L7: Ovary 2: Oocyte and Ovulation

Question 1

What happens to oocyte during follicle development?

Answer 1

• it maintains meiotic arrest
• but at same time, it develops the competence to resume meiosis later
• it grows, and acquires more components as it grows
• it undergoes genomic imprinting
• around the time of ovulation, it resumes meiosis
• crucially, it acquires developmental competence (ability to support the development of a viable embryo and then healthy individual if fertilised)

Question 2

What is germinal vesicle?

Answer 2

Simply the term for the nucleus of an oocyte

Question 3

What is nuclear maturation of an oocyte?

Answer 3

only processes within germinal vesicle (GV)

Question 4

What is cytoplasmic maturation of an oocyte?

Answer 4

Excludes processes within GV

Question 5

What is zona pellucida?

Answer 5

Specialised ECM formed from secretion from the oocyte and surrounding granulosa cells (both cell types are needed!)

Question 6

When is zona pellucida formed?

Answer 6

As follicles start to grow

Question 7

What is zona pellucida made of?

Answer 7

Repeating dimers of ZP2 and ZP3 linked together by occasional cross-linking of ZP1

Question 8

How is ZP3 important?

Answer 8

At fertilisation ZP3 is key - stops polyspermy

Question 9

Why does oocyte accumulate mRNAs and organelles as it grows?

Answer 9

• the oocyte is in meiotic arrest, with an extra set of all chromosomes, and so has four copies of each chromosome.
• While meiosis is arrested, the chromosomes themselves are active, including mRNA production and transcription of proteins.
• Some proteins are used as the oocyte grows, other mRNAs and proteins are used only after fertilisation, providing a maternal source of RNAs that can support the embryo until its own genome is activated.

Question 10

What are the cytoplasmic organelles of an oocyte?

Answer 10

• mitochondria
• lipids
• cortical granules
• vesicles

Question 11

How is mitochondria important for an oocyte?

Answer 11

• site of energy production
• circular DNA, high mutation rate (particularly susceptible to reactive oxygen species - likely why mitochondria from sperm do not contribute to embryo)
• as mammals become older, mitochondria in oocytes become increasingly vacuolated - poor quality - and reduced in number

Question 12

What is mitochondrial bottleneck?

Answer 12

Each oocyte inherits only a small proportion of a female’s mitochondria: has important implication in mitochondrial diseases, depending on the mitochondrial DNA that the oocyte receives –> depends the severity of the disease
That’s why three parent embryos created, also with mitochondrial DNA, so diseases not inherited
- As PGCs are dividing at each division you will only get a few mitochondria going into oocytes – bottleneck

Question 13

How are cortical granules important for an oocyte?

Answer 13

secretory organelles produced by Golgi, needed to block polyspermy along with ZP3

Question 14

How is the interaction between oocyte and granulosa cells important?

Answer 14

Now very clear that both cell types (granulosa and oocyte) are dependent on each other for their development.
Oocyte regulates:
- differentiation,
- follicle organization,
- steroidogenesis,
- proliferation,
- expansion.
Granulosa cells regulate:
- meiotic arrest,
- oocyte growth,
- metabolic substrates,
- meiotic maturation.

Without granulosa cells the oocyte doesn’t stay in meiotic arrest.

Question 15

How do granulosa cells and oocyte communicate?

Answer 15

Via gap junctions - collections of intracellular channels

Question 16

What are gap junctions composed of?

Answer 16

12 connexins - six coming from each of the two contacting cells.
Cx43: granulosa cell to granulosa cell
Cx37: granulosa cell to oocyte

Question 17

What is the function of gap junctional communication?

Answer 17

Supports metabolism and prevents meiotic resumption (keep oocyte in meiotic arrest), removal of granulosa cells leads to resumption of meiosis

Question 18

Where are the oocyte-granulosa cell gap junctions located?

Answer 18

They sit on unusual structures called trans-zonal projections, that go between the oocyte and the granulosa cells

Question 19

What is genomic imprinting of oocytes?

