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RED > Fertilisation > Flashcards

Fertilisation Flashcards

1
Q

What are the conditions required for fertilisation?

A

Sperm must be mature and capacitated
Egg must be arrested in metaphase II
Endometrium must be receptive

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2
Q

What happens during ovulation and how is the movement of the oocyte controlled?

A

The cumulus-oocyte complex is picked up by fimbriae on the infundibulum, by stroking across the ovary.
Movement is controlled by the menstrual cycle with increased movement and decreased distance between ovary and fimbriae at ovulation. A -ve pressure in the uterine tube helps to pull the oocyte in.

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3
Q

What is the purpose of seminal fluid?

A

To provide a short term buffer to protect against acidic environments

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4
Q

What causes the seminal plasma to coagulate? What causes the breakdown of the clot?

A

Semenogelin causes coagulation within 1 minute of deposition in the vagina. It gives the sperm 30 minutes to get out of the clot. PSA breaks down the clot and sperm leaves the vagina.

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5
Q

When is cervical mucus least viscous?

A

Days 9-16 to give maximum chance of optimal sperm passing through = sperm selection

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6
Q

What does the oral contraceptive do to the mucus?

A

Thickens it to reduce the chance of sperm passing through

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7
Q

What helps the sperm move towards the ovary within the uterus?

A

Pro-ovarian contractions in the myometrium in the late follicular phase

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8
Q

What helps a sperm to decide which uterine tube to take?

A

Progesterone production from granulosa cells of the ovulating ovary provides a chemotaxis gradient.

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9
Q

What must the sperm pass through whilst travelling to the uterine tube?

A

Uterotubual junction which funnels sperm as it passes through slowly

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10
Q

What assists sperm movement in the uterine tube?

A

Cilia lining the tubes move fluid to assist sperm

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11
Q

Why doesn’t primary cilia dyskinaesia cause infertility?

A

Sperm can move themselves

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12
Q

What barriers must sperm pass through to reach the egg?

A

Acidic vagina, mucus, uterus, uterotubual junction, viscous fluid, invaginations of uterine tube that increase in number along the length

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13
Q

Where does fertilisation usually occur?

A

Ampulla

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14
Q

How long can sperm stay within the female tract? But how long is an oocyte viable for?

A

5 days

24hrs

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15
Q

What will increase a couples chance of fertilisation?

A

Having sexual intercourse upto 5 days before ovulation

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16
Q

What do ovulation kits use to predict?

A

LH surge seen 24hrs prior to ovulation

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17
Q

Why are ovulation kits not recommended?

A

Induces stress and anxiety

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18
Q

What obstacles must the sperm pass through for fusion with the egg?

A 
Detection of oocyte-cumulus complex
Penetration of cumulus
Zona binding
Acrosomal reaction
Zona pellucida pentration
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19
Q

What is the cumulus?

A

3000 cells embedded in a gelatinous matrix of hyaluronic acid which holds the cells together. The closer to the egg the tighter the layer is held.

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20
Q

How does a sperm penetrate the cumulus?

A

Hyalurodiase in the sperm breaks down the matrix causing the cells to disperse and exposes ZP.

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21
Q

What does Zona pellucida binding induce?

A

The acrosomal reaction

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22
Q

Why is the acrosomal reaction important?

A

It releases the hydrolytic enzymes needed for penetration of the ZP and exposes a new membrane on the sperm for oocyte fusion

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23
Q

What is the zona pellucida?

A

An ECM of proteins surrounding the egg. The proteins consist of ZP1-4.

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24
Q

Why are the ZP proteins important?

A

Needed for sperm-egg binding and induction of AR

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25
Q

Why does the ZP persist post fertilisation?

A

To hold the cells of the embryo together until implantation

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26
Q

What happens during fusion?

