6. Ovarian oocyte development

Question 1

What are the main events in oocyte during follicle development?

Answer 1

• 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

Question 2

What is the germinal vesicle?

Answer 2

Germinal vesicle (GV) - oocyte nucleus

Question 3

What are the two indicators of oocyte maturation?

Answer 3

• Nuclear maturation - processes within germinal vesicle (GV)
• Cytoplasmic maturation - processes excluding GV

Question 4

What structure separates the oocyte from other cell types in the follicle?

Answer 4

Zona pellucida (ZP) lies in between the oocyte and granulosa cell layer

Question 5

What is the structure of zona pellucida?

Answer 5

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

Question 6

Why does a growing oocyte have four copies of each chromosome?

Answer 6

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)

Question 7

How is oocyte supported if chromosomes are in meiotic arrest?

Answer 7

Although in meiotic arrest chromosomes are active for transcription

Question 8

How is early embryo development supported?

Answer 8

Maternally deposited mRNAs are transcribed until zygotic transcription starts after fertilisation

Question 9

What are the main organelles in oocytes and what are their functions?

Answer 9

• germinal vesicle - nucleus
• mitochondria - energy production
• lipids - function not clear
• cortical granules - secretory organelles - block polyspermy
• vesicles

Question 10

What are mitochondrial bottlenecks?

Answer 10

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

Question 11

Why are mitochondria only inherited maternally?

Answer 11

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

Question 12

What is the effect of maternal age on mitochondria and how can it be solved?

Answer 12

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

Question 13

How long does it take for a follicle to mature?

Answer 13

Humans - ~3 months - primordial follicle activated before two previous periods

During development greatly increases in size

Question 14

How was the importance of oocyte-granulosa interaction studied?

Answer 14

Oocyte taken out of follicle - cytoplasm+oocyte sucked out - no development - cumulus (granulosa)-oocyte interaction necessary for oocyte development

Question 15

Why are granulosa cells and oocyte interaction necessary?

Answer 15

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

Question 16

How do oocytes and granulosa cells communicate? What are the methods

Answer 16

Inhibitory / stimulatory signals transmitted through gap junctions in trans-zonal projections (TZPs) (extend from granulosa / cumulus cells)

Question 17

What is the main mechanism of genomic imprinting?

Answer 17

DNA methylation - silencing

Question 18

What is parental gene imprinting?

Answer 18

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

Question 19

How are the imprinted genes organised?

Answer 19

Maternally/paternally imprinted genes are usually clusterred in chromosome regions - imprinting controlled by same mechanisms in the region

Question 20

Are all genes affected in genomic imprinting?

Answer 20

No, only key developmental genes are genomically imprinted to separate maternal and paternal expression needed of certain genes

Question 21

How is genomic imprinting removed?

Answer 21

Genomic imprinting is removed by global demethylation in migrating PGCs - re-establishment of the genomic imprint in germ cells based on embryo sex

Question 22

Explain the oogonia to oocyte developmental process considering mitosis and meiosis

Answer 22

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

Question 23

In meiotic arrest how are sister chromatids held together?

Answer 23

Sister chromatids are held by cohesin protein rings

Question 24

What is the most important condition for maintaining meiotic arrest in oocytes?

Answer 24

Meiotic arrest in oocytes is mainly maintained by signalling from granulosa (cumulus) cells through gap junctional contacts - if granulosa are lost - meiosis resumes

Question 25

What signalling mechanism maintains meiotic arrest?

Answer 25

**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**

Question 26

What happens to meiotic arrest regulating mechanism once ovulation approaches?

Answer 26

As ovulation approaches - **GPCRs no longer activated** - **cAMP levels fall** - downstream pathway no longer stimulated - **oocyte exits meiotic arrest** - meiosis resumes

Question 27

Explain how chromosomes move throughout oocyte development form fetal to adult stages

Answer 27

Question 28

What is germinal vesicle breakdown?

Answer 28

In **oocyte maturation** - **germinal vesicle breakdown (GVBD)** - as **meiosis resumes** in oocyte - **spindle assembles, chromosomes condense** - germinal vesicles break down

Question 29

What is the role of polar bodies?

Answer 29

**Not known** - but the genetic material seems completly fine - because **oocyte regeneration was possible using polar body genomes**

Question 30

What are the processes that oocytes must complete after ovulation?

Answer 30

After ovulation the oocyte must: - **resume meiosis** - get **fertilised** - undergo **early divisions** - **implant** in uterus - **continue to develop** to the stage of birth

Question 31

What is the order of acquiring developmental competence for further development in oocyte maturation from primordial to Graafian follicle?

Answer 31

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

Question 32

How are mature oocytes released from Graafian follicles?

Answer 32

**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**

Question 33

What are the synonyms to decribe mature Graafian follicles?

Answer 33

Graafian = pre-ovulatory = mature follicles

Question 34

What happens in the ovary after an oocyte is ovulated?

Answer 34

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**

Question 35

How is the inflammation caused by ovulation controlled?

Answer 35

Inflammation controlled by **increased levels of cortisol in follicular fluid after LH surge** - provides **anti-inflammatory environment** to **limit ovarian damage**

Question 36

Why is ovarian inflammation dangerous?

Answer 36

Can **cause cancer** - **90% ovarian cancers are in ovarian surface epithelium (OSE)** - **repeating OSE cell inflammation** in each ovulation **induces DNA damage** - potential of mis-repaired DNA -> **cancer** **Pregnancy + breast-feeding** - fewer ovulations - **protect against ovarian cancer** - the pill could be taken as prevention of ovarian cancer

Question 37

Explain the process of embryo release from ovary

Answer 37

**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**