Answer 19

Usually we have two copies of each gene (other than XY sex chromosomes)
But not all genes are equally transcribed across the two copies!
Happens with key developmental genes (only small amount of genes). Expression of one copy of the gene is usually repressed, this happening either in the sperm or in the egg (during sperm or egg development)

Question 20

At which stage of meiosis does meiotic arrest take place? When does meiotic arrest take place?

Answer 20

Oocytes held in meiotic arrest (at dictyate stage of Prophase 1) from time of follicle formation (before birth) all the way through to ovulation

Question 21

What maintains meiotic arrest?

Answer 21

• Activation of GPCRs on the oocyte leads to production of cAMP, keeping levels high, resulting in downstream pathway together maintaining meiotic arrest.
• The oocyte needs to maintain high levels of cAMP (dependant on granulosa cells, that gap junction lets keep high levels of cAMP) if cAMP levels fall, they resume meiosis

Question 22

What happens to meiotic arrest as ovulation approaches?

Answer 22

GPCR receptors are no longer activated, production of cAMP falls, downstream pathway no longer stimulated. Meiosis resumed

Question 23

What is the function of polar bodies?

Answer 23

Is not clear, one theory was that it was used for defective chromosomes to be thrown out, however Ma et al. showed that the chromosomes in an egg were not defective.

Question 24

What is the usual sign that meiosis has resumed?

Answer 24

Germinal vesicle breakdown

Question 25

What are the steps of an oocyte once it ovulates? What is the sequence of these events?

Answer 25

1. resume meiosis
2. get fertilised
3. undergo early cell divisions
4. implant
5. continue to develop properly to the stage of birth

it turns out that it acquires the ability to carry out these processes step-by-step, and in the same order that it will later fo on to do them (if it is ovulated)

Question 26

What happens to a follicle as ovulation approaches?

Answer 26

• Antrum enlarges (cumulus cell expand) and pre-ovulatory follicle diameter increases.
• cumulus cells secrete hyaluronan in response to factors secreted from the oocyte (GDF9 and BMP15)

Question 27

How is ovulation induced?

Answer 27

Ovulation is induced by the LH surge. Positive feedback effect of very high levels of follicular E2 induces a dramatic increase in hypothalamic GnRH and a huge rise in serum LH concentration

Question 28

What is the cause of follicle rupture?

Answer 28

Just before ovulation, the ovulatory ‘stigma’ (a bit of basal membrane) bulges out just prior to rupture, strongly suggesting that this area of the follicle has become weakened and the connective tissue has been broken down

Question 29

What happens after ovulation to corpus luteum and the follicle wall?

Answer 29

Formation of the corpus luteum and repair of the damage from ovulation.
In the follicle wall, there is:
- increased blood flow
- increased ingress of white blood cells in the ovarian capillaries
- thinning of connective tissue around the follicle wall
- apoptosis of ovarian surface epithelial (OSE) cells imediately above the stigma

Question 30

How is ovulation and follicular rupture characterised?

Answer 30

• By vasodilatation, increased vascular permeability, tissue remodelling - similar to events involved in acute inflammatory process
• cytokines, prostglandin and proteases are elevated in the region of the follicular rupture

Question 31

How is inflammation after follicle rupture controlled?

Answer 31

Cortisol in follicular fluid rises after LH surge and provides an anti-inflammatory environment so that ovarian damage is limited

Question 32

Why worry about ovarian inflammation?

Answer 32

• 90% of ovarian cancers originate in the ovarian surgace epithelium (OSE)
• successive bouts of OSE cell inflammation have been shown to induce DNA damage, with the potential to lead to mis-repair of DNA and subsequent malignant changes
• pregnancy and breast-feeding (fewer ovulations) protect against ovarian cancer

Question 33

What are the final steps of ovulation?

Answer 33

• the oocyte is picked up by the fimbria of the Fallopian tube and transported by ciliary action into the open end of oviduct
• the oocyte then travels down the Fallopian tube (by ciliary action and peristalsis) to the isthmus (the site of fertilization) where its progress is blocked (unless fertilisation occurs)
• the oocyte remains viable for about 24 hours