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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
Q

Give an overview of the processes that block polyspermy upon sperm fusion to an oocyte.

A

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
Q

Describe the cortical reaction.

A
  • 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
Q

Describe the zona reaction.

A
  • 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
Q

What causes loss of juno from the oocyte plasma membrane?

A

It is shed from the oocyte with the cortical granules.

29
Q

List the structures delivered to an oocyte by a sperm.

A

1 - Haploid male genome.

2 - Centrioles to enable spindle for the first cell division.

30
Q

List the processes that occur during the zygotic / pronucleate stage.

A

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
Q

Define syngamy.

List the processes that occur during syngamy.

A
  • 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
Q

Give an overview of the processes that occur from ovulation at day 1 to implantation.

A
  • 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
Q

Where is the isthmic spgincter?

A

At the uterotubal junction.

34
Q

Describe the process of transport of a fertilised embryo to the uterus.

A
  • 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
Q

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

A

Cells are harvested for pre-implantation genetic testing at the 4-8 cell stages.

36
Q

Why does blastomere size decrease with each division?

A

Because there is no cytoplasmic synthesis at the cleavage stages.

37
Q

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

A

It keeps the blastomere cells together, maintaining cell-cell contact.

38
Q

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

What happens after this?

A
  • 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
Q

Define compaction.

Describe the process of compaction.

A
  • 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
Q

What is the function of the trophoblast?

A

The trophoblast forms the placenta.

41
Q

What is a blastocoel?

A

The fluid-filled cavity at the centre of a blastocyst.

42
Q

When is hCG produced?

Which cells produce hCG?

What is the function of hCG?

A
  • 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
Q

Which structures are found at the embryonic and abembryonic poles of the early blastocyst?

A
  • The inner cell mass is found at the embryonic pole.

- The single layer of trophoblast cells is found at the abembryonic pole.

44
Q

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

A

When the embryonic and abembryonic poles are established.

45
Q

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

A

The abembryonic pole migrates out of the zona pellucida first, followed by the embryonic pole.

46
Q

What is the difference between monozygotic and dizygotic twins?

What increases the risk of having monozygotic and dizygotic twins?

A
  • 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
Q

What is the difference between dichorionic and diamniotic?

What determines whether monozygotic twins will be di/mono chorionic/amniotic?

A
  • Dichorionic means having two placentas.
  • Diamniotic means having two amniotic sacs.
  • It is determined by the stage at which the blastocyst splits.
48
Q

List the possible combinations of chorionicity and amnionicity for monozygotic twins based on the stage at which the blastocyst splits.

A
  • 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
Q

List the layers of the endometrium.

A

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
Q

Which arteries supply the endometrium?

A

The spiral arteries.

51
Q

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

A
  • 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
Q

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

A
  • 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
Q

When is the window of implantation.

A

Days 20-24 of the menstrual cycle.

54
Q

Give an example of a characteristic of the endometrium during the window of implantation.

A
  • 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
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 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
Q

What proportion of pregnancies end in miscarriage?

A

1/4.

57
Q

What is an ectopic pregnancy?

A

Implantation of the blastocyst into a site other than the endometrium.

58
Q

Why is an ectopic pregnancy life threatening to the mother?

A

Because it can cause rupture of blood vessels, resulting in substantial loss of blood.

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