2. folliculogenesis and ovulation Flashcards
describe the stages of follicle development
- primordial follicle
- primary follicle
- secondary follicle
- developing antral follicle
- antral follicle
- ovulation
- corpus luteum
- corpus albicans
what is a follicle
- contains the oocyte
- place for oocyte to mature
- fluid filled
- secretes oestradiol => oestrus
- multiple stages of development
- lined with granulosa, -> basement membrane -> theca cells
what is the corpus luteum
- highly vascularised, transient endocrine gland
- solid
- progesterone production
how do the early germ cells develop
- primordial germ cells migrate from yolk sac –> hind gut –> developing gonad
- proliferate by mitosis to become oogonia
- oogonia develop in interconnected clusters of “egg nests”
- oogonia enter meiosis to become primary oocytes
how do oocytes develop
- egg nest breaks apart
- oocytes associate with somatic granulosa cells to form primordial follicles
- entire population undergo meiotic arrest (prophase 1) until puberty
- oogenesis is the process by which oocytes are formed
how long is the preantral follicle growth period
months
what stimulates primordial follicles to grow
intra-ovarian factors - NOT gonadotrophins
what stimulates antral follicle growth
gonadotrophins
primordial follicle characterised by
- single layer of squamous granulosa
- found clustered near the outer edge of ovary
20 mm
primary follicle characterised by:
single layer of cuboidal granulosa
200-400 mm
secondary follicle characterised by:
- multiple layers of granulosa
- thin layer of outer theca cells
antral follicle
- clear oocyte
- fluid
- stratified granulosa
>500 mm
how do oocytes grow
- oocyte grows rapidly during early follicular development
- increase size and RNA
- most oocyte growth occurs in pre-antral stages
- zona pellucida: specialised glycoprotein matrix (oocyte protection, sperm recognition and binding)
- communication between oocytes and granulosa is bi-directional: growth factors, projections and gap junctions (both physical and chemical interaction)
what are the phases of antral follicle growth
- recruitment
- selection
- dominance
takes several days - most follicles die by artresia or degeneration
what happens in antral follicle recruitment phase
- corhort of small (antral) follicles start to develop (10-20)
- initiated by FSH wave
- secrete small amount of oestradiol
- occurs in luteal as well as follicular phase
- some undergo artresia
what happens during the antral follicle selection phase
- cohort of growing follicles
- dependent on FSH for supprt
- produce inhibin and oestradiol (neg feedback on pituitary, supresses FSH)
- shift to LH dependence
- some will start to undergo artresia
what happens during the antral follicle dominance stage
- transition from FSH to LH dependence (LH receptors appear in granulosa cells)
- increased oestradiol and inhibin production –> inhibits growth of other follicles
- growth is rapid
- remain for about 6 days and either ovulate or undergo artresia (depends on whether progesterone falls)
explain antral follicle waves in cattle
First Wave: The first wave of follicular development typically begins soon after ovulation, around days 1 to 3 of the cycle. A dominant follicle emerges, but it regresses due to the presence of high progesterone levels from the corpus luteum.
Second Wave: The second wave starts around mid-cycle, around days 9 to 12. Again, a dominant follicle develops, but it usually regresses unless it’s the final wave leading to ovulation.
Third Wave (if present): In cows with three waves, the third wave begins around days 15 to 18. This wave is crucial because the dominant follicle in this wave is typically the one that will ovulate, triggered by a surge in luteinizing hormone (LH).
ovary is in which stage of follicle development
recruitment: small (<4mm) follicles, cohort of 10-15
ovary is in which stage of follicle development
selection
4-9mm med. follicles, cohort of 5-8
ovary is in which stage of follicle development
> 9mm large follicles, 1-2 follicles
how is oestradiol produced
- the coordinated action of the theca and granulosa layers
- requires aromatase
- stimulated by FSH and LH
outline the pathway of oestradiol production
the theca layer is vascular or avascular
vascular
is there vasculature in the granulosa layer
no
dicuss ovulation control and timings
release of the oocyte
- occurs over the entire surface of the ovary (except in mares)
- initiated by the LH surge (stimulated bu oestradiol)
- LH surge usually occurs around start of oestrus (except in mares where gradual increase in LH occurs during oestrus)
- LH surge to ovulation interval
how does the oocyte mature
discuss how induced ovulation occurs
nerve ending in the vagina send signals to spinal cord which stimulates surge centre to release GnRH to surge LH
antral follicle growth is initiated by:
an FSH wave
which cell type within the follicle synthesises aromatase
granulosa cells
in which phase of cell division is the oocyte arrested until the LH surge
meiosis 1
the LH surge is absent in induced ovulators: true or false
false
