L06 - Fertilisation Flashcards by J M

What processes occur during ovulation to transport a cumulus-oocyte complex to the endometrium?

1 - The cumulus-oocyte complex is picked up by ciliated fimbriae on the infundibulum at the end of the uterine tube

2 - The cumulus-oocyte moves through the uterine tubes due to:

Muscular contractions of the uterine tube, which create a negative pressure to suck the oocyte through the tube
Chemoattraction

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What is the (qualitative) pH of seminal plasma?

Why is this necessary?

The seminal plasma is alkaline

- This is necessary because the vagina is acidic due to the presence of lactobacilli, which serve a protective function

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Why is coagulation of sperm necessary and how does it occur?

Semenogelin enzyme is released in ejaculate
Coagulation keeps the semen in place next to the cervix
It is broken down by PSA to enable semen to flow out of the vagina

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How does the viscosity of the cervical mucus change with the menstrual cycle?

During ovulation, it is thin and enables the passage of sperm (least viscous during days 9-16)
During the rest of the menstrual cycle, it is viscous and does not enable the passage of sperm

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What assists the sperm in reaching the egg?

Pro-ovarian contractions of the myometrium (contractions are increased in the late follicular phase)
Chemotaxis towards follicular fluid / cumulus cells
Cilia

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What processes occur once a sperm comes into contact with an oocyte?

1 - Penetration and dispersion of the surrounding cumulus cells

2 - Binding of the zona pellucida

3 - Acrosome reaction

4 - Penetration of the zona pellucida

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What is the function of hyaluronic acid in fertilisation?

Hyaluronic acid is the gelatinous matrix in which the cumulus cells surrounding the oocyte are embedded
Sperm contain the enzyme hyaluronidase to break down the hyaluronic acid upon contact with the oocyte

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What are the functions of cumulus cells?

1 - Cumulus cells cause entrapment of spermatozoa at the oocyte

2 - Cumulus cells guide spermatozoa towards the oocyte

3 - Cumulus cells create a micro‐environment for the spermatozoa which favours their capacitation and penetration into the oocyte

4 - Cumulus cells prevent changes in the oocyte which are unfavourable for normal fertilisation

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What are the functions of the zona pellucida?

1 - It facilitates sperm-egg binding

2 - It induces the acrosome reaction

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What processes occur during the fusion of sperm to an oocyte?

1 - Sperm penetrates the zona pellucida and enters the perivitelline space

2 - The equatorial segment of the sperm head fuses with the oocyte plasma membrane

3 - The sperm nucleus encased by a vesicle composed of the oocyte membrane

4 - There is a large increase in intracellular calcium across the oocyte from the point of sperm fusion

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What are izumo and juno?

Izumo is a protein on the surface of a sperm that is only present after the acrosome reaction
Izumo enables sperm fusion with the oocyte by binding with a protein on the surface of the oocyte known as juno

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How does intracellular Ca2+ change in the oocyte during and after sperm fusion?

There is a large rise in calcium during sperm-egg binding
There is followed by oscillations in calcium every 3-15 minutes for several hours
Calcium release is mediated by phospholipase C zeta, which is a form of PLC delivered by and specific to sperm cells

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What are the functions of intracellular calcium release after sperm binds to the oocyte?

1 - Release of the oocyte from meiotic block

2 - Blockage of polyspermy

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What is M-phase promoting factor (MPF)?

How does MPF respond to changes in intracellular calcium?

M-phase promoting factor (MPF) is a protein that blocks metaphase to anaphase transition
MPF is composed of cdk1 and cyclin B
MPF is stabilised by cytostatic factor (CSF)
Raised intracellular calcium suppresses CSF activity
Raised intracellular calcium also destroys cyclin B by activating anaphase-promoting complex / cyclosome (APC/C) (= E3 ubiquitin ligase), which ubiquitinates cyclin B

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What are cohesin protein complexes?

How do cohesin protein complexes respond to changes in intracellular calcium?

Cohesin protein complexes are ring-like structures that hold sister chromatids together, opposing the pulling force of microtubules
Separase is an enzyme that cleaves the ssc1 subunit of cohesin, enabling meiosis to continue
However, separase is normally inhibited by securin
APC/C, which is activated when intracellular calcium increases, ubiquitinates securin, disabling it
This stops securin from inhibiting separase, allowing spearase to release the oocyte from meiotic block

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How is polyspermy blocked upon sperm fusion to an oocyte?

Fast block - electrical block by membrane depolarisation (due to increased calcium)
that occurs as soon as sperm and egg fuse (within minutes)
Slow block - via cortical reaction triggered by increased calcium concentration and
occurs over an hour or more

What is the cortical reaction?

Release of cortical granules underneath the oocyte plasma membrane, containing enzymes such as proteases
They diffuse into the zona pellucida and induce the zona reaction

What is the zona reaction?