A

As the sperm penetrates ZP it occupies the perivitelline space. The equatorial segment of the sperm head fuses with the oocyte plasma membrane. The oocyte engulfs the sperm head by folding its internalised membrane into a vesicle to encase the sperm nucleus. The tail and male mitochondria are lost.

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27
Q

What happens to the egg on fusion?

A

It induces a large increase in free [Ca] which sweeps across the egg from the point of fusion.

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28
Q

What is the name of the sperm membrane receptor needed for fusion? When is it detectable?

A

Izumo, detectable after AR

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29
Q

What is the name of the receptor found on the oocyte plasma membrane for fusion?

A

Juno

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30
Q

What would happen if no Juno was present?

A

Sperm would continue to accumulate in the perivitelline space and not be able to fuse

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31
Q

How long does the Ca surge last? What follows the initial surge?

A

2-3 minutes followed by Ca oscillations every 3-15 minutes. It can last several hours post fertilisation.

32
Q

What triggers the Ca surge?

A

Internalisation of the male nucleus and Phospholipase C zeta from the sperm

33
Q

What do the Ca oscillations result in?

A
  • Release the meiotic block for completion of meiosis II

- Blocks polyspermy

34
Q

How is the meiotic block normally regulated?

A

Regulated by MPF for the CyclinB/cdk1 complex. MPF is stabilised by CSF.

35
Q

How is the meiotic block removed?

A

The raised Ca levels suppress CSF activity and destroy CyclinB to trigger the cell cycle

36
Q

How is polyspermy blocked?

A

Fast block occurs in minutes due to the initial raise in Ca causing membrane depolarisation.
Slow block is triggered by the oscillations after 1 hr to induce the cortical reaction

37
Q

What is the cortical reaction?

A

Cortical granules leave the egg via exocytosis and diffuse into the ZP to induce the zona reaction. The cortical granules contain enzymes.

38
Q

What is the zona reaction?

A

Proteases from the granules catalyse an alteration in ZP structure.
Ovastacin cleaves ZP2 so that no more sperm can penetrate.

39
Q

Why is juno lost after fusion?

A

The protein is shed from the oocyte plasma membrane with the granules after 40mins of the initial fusion to ensure no further sperm can fuse. A polyspermy egg is no viable.

40
Q

What does the sperm provide for the embyro?

A

Haploid male genome that determines sex.

Centrioles to from the spindle for the first cell division.

41
Q

What does the egg provide for the embryo?

A

Haploid female genome

Cytoplasm, organelles and all mitochondria

42
Q

What happens during the zygotic/pronucleate stage?

A

2 pronuclei are present.
Sperm DNA decondenses by exchanging the protamines back to histones.
The pronuclei replicate their DNA an migrate towards each other guided by sperm aster

43
Q

What is sperm aster?

A

Microtubules radiating from centrosome.

44
Q

What happens in the syngamy phase?

A

Pronuclei membranes breakdown as the sperm aster draws the nuclei together for chromatid mixing. Takes 18-24 hours. A nuclear envelope reforms around the zygote nucleus concluding fertilisation.

45
Q

Where do all of the stages of fertilisation occur?

A

In the Ampulla

46
Q

When does the embryo start to divide?

A

As it travels down the uterine tube

47
Q

When does the embryo implant?

A

Day 7

48
Q

What is the function of an increase in prog:oestro ratio?

A

It relaxes the musculature of the female tract to widen the isthmic sphincter to let the dividing embryo pass.

49
Q

Where is the isthmic sphincter?

A

At the entrance to the uterus from the uterine tubes.

50
Q

How does the embryo move?

A

Cilia action

51
Q

What are the stages of cell division in the embryo?

A

Zygote —> Blastomere (+2 cells) —> Morula (16-32 cells) —> Blastocyst (day 5)

52
Q

When does preimplantation genetic diagnosis take place?

A

At the 4-8 cell stage by removing one of the blastomeres.

53
Q

Why does the blastomere decrease in size?

