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Fertilisation Flashcards

1
Q

In which stage of the cell cycle must an oocyte be suspended to enable it to be fertilised?

A

Metaphase II.

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2
Q

List the processes that occur during ovulation to transport a cumulus-oocyte complex to the endometirum.

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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3
Q

What is the average number of sperm in one ejaculate?

A

200 million sperm in one ejaculate.

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4
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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5
Q

Which enzyme is released in the ejaculate to coagulate semen?

Why is coagulation necessary?

A
  • Semenogelin.

- Coagulation keeps the semen in place next to the cervix.

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6
Q

What is PSA?

What is its function?

A
  • An enzyme released in the prostatic fluid.

- It breaks down semenogelin to enable semen to flow out of the vagina.

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7
Q

What is the cervix?

A

The channel that links the vagina and the uterus.

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8
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.
  • During the rest of the menstrual cycle, it is viscous and does not enable the passage of sperm.
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9
Q

What proportion of sperm in the average ejaculate reach the uterus?

A

100,000 sperm out of 2,000,000 reach the uterus.

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10
Q

What proportion of sperm in the uterus reach the uterine tubes?

A

1000 sperm out of 100,000 reach the uterine tubes.

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11
Q

What assists the sperm in reaching the uterine tubes?

A
  • Pro-ovarian contractions of the myometrium.

- Contractions are increased in the late follicular phase.

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12
Q

List the regions of the uterine tubes from the region closest to the uterus to the region closest to the ovaries.

A

1 - Isthmus.

2 - Ampulla.

3 - Infundibulum.

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13
Q

Give an overview of the processes that 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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14
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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15
Q

List 4 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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16
Q

List the components of the zona pellucida.

A

4 glycoprotein components:

1 - ZP1.

2 - ZP2.

3 - ZP3.

4 - ZP4.

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17
Q

List 2 functions of the zona pellucida.

A

1 - It facilitates sperm-egg binding.

2 - It induces the acrosome reaction.

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18
Q

List the processes that 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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19
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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20
Q

Describe how intracellular Ca2+ changes in the oocyte during and after sperm fusion.

Which protein mediates this calcium release?

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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21
Q

List the functions of intracellular calcium release after sperm binding to the oocyte.

A

1 - Release of the oocyte from meiotic block.

2 - Blockage of polyspermy.

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22
Q

List 2 proteins that are involved in the maintenance of oocyte meiotic block.

A

1 - M-phase promoting factor.

2 - Cohesin protein complexes.