Alteration in the structure of the ZP is catalysed by proteases from cortical granules
Cleavage of ZP2 (protein key for sperm binding) by ovastacin protease
Sperm can no longer bind or penetrate

What happens to Juno following the cortical reaction?

Juno protein is shed from oocyte plasma membrane with the cortical granules
Undetectable within 40mins of fusion
No further sperm can fuse

What do the sperm and oocyte contribute to a zygote?

Sperm contributes haploid male genome (determines sex) & centrioles (forms spindle of first cell division)
Oocyte contributes haploid female genome, cytoplasm, all organelles & mitochondria

What processes occur during the zygotic / pronucleate stage?

Decondensation of sperm DNA - protamine-histone exchange
Male & female pronuclei replicate DNA & migrate towards each other, guided by sperm aster (microtubules radiating from the centrosome)

What is syngamy?

The fusing of male and female pronuclei
18-24hrs post-fertilisation: pronuclear membranes break down & chromatin from both pronuclei intermixes
Nuclear envelope reforms around new zygote nucleus & cleavage begins (end of fertilisation; start of embryogenesis)

How is an embryo transported to the uterus?

Increased progesterone:oestrogen ratio relaxes musculature of the reproductive tract and relaxes the isthmic sphincter at the UTJ
Mostly transported under action of cilia -> beat towards uterus

During IVF, at which cleavage stage are cells harvested for pre-implantation genetic testing?

8-cell stage

Why does blastomere size decrease with each division?

There is no cytoplasmic synthesis so blastomere size decreases with each division

What is the function of the zona pellucida as the oocyte develops pre-implantation?

ZP surrounds & holds blastomeres tightly together

What controls development of the oocyte until the 2-cell stage? What happens after this?

- Up to 2 cell stage – dependent on oocyte cytoplasm - 4 to 8 cell stage – major burst of transcription - Many maternally derived proteins persist until blastocyst stage

What is compaction?

- 8-cell stage onwards – inside-outside polarity develops - Outer cells = trophoblast -> placenta - Inner cells = inner cell mass -> embryoblast - Late morula stage: water drawn into centre of embryo via Na+K+ATPase forming blastocoel (day 5) & formation of intercellular junctions between trophoblast cells - Distinct ICM & single-layered epithelial trophoblast layer (hCG to maintain corpus luteum) - Embryonic (ICM) & abembryonic poles present

At which point in IVF is the blastocyst returned to the uterus?

- Late day 5 | - The embryonic genome is activated and they are past the stage of totipotency and hence are now pluripotent

During hatching, which pole of the blastocyst migrates out of the zona pellucida first?

Abembryonic pole

What is the difference between monozygotic and dizygotic twins? What increases the risk of having monozygotic and dizygotic twins?

- Dizygotic = non-identical - derived from two separate ova - There is an increased likelihood of dizygotic twins with increased maternal age and fertility treatment (due to ovulation induction and the use of multiple embryos) - Monozygotic = identical - derived from same ovum - There is an increased likelihood of monozygotic twins with in vitro embryo culture as they often implant multiple embryos in order to improve success rates

How can monozygotic twins be further classified?

- Monochorionic - sharing the same placenta although this can leads to twin-twin transfusion syndrome where there is an imbalance of blood flow to one of the twins - Monoamniotic - sharing an amniotic sac although the umbilical cord can become tangled leading to possible strangulation and other serious consequences

What are the layers of the endometrium?

- Upper functional layer – undergoes proliferation then shedding (menstruation) - Lower basal layer – attached to myometrium -> remains intact during menstruation; helps reconstitute functional layer

Which arteries supply the endometrium?

Spiral arteries

What happens to the endometrium in the first 14 days of the menstrual cycle (the follicular phase)?

- After menstruation, endometrium is very thin & consists of only a few cell layers (basal layer) - First 14 days: endometrial cells proliferate due to influence of oestrogen - Thickening due to stromal cell proliferation & stromal oedema - Increased SA of surface epithelium & metabolic activity - Increase in the number & size of glandular invaginations of stroma - Expression of intracellular progesterone receptor

What happens to the endometrium in the last 14 days of the menstrual cycle (the secretory phase) if fertilisation occurs?

- After ovulation, ovaries produce progesterone which stimulates the synthesis of secretory material by glands, which is rich in glycogen, glycoproteins & AAs, providing nutrition to blastocyst & early embryo - Stromal cells become larger & plumper & spiral arteries fully develop - Cellular secretions released into glandular lumen - Must be oestrogen-primed = receptive endometrium

What happens to the endometrium in the last 14 days of the menstrual cycle (the secretory phase) if fertilisation does not occur?

- Spiral arteries of the functional layer are hormone sensitive and constrict when progesterone falls - This causes necrosis of the functional layer - Blood & necrotic tissue lost (menstruation)

What is an ectopic pregnancy?

- A pregnancy in which the blastocyst implants outside of the uterus - It can occur anywhere the epithelium provides sufficiently increased vascularity to support early development - It can lead to maternal blood vessel rupture which is life-threatening