Folliculogenesis Flashcards
1
Q
Name 5 things that have to be achieved in order to be able to reproduce
A
- sexual differentiation
- sexual maturation
- production, storage and release of sufficient supply of eggs and sperm
- correct number of chromosomes in eggs and sperm (haploid - 23 otherwise there will be aneuploidy)
- eggs and sperm have to meet
- creation of new individual from genes from each parent
- to nurture individual until capable of independent life
2
Q
What are primordial germ cells?
A
- cells that will become eggs or sperm
- first identifiable in the yolk sac of the developing foetus at 3 weeks after conception
- undergo many cycles of mitosis and then migrate to the genital ridge of the foetus
- this genital ridge then becomes the gonad
- further differentiation of the PGC depends on the development of the gonad (ovary or testis)
3
Q
What is the cell lineage of primordial germ cells in females?
A
- PGCs
- if they go to the ovary, they become oogonia
- these are egg-precursors which are diploid and multiply by mitosis. Once this stops, they enter meiosis and become primary oocytes
- these get surrounded by granulosa cells to make the primordial follicles
4
Q
How many oocytes to females have before vs after birth?
A
- before = about 7 million
- lots apoptose before birth, so left with about 500,000
- through puberty and up to menopause, this number declines
- cannot make anymore after birth
5
Q
How are chromosomes duplicated?
A
- chromosomes replicate during S-phase of the cell cycle
- they remain attached at the centromere
- each copy is a chromatid, the two copies are identical and so known as sister chromatids
- during mitosis, spindles attach to these and pull them apart, producing two identical chromosomes
6
Q
Meiosis overview
A
- There is a replication of chromosomes
- Unlike mitosis, this involves two divisions
- 1st - chromosome number is reduced as each homologous chromosome pair is separated into two different cells, leaving each one with a member of each of the homologous pair
- 2nd - the replicated chromatids of each chromosome are separated
- meiosis begins with one diploid cell, and two divisions later gives 4 haploid cells
7
Q
Why is there an uneven division of cytoplasm during cytokinesis?
A
- After first division, there is the secondary oocyte and the first polar body
- in the second division, the polar body produces two polar bodies, and the secondary oocyte produces the mature ovum and a third polar body
- there is this uneven division as the mature ovum needs as much cytoplasm and cellular machinery as possible, it doesnt need to waste it in the polar bodies
8
Q
Why are oocytes vulnerable in meiosis I?
A
- chromatids are lined up on the spindle
- there is more risk of chromosome abnormalities if there are errors in spindle dividing - don’t get proper movement of chromosomes - more or less in one of the divided cells than the other
9
Q
How are the oocytes protected as the primordial follicle?
A
- each becomes surrounded by protective layers and cells
- in the foetal ovary, the surrounding cells condense around the oocyte and differentiate into the granulosa cells
- these granulosa cells then secrete an acellular layer called the basal lamina
- the whole structure is called the primordial follicle
10
Q
What are the first stages in folliculogenesis?
A
- as the follicles start to grow, the granulosa cells multiply and the oocyte secretes another protective acellular layer called the zona pellucida - which stays attached after ovulation
- once growth of the follicles has started, a second layer of cells then differentiate around the basal lamina - the theca, which becomes vascularised
11
Q
Is folliculogenesis driven by FSH?
A
YES
- but early growth is independent of FSH and driven by other local factors
- we can see this in FSH-deficient patients and people on the COCP, where the follicles will continue early growth, but then die
12
Q
What happens as the follicle continues to grow?
A
- It rapidly increases in diameter and granulosa cell divisions increase
- Gaps begin to form in the granulosa cell layers
- these gaps consist of flud-filled space which form an antrum
- follicles with an antrum are known as antral or secondary follicles
13
Q
How does the antrum develop?
A
- gaps begin to form in the granulosa cell layers
- these are initially found when the follicle is around 0.2-0.4mm
- these join and make a larger, single antrum at 0.4-0.9mm
- as the follicle grows, the antrum grows and displaces the oocyte to one side
- the follicle can get 2-20mm, with the antrum displacing the oocyte to the side
14
Q
How do we classify follicles?
A
- Preantral or primary
- antral or secondary (has an antrum)
- preovulatory or Graafian (has an antrum, basal lamina, theca etc)
15
Q
What are the stages of follicle growth?
A
- Start off with a resting follicle
- unknown factors trigger the initiation to grow into a preantral (larger) follicle
- these two steps are FSH independent and take around 65 days
- when they reach a certain size, they are recruited into the cycle
- from these, one is selected to be the dominant follicle, which multiplies a lot and pumps out a load of oestrogen - this is destined to be ovulated
- this process of recruitment to selection takes 15 days and is FSH/LH dependent
- The egg that is ovulated will have started its growth about 3 months ago
