The Ovary: The Oocyte and Ovulation

Question 1

What happens to the oocyte during follicle development?

Answer 1

It remains in meiotic arrest but could resume this later
It grows larger and undergoes genomic imprinting
Acquired developmental competance (can now support a vaible embryo if fertilised).

Question 2

What are things found in a single follicle?

Answer 2

Oocyte
Zona Pellucida
Germinal Vesicle (Nucleus)

Question 3

What is the zona pellucida and what does it do?

Answer 3

It is an extracellular complex membrane made from secretions from oocyte and granulosa cells.

ZP1 and ZP3 and is joined together with ZP1 interlinking them.

During fertilisation ZP3 cross links stopping additional sperm from entering the oocyte and causing polyspermy.

Question 4

Why does the growing oocyte accumulate mRNA’s?

Answer 4

They have 4 chromosomes as they have not undergone meiosis and therefore make lots of proteins which helps it grow.

After fertilisation the embryo does not have its own genome for a few days and needs to rely on these proteins.

Question 5

What are the cytoplasmic organelles?

Answer 5

Mitochondria
Lipids
Cortical granules
Vesicles

Question 6

What is mitochondria, where does it come from and why?

Answer 6

Site of energy production

The mother as mitochondria are easily destroyed by ROS (reactant oxygen species) and the sperm will meet many of these whilst travelling to the egg destroying its mitochondria.

Question 7

What happens to mitochondria as mammals become older?

Answer 7

They are increasingly vacoulated reducing their quality and number. This makes it more likely you pass on mitochondrial mutations.

Question 8

Describe the mitochondrial bottleneck?

Answer 8

The mother has some mutated mitochondria and some healthy. When creating oocytes some of the mothers mitochondria (whether healthy or not) go into each cell randomly. These mitochondria then proliferate. This means one oocyte might have got lots of mutatated mitochondria and the other only healthy mitochondria.

Question 9

What do cortical granules do in an oocyte?

Answer 9

Can stop polyspermy along with ZP3

Question 10

What is the growth rate in follicles?

Answer 10

This can differ between species but in humans it can be 2 - 4 cm.

Question 11

Is the oocyte passive and how did they work this out?

Answer 11

It is not as when they removed the oocyte from a follicle they noticed that the granulosa cells needed something to communicate to

Question 12

What things do granulosa cells need oocytes for?

Answer 12

Differentiation,
Expansion,
Follicle organisation
Steroidogenesis,
Proliferation

Question 13

What do oocytes need granulosa cells for?

Answer 13

Oocyte Growth,
Meiotic arrect
Meiotic maturation
Metabolic substrates

Question 14

How do cells communicate with one another and how does this work?

Answer 14

Gap junctions. These work by intracellular channels composed of 12 connexins which support metabolism and prevents meiotic starting back up again.

Question 15

What happens if you remove the granulosa cells?

Answer 15

Meiosis starts again

Question 16

What are the 2 connexions and what do they connect to?

Answer 16

Cx43 = granulosa cell - granulosa cell
Cx37 = granulosa cell - oocyte

Question 17

What do zona pellucida gap junctions sit on and where do these connect?

Answer 17

Trans-zonal projections which go between the cumulus granulosa cells and the oocyte.

Question 18

What can the communication of factors by oocytes and granulosa cells do to the other?

Answer 18

Inhibit or stimulate

Question 19

What is genomic imprinting of oocytes and how does this happen?

Answer 19

Not all genes on all chromosomes are transcribed (as you get 2 of most chromosomes and 2 of most genes) therefore some genes on one of the chromosomes from each pair has imprinted regions. These genes are covered in DNA methylation and therefore cannot be transcribed.

For some genes it will always be the gene on the paternal chromosomes and for some the maternal ones. This only occurs however of developmentally important genes and not all.

Question 20

How does genetic imprinting change from fertilisation to the embryo becoming its sex?

Answer 20

After fertilisation all genes are methylated but some have imprinted regions. When demethylation of all genes occurs the imprinted regions stay methylated (off). However, once the embryo is developed enough all imprintings are removed so it can make it’s own (either male or female) imprints on their own follicles. (follicles within the developing embryo).

Question 21

What is meiotic arrest and what does this depend on?

Answer 21

In follicles the oocytes do not undergo meiosis until they ovulate. This depends on the somatic cells circuling the oocyte (e.g. granulosa cells) and gap junctions.

Question 22

How does G-protein couples receptors help the oocyte stay in meiotic arrest and what happens as you approach ovulation?

Answer 22

G-protein coupled receptor produce cAMP which is needed for the meiotic rest.

At ovulation G-protein coupled receptors are no longer activated and cAMP levels fall oocyte comes out of meiotic rest.

Question 23

What happens to the polar bodies in meiosis?

Answer 23

Nobody really knows. They thought it was mutated chromosomes and therefore was good it was away, however they then found that the chromosomes within still had genetic integrity.

Question 24

What does the oocyte need to do when ovulated and does it need to do it in this exact order?

Answer 24

Yes as if you stop the cell at any of these times you will not get a baby.

Meiosis continues
Fertilisation occurs
Undergo early cell division
Implant
Develop until birth

Question 25

Describe the hormones of the menstrual cycle which trigger ovulation?

Answer 25

The LH surge triggers ovulation but FSH increases slightly, progesterone and oestrogen drop.

Question 26

What is mono-ovular and poly-ovular?

Answer 26

This is species dependant

Mono-ovular species ovulate one oocyte
Poly-ovular species ovulare more than one oocyte

Question 27

What two types of granulosa cells do mature graafian follicles that are ready to ovulate have?

Answer 27

Cumulus granulosa cells which sit near the oocyte
mural granulosa cells which sill away from the oocyte

Question 28

What happens to the Graafian follicles when ovulation approache?

Answer 28

The antrum and follicles expands, cumulus cells secrete hyaluronon in response to GDF9 and BMP15 from the oocyte.

Question 29

Why and how does the follicle rupture?

Answer 29

It ruptures to release the oocyte from the follicle and occurs when the ovulatory stigma bulges prior to rupture indicating there is weakening of the basal membrane and connective tissue is broken.

Question 30

After ovulation the corpus luteum is made and repair of damage needs to happen - what happens to the follicle wall?

Answer 30

Increased blood flow
More white blood cells in the ovarian capillaries
Connective tissue thinning around wall
Apoptosis of ovarian surface epithelial (OSE) cells above the stigma

Question 31

How is inflammation and ovulation similar?

Answer 31

Ovulation causes vasodilation, increased vascular permeability and tissue remodelling which is also seen in inflammation.

Cytokines, prostaglandins and proteases are higher where the follicle ruptures

Question 32

How is ovulation inflammation controlled?

Answer 32

Cortisol rises after LH surge providing anti-inflammatory environments so that the ovarian is less damaged.

Question 33

Why would inflammation in the ovary cause ovarian cancer and what would protect you from this?

Answer 33

Lots of ovarian surface epithelium (OSE) inflammation damages DNA which could cause mis-repair of DNA and malignancies.

Pregnancy and extended breast feeding protects you from this. So does the pill but that increases your odds of cervical and breast cancer.

Question 34

Describe the final steps of ovulation?

Answer 34

The oocyte is picked up by fimbria in the fallopian tube and moved by cillia action along this tube until reaching the endometrium isthmus (site of fertilisation) where it is blocked unless fertilisation happens.