng/ml. High rate of atresia.
5) . Ovulatory follicles. Have LH receptors on granulosa cells. Can survive if FSH levels fall below 1ng/ml.
6) . Ovulation in the presence of an LH surge or else atresia after 72 hours.
Describe the transition of a primordial follicle to a secondary oocyte.
Primordial follicle»_space;> Primary follicle»_space;> Secondary follicle»_space;> Pre-antral»_space;> Early antral»_space;> Late antral»_space;> Pre-ovulatory follicle»_space;> Secondary Oocyte (Metaphase 2).
The follicle is clearly exerting a negative influence in terms of maintaining it in meiotic arrest because when we remove the oocyte from the follicle it will begin to develop.
There is an intra-follicular cascade of factors expressed during early folliculogenesis.
Once a follicle has started growing and becomes primary it is either committed to keep going or atresia takes place. It is not clear which factors are actually key in the transition from one stage to the next.
The very early stages of follicle development are completely gonadotrophin-independent (primordial, primary, secondary phase).
The following stages of follicle development are gonadotrophin-responsive (preantral, early antral phase).
The gonadotrophin-dependant stage is the preovulatory large antral phase.
How do we known that the follicle is exerting a negative influence in terms of maintaining the oocyte in meiotic arrest?
The follicle is clearly exerting a negative influence in terms of maintaining it in meiotic arrest because when we remove the oocyte from the follicle it will begin to develop.
Describe activation of the primordial follicle.
1) . Flattened granulosa cells become cuboidal and proliferate.
2) . Oocyte begins to grow and becomes synthetically active.
3) . Zona pellucida forms around oocyte.
4) . Granulosa cells continue to proliferate and Theca layer forms on the follicle.
5) . Antrum forms (200-500um). Rate of oocyte growth declines and rate of follicle growth accelerates.
This whole process takes 2-4 months.
As soon as the Antrum forms the oocyte tends to flatten out and doesn’t increase very much.
As the rate of oocyte growth declines the rate of follicular growth accelerates.
As the oocyte starts to slow down the mitotic index in the surrounding granulosa cells tends to increase.
As soon as we get to a stage where the follicle switches from gonadotrophin-responsive to gonadotrophin-dependent we get a decline in this granulosa cell proliferation as the somatic cells start to differentiate and secrete ovarian steroids and peptides.
How does the growth rate of the oocyte and follicle change throughout follicular development? How does the oocyte mature through follicular development?
Fast oocyte growth and slow granulosa cell growth»_space;> slow oocyte growth and fast granulosa cell growth»_space;> declining granulosa cell growth and somatic cell differentiation.
We have fast growth of the oocyte when the granulosa cells are not dividing that fast. Then the oocyte stops growing and the granulosa cells start to proliferate very rapidly. As the follicle then moves into the small/large antral phase the rate of granulosa cell proliferation declines as the somatic cells start to differentiate and begin to secrete steroids and peptides.
There is quite a lot of protein synthesis during oocyte development.
The oocyte matures during follicle development. It is not mature during the primordial and primary phase. The secondary (late preantral) phase the oocyte is pretty close to mature size and will undergo germinal vesicular breakdown but won’t progress to metaphase 2. In early antral phase the oocyte will progress to metaphase 2 and is capable of fertilisation. It is is really only the preovulatory follicles that are fully developmentally competent and can produce an embryo.
At what stage in follicular development with the oocyte progress to metaphase 2?
The early antral phase.
At what point on follicle development does growth of the oocyte stop?
As soon as the Antrum forms the oocyte size tends to flatten out and doesn’t increase very much. Rate of granulosa cell proliferation then accelerates.
What stages of follicle development are gonadotrophin-independent?
The primordial, primary and secondary phases.
What phases of follicle development are gonadotrophin-responsive?
The pre-antral and early antral phase.
What stages of follicle development are gonadotrophin-dependent?
The late-antral and preovulatory phases.