A

There is no cytoplasmic synthesis so with each division, the blastomeres decrease in size.

54
Q

What is the 2 cell stage dependent on?

A

Oocyte cytoplasm affecting the quality of the egg

55
Q

What happens in the 4-8 cell stage?

A

Burst of transcription as the paternal genes are activated

56
Q

What stage do maternally derived proteins remain until?

A

Until blastocyst stage

57
Q

What stimulates embryo metabolism and growth?

A

GF - autocrine and paracrine

58
Q

What is inside-outside polarity?

A

From the 8 cell stage onwards, inside-outside polarity starts to develop with the outer cells forming the trophoblast and the inner cell mass forming the embryoblast.

59
Q

What is compaction?

A

In the late morula stage, fluid absorption occurs and intracellular junctions form between the trophoblast cells to form a sac around the embryo. Na/K ATPase expression increases on the cells so that as Na is pumped out, water follows forming the blastoceol.

60
Q

When does blastocyst formation occur?

A

Day 5 - The blastoceol is made of a distinct inner cell mass and single layer of epithelial trophoblasts.

61
Q

What do the trophoblasts secrete?

A

hCG to maintain the corpus luteum until the placenta forms.

62
Q

Why is it better to implant blastocysts rather than 8 cell stage in IVF?

A

Ensuring the embryo has passed the embryonic genome activation and stage of totipotency, the chance of success is increased.

63
Q

What is hatching?

A

From day 6 onwards the blastocyst expands out of a hole in the ZP at the abembryonic pole ready for implatation.

64
Q

What increases the risk of twins?

A

Dizygotic with maternal age and fertility treatment

Monozygotic with IVF

65
Q

The day of division determines the type of monozygotic twins. What results from a division on

a) day 1-3
b) day 4-8
c) day 8-13
d) day 13-15

A

a) morulla stage = Dichorionic-diamniotic
b) blastocyst stage = Monochorionic-diaminotic
c) Implanted = Monochorionic-monoamniotic
d) Embryonic disc formed = conjoined twins.

66
Q

What is the structure of the endothelium?

A

2 layers:
-Upper functional that undergoes proliferation and shedding
-Lower basal that is attached to myometrium and remains intact. Produces a new upper layer.
Basal layer contains stroma which glands penetrate into.

67
Q

What artery supplies the endometrium?

A

Spiral arteries

68
Q

What happens during the proliferative/follicular phase of the menstrual cycle?

A

The endometrium proliferates as the oestrogen levels rise. The stromal cells proliferate and have stromal oedema. SA and metabolic activity increases.
Increase in number and size of glandualr invaginations.
Expression of progesterone R.

69
Q

What happens during the secretory/luteal phase?

A

After ovulation the ovaries produce progesterone to stimulate synthesis of secretory material by the glands.
Stromal cells enlarge and spiral A develop.

70
Q

Why is the secretory material important?

A

Glycogen, glycoproteins and AA provide nutrition for blastocyst until placenta develops.

71
Q

What makes the endometrium receptive?

A

Oestrogen causing successful proliferation and progesterone causing secretion

72
Q

What happens to the endometrium during the window of implantation? How does this assist implantation?

A

During days 20-24 pinopodes appear to absorb the uterine fluid and increase the chance of implantation by pulling the blastocyst to the endometrium and immobilising it by preventing it from floating away.

73
Q

What happens during menstruation?

A

Spiral A constrict, progesterone falls and as the A collapse the upper layer necroses and is lost as menses.

74
Q

What is an ectopic pregnancy?

A

Implantation outside of uterus in tube, abdominal cavity, ovaries or peritoneum. Vessels will rupture as the embryo invades.

75
Q

Why can an embryo implant outside of the uterus?

A

It can implant as the blood supply is sufficient to supply the early development.

76
Q

What are the RF for ectopic pregnancy?

A

Hx of PID, tubual surgery, failed sterilisation, IUD = scarring

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