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RED > L06 - Fertilisation > Flashcards

L06 - Fertilisation Flashcards

1
Q

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

A

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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2
Q

What is the (qualitative) pH of seminal plasma?

Why is this necessary?

A
  • 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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3
Q

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

A
  • 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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4
Q

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

A
  • 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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5
Q

What assists the sperm in reaching the egg?

A
  • 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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6
Q

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

A

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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7
Q

What is the function of hyaluronic acid in fertilisation?

A
  • 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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8
Q

What are the functions of cumulus cells?

A

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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9
Q

What are the functions of the zona pellucida?

A

1 - It facilitates sperm-egg binding

2 - It induces the acrosome reaction

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10
Q

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

A

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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11
Q

What are izumo and juno?

A
  • 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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12
Q

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

A
  • 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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13
Q

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

A

1 - Release of the oocyte from meiotic block

2 - Blockage of polyspermy

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14
Q

What is M-phase promoting factor (MPF)?

How does MPF respond to changes in intracellular calcium?

A
  • 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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15
Q

What are cohesin protein complexes?

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

A
  • 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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16
Q

How is polyspermy blocked upon sperm fusion to an oocyte?

A
  • 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
17
Q

What is the cortical reaction?

A
  • 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
18
Q

What is the zona reaction?

A
  • 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
19
Q

What happens to Juno following the cortical reaction?

A
  • Juno protein is shed from oocyte plasma membrane with the cortical granules
  • Undetectable within 40mins of fusion
  • No further sperm can fuse
20
Q

What do the sperm and oocyte contribute to a zygote?

A
  • Sperm contributes haploid male genome (determines sex) & centrioles (forms spindle of first cell division)
  • Oocyte contributes haploid female genome, cytoplasm, all organelles & mitochondria
21
Q

What processes occur during the zygotic / pronucleate stage?

A
  • 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)
22
Q

What is syngamy?

A
  • 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)
23
Q

How is an embryo transported to the uterus?

A
  • 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
24
Q

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

A

8-cell stage

25
Q

Why does blastomere size decrease with each division?

A

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

26
Q

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

A

ZP surrounds & holds blastomeres tightly together

27
Q

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

What happens after this?

A
  • 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
28
Q

What is compaction?

A
  • 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
29
Q

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

A
  • Late day 5

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

30
Q

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

A

Abembryonic pole

31
Q

What is the difference between monozygotic and dizygotic twins?

What increases the risk of having monozygotic and dizygotic twins?

A
  • 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
32
Q

How can monozygotic twins be further classified?

A
  • 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
33
Q

What are the layers of the endometrium?

A
  • Upper functional layer – undergoes proliferation then shedding (menstruation)
  • Lower basal layer – attached to myometrium -> remains intact during menstruation; helps reconstitute functional layer
34
Q

Which arteries supply the endometrium?

A

Spiral arteries

35
Q

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

A
  • 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
36
Q

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

A
  • 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
37
Q

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

A
  • 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)
38
Q

What is an ectopic pregnancy?

A
  • 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

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