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23
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), which ubiquitinates cyclin B.
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24
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 of meiotic block.
25
Give an overview of the processes that block polyspermy upon sperm fusion to an oocyte.
1 - Fast block: Membrane depolarisation that occurs in minutes. 2 - Slow block: Cortical reaction triggered by an increase in Ca2+ that takes >1 hour. 3 - Loss of juno from the oocyte plasma membrane.
26
Describe the cortical reaction.
- Cortical granules are vesicles found under the oocyte plasma membrane. - They contain a mixture of proteases, - They are released from the plasma membrane in response to the increase in intracellular calcium caused by fusion of a sperm to the oocyte. - They release their contents into the zona pellucida following exocytosis to induce the zona reaction.
27
Describe the zona reaction.
- Ovastacin, a proteolytic enzyme released in the cortical reaction, cleaves ZP2 in the zona pellucida. - This prevents sperm from binding or penetrating the zona pellucida.
28
What causes loss of juno from the oocyte plasma membrane?
It is shed from the oocyte with the cortical granules.
29
List the structures delivered to an oocyte by a sperm.
1 - Haploid male genome. 2 - Centrioles to enable spindle for the first cell division.
30
List the processes that occur during the zygotic / pronucleate stage.
1 - Decondensation of sperm DNA by exchange of protamines for histones. 2 - Male and female haploid pronuclei replicate their DNA. 3 - Pronuclei migrate towards each other by guidance from the sperm aster, which is the collection of microtubules radiating from the sperm centrioles. 4 - This is followed by syngamy.
31
# Define syngamy. List the processes that occur during syngamy.
- The fusion of male and female pronuclei. 1 - Pronuclear membranes break down. 2 - Chromatin mixes. 3 - Nuclear envelope reforms around the zygote nucleus. 4 - Cleavage begins, marking the end of fertilisation and the beginning of embryogenesis.
32
Give an overview of the processes that occur from ovulation at day 1 to implantation.
- Day 1: The oocyte is released from the ovary and fertilisation occurs. - Day 2: The first cleavage occurs, resulting in 2 embryonic cells. - Day 3: The second cleavage occurs, resulting in 4 embryonic cells. - Day 4: The third cleavage occurs, resulting in an 8-cell uncompacted morula, which then becomes an 8-cell compacted morula. - Day 5: An early blastocyst forms, containing blastomere cells. - Day 6: A late blastocyst forms and hatches from the zona pellucida. - Day 7: The blastocyst implants into the endometrium.
33
Where is the isthmic spgincter?
At the uterotubal junction.
34
Describe the process of transport of a fertilised embryo to the uterus.
- An increased progesterone:oestrogen ratio following ovulation relaxes the musculature of the female reproductive tract, including the isthmic sphincter. - Therefore the fertilised oocyte is mostly transported under the action of cilia on the fallopian tubes.
35
During IVF, at which cleavage stage are cells harvested for pre-implantation genetic testing?
Cells are harvested for pre-implantation genetic testing at the 4-8 cell stages.
36
Why does blastomere size decrease with each division?
Because there is no cytoplasmic synthesis at the cleavage stages.
37
What is the function of the zona pellucida as the oocyte develops pre-implantation?
It keeps the blastomere cells together, maintaining cell-cell contact.
38
What controls development of the oocyte until the 2-cell stage? What happens after this?
- Both before and after the 2-cell stage, embryo metabolism and growth is stimulated by both autocrine signalling (between blastomere cells) and paracrine signalling (between the oocyte and cells of the female reproductive tract). - Before the 2-cell stage, development is dependent on the contents of the oocyte cytoplasm. - After this, there is a major burst of transcription.
39
# Define compaction. Describe the process of compaction.
- The process by which an 8-cell compacted morula is formed. - This is driven by fluid absorption and formation of intercellular junctions between trophoblast cells. - This is facilitated by expression of Na/K+ ATPases on trophoblast cells, which pump Na+ into the central cavity of the morula, drawing in water. This forms the blastocoel. - Compaction also establishes polarity, such that there is a single outer layer of cells forming the trophoblast, and an inner cell mass.
40
What is the function of the trophoblast?
The trophoblast forms the placenta.
41
What is a blastocoel?
The fluid-filled cavity at the centre of a blastocyst.
42
When is hCG produced? Which cells produce hCG? What is the function of hCG?
- hCG is produced after 5 days of fertilisation, during the early blastocyst stage. - The single layer of trophoblast cells produce hCG. - hCG signals back to the ovary, where it interacts with the LHCG receptor to promote the maintenance of the corpus luteum. - This allows the corpus luteum to secrete progesterone, maintaining the lining of the endometrium.
43
Which structures are found at the embryonic and abembryonic poles of the early blastocyst?
- The inner cell mass is found at the embryonic pole. | - The single layer of trophoblast cells is found at the abembryonic pole.
44
At which point in IVF is the blastocyst returned to the uterus?
When the embryonic and abembryonic poles are established.
45
During hatching, which pole of the blastocyst migrates out of the zona pellucida first?
The abembryonic pole migrates out of the zona pellucida first, followed by the embryonic pole.
46
What is the difference between monozygotic and dizygotic twins? What increases the risk of having monozygotic and dizygotic twins?
- Monozygotic twins are formed from one fertilised oocyte. - IVF increases the risk of having monozygotic twins. - Dizygotic twins are formed from two fertilised oocytes. - Both IVF and increasing maternal age increases the risk of having dizygotic twins.
47
What is the difference between dichorionic and diamniotic? What determines whether monozygotic twins will be di/mono chorionic/amniotic?
- Dichorionic means having two placentas. - Diamniotic means having two amniotic sacs. - It is determined by the stage at which the blastocyst splits.
48
List the possible combinations of chorionicity and amnionicity for monozygotic twins based on the stage at which the blastocyst splits.
- If the split occurs at days 1-3, the twins will be dichorionic and diamniotic. - If the split occurs between days 4-8, the twins will be monochorionic and diamniotic. - If the split occurs between days 8-13, the twins will be monochorionic and monoamniotic. - If the split occurs between days 13-15, the twins will be conjoined.
49
List the layers of the endometrium.
1 - Outer layer of endometrium, which contains two layers: - An outer functional layer which undergoes proliferation and shedding. - An inner basal layer that is attached to the myometrium and remains intact during menstruation. The functional layer is reconstituted from this basal layer. 2 - Inner muscular layer of myometrium.
50
Which arteries supply the endometrium?
The spiral arteries.
51
What happens to the endometrium in the first 14 days of the menstrual cycle (the follicular phase)?
- Endometrial cells proliferate in response to rising levels of oestrogen. - The increase in cells in combination with stromal oedema causes thickening of the endometrium. - The surface area of the surface epithelium increases. - There is an increase in metabolic activity of the surface epithelium. - The endometrium begins to express the intracellular progesterone receptor.
52
What happens to the endometrium in the last 14 days of the menstrual cycle (the secretory phase) if fertilisation occurs?
- After ovulation, the ovaries begin to produce progesterone. - Progesterone stimulates the synthesis of secretory material in the glands of the endometrium that are rich in nutrients for the blastocyst. - This makes the endometrium even thicker. - The spiral arteries become fully developed (prepared for the formation of the placenta).
53
When is the window of implantation.
Days 20-24 of the menstrual cycle.
54
Give an example of a characteristic of the endometrium during the window of implantation.
- There are pinopodes present, which are small elevations at the apical poles of the epithelial endometrial cells. - Pinopodes absorb fluid from the uterus, drawing in the blastocyst and immobilising it.
55
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 endometrium constrict when progesterone falls. - This results in collapse and necrosis of the functional layer of endometrium. - The blood and necrotic tissue is lost by menstruation.
56
What proportion of pregnancies end in miscarriage?
1/4.
57
What is an ectopic pregnancy?
Implantation of the blastocyst into a site other than the endometrium.
58
Why is an ectopic pregnancy life threatening to the mother?
Because it can cause rupture of blood vessels, resulting in substantial loss of blood.

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