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Human Reproduction > Fertilisation > Flashcards

Fertilisation Flashcards

1
Q

What are the events that occur during the process of fertilisation?

A
  1. Capacitation & activation of sperm
  2. Penetration and fusion
  3. Activation of ovum and establishment of diploidy
  4. Syngamy
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2
Q

What are the overall types of changes that occur during sperm capacitation?

A
  1. Change of motility pattern: Beating of flagellum changes from gentle wave-like motion to higher amplitude whiplash motion.
  2. Change in surface membrane: Allowing sperm to respond to chemical factors released by the oocyte as well as fuse with it.
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3
Q

What are the changes that occur in a sperm during capacitation?

A
  1. Loss of membrane cholesterol: Binds to albumin in uterine secretions.
  2. Loss of glycoproteins acquired in epididymis and seminal fluid: Changes in charge and organisation of the sperm PM, partly driven by loss of cholesterol and enzymes in uterine secretions.
  3. Increased fusogenicity with oocyte: Loss of cholesterol causes increased PM fluidity, resulting in instability.
  4. Biochemical changes: Removal of glycoproteins increase Ca2+ entry into the sperm, causing ↑ [cAMP]i and subsequently ↑ PKA activity.
  5. Increased motility: Caused by opening of Ca2+ channels.
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4
Q

Where does spermatozoal capacitation take place?

A

In uterus and tubal-uterine junction

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5
Q

What properties of the uterus allow for it to facilitate capacitation?

A
  1. Secretion of proteolytic enzyme
  2. Providing cholesterol-binding sinks
  3. High ionic strength
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6
Q

How can capacitation be reversed?

A

When capacitated sperm is added back to semen

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7
Q

What are the changes that occur during sperm activation?

A
  1. Change in spermatozoa tail motion
  2. Acrosome reaction
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8
Q

What changes to sperm tail motion occur during capacitation?

A

Wave-like movement → Whiplash movement (hyperactivity)

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9
Q

What mediates changes in sperm tail motion during activation?

A

Ca2+ influx causing activation of PKC

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10
Q

What is the acrosome reaction?

A

Fusion of outer acrosome membrane and sperm PM

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11
Q

What are the changes that occur during the acrosome reaction?

A
  1. Exocytosis of hyaluronidase: Facilitates penetration through cumulus cells around the oocyte.
  2. Exocytosis of β-hexosaminidase: Breaks down ZP3 and prevents further binding of sperm.
  3. Exposure of inner acrosomal membrane: Contains acrosin that facilitates penetration through zona pellucida.
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12
Q

How many types of zona pellucida proteins (ZPs) are there?

A

4 (ZP1-4)

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13
Q

What are the functions of the different ZP proteins?

A
  • ZP1, 3