6. Ovarian oocyte development Flashcards
What are the main events in oocyte during follicle development?
- maintains meiotic arrest until ~ovulation
- develops competence to complete meiosis II after ovulation
- undergoes genomic imprinting
- develops competence to support development of a viable embryo -> healthy baby
What is the germinal vesicle?
Germinal vesicle (GV) - oocyte nucleus
What are the two indicators of oocyte maturation?
- Nuclear maturation - processes within germinal vesicle (GV)
- Cytoplasmic maturation - processes excluding GV
What structure separates the oocyte from other cell types in the follicle?
Zona pellucida (ZP) lies in between the oocyte and granulosa cell layer
What is the structure of zona pellucida?
Zona pellucida (ZP) - specialised ECM formed from oocyte secretion and surrounding granulosa cells - made from ZP1, ZP2, ZP3 proteins - repeating dimers of ZP2 and ZP3 + ZP1 cross-linking - at fertilisation ZP3 is key - conformational change stops polyspermy
Why does a growing oocyte have four copies of each chromosome?
After meiosis I: 2 copies in oocyte + 2 copies in polar body -> in meiosis II arrest - polar body DNA not separated yet - 4 chromosomes (oocyte x2 + polar body x2)
How is oocyte supported if chromosomes are in meiotic arrest?
Although in meiotic arrest chromosomes are active for transcription
How is early embryo development supported?
Maternally deposited mRNAs are transcribed until zygotic transcription starts after fertilisation
What are the main organelles in oocytes and what are their functions?
- germinal vesicle - nucleus
- mitochondria - energy production
- lipids - function not clear
- cortical granules - secretory organelles - block polyspermy
- vesicles
What are mitochondrial bottlenecks?
Mitochondrial bottlenecks - oocyte inherits only a proportion of mt - chance how many good:mutant mt will end up in the oocyte - kind of natural selection
Why are mitochondria only inherited maternally?
Mitochondria have a circular DNA - 37 genes without introns - high mutation rate - very susceptible to ROS - mt in sperm encounter many ROS on the way to the egg - more likely to be faulty than oocyte mt - paternal mt excluded from embryo
What is the effect of maternal age on mitochondria and how can it be solved?
As mammal becomes older - more mt mutations accumulate + mt become increasingly vacuolated - poor quality + decrease in numbers -> reduced quality / viability oocytes in older women - mitochondrial donors - 3 parent babies => reduced egg quality / viable egg quantity in older women
How long does it take for a follicle to mature?
Humans - ~3 months - primordial follicle activated before two previous periods
During development greatly increases in size
How was the importance of oocyte-granulosa interaction studied?
Oocyte taken out of follicle - cytoplasm+oocyte sucked out - no development - cumulus (granulosa)-oocyte interaction necessary for oocyte development
Why are granulosa cells and oocyte interaction necessary?
Granulosa cells and oocyte are dependent on each other for their development because signal to each other:
- oocyte signals: differentiation, expansion, steroidogenesis, follicle organization
- granulosa signal: meiotic arrest, meiotic maturation, oocyte growth, metabolic substrates
How do oocytes and granulosa cells communicate? What are the methods
Inhibitory / stimulatory signals transmitted through gap junctions in trans-zonal projections (TZPs) (extend from granulosa / cumulus cells)
What is parental gene imprinting?
Parental gene imprinting: some genes activated / repressed depending on parental origin -> monoallelic uniparental expression
Imprinting occurs in gametogenesis exclusively in eggs / sperm -> different imprinting marks -> persist into the zygote
Both maternal and paternal needed because gene expression must be complementary
How are the imprinted genes organised?
Maternally/paternally imprinted genes are usually clusterred in chromosome regions - imprinting controlled by same mechanisms in the region
Are all genes affected in genomic imprinting?
No, only key developmental genes are genomically imprinted to separate maternal and paternal expression needed of certain genes
How is genomic imprinting removed?
Genomic imprinting is removed by global demethylation in migrating PGCs - re-establishment of the genomic imprint in germ cells based on embryo sex
Explain the oogonia to oocyte developmental process considering mitosis and meiosis
Fetal: mitosis in oogonia until signalled by RA to stop mitosis and start meiosis - meiotic arrest
Fetal - puberty: meiotic arrest
Post puberty: meiotic arrest persists until the primordial follicle is recruited to mature and ovulate post-puberty
In meiotic arrest how are sister chromatids held together?
Sister chromatids are held by cohesin protein rings
What signalling mechanism maintains meiotic arrest?
G protein-coupled receptors (GPCRs) are involved - activation signal from granulosa (cumulus) to GPCRs on the oocyte leads to production of cAMP - high levels regulate downstream pathway which maintains meiotic arrest
What happens to meiotic arrest regulating mechanism once ovulation approaches?
As ovulation approaches - GPCRs no longer activated - cAMP levels fall - downstream pathway no longer stimulated - oocyte exits meiotic arrest - meiosis resumes
Explain how chromosomes move throughout oocyte development form fetal to adult stages
What is germinal vesicle breakdown?
In oocyte maturation - germinal vesicle breakdown (GVBD) - as meiosis resumes in oocyte - spindle assembles, chromosomes condense - germinal vesicles break down
What is the role of polar bodies?
Not known - but the genetic material seems completly fine - because oocyte regeneration was possible using polar body genomes
What are the processes that oocytes must complete after ovulation?
After ovulation the oocyte must:
- resume meiosis
- get fertilised
- undergo early divisions
- implant in uterus
- continue to develop to the stage of birth
What is the order of acquiring developmental competence for further development in oocyte maturation from primordial to Graafian follicle?
The oocyte acquires the potential to undergo processes after ovulation in step-by-step sequential manner - same as will later undergo the processes - while it develops from primordial to Graafian follicle
How are mature oocytes released from Graafian follicles?
Antrum in Graafian follicle enlarges - follicle diamater increases - oocyte secretes factors GDF9 and BMP15 - induces granulosa (cumulus) to secrete hyaluronan => cumulus-oocyte complex ruptures:
- ovulatory ‘stigma’ bulges out due to follicle wall weakening - connective tissue broken down
What happens in the ovary after an oocyte is ovulated?
After ovulation rupture damage is controlled at the follicle wall:
- increased blood flow
- increased ingress of white blood cells in ovarian capillaries
- thinning of connective tissue around follicle wall
- apoptosis of ovarian surface epithelium (OSE) above the stigma
- cytokines, prostaglandin, proteases elevated in region of follicular rupture
-> typical inflammation process
+ corpus luteum (CL) formation
How is the inflammation caused by ovulation controlled?
Inflammation controlled by increased levels of cortisol in follicular fluid after LH surge - provides anti-inflammatory environment to limit ovarian damage
Explain the process of embryo release from ovary
Oocyte picked up by fimbria of oviduct - transported by ciliary action + peristalsis into open end of oviduct - travels to isthmus - site of fertilisation - progress is blocked unless fertilized - oocyte remains viable for 24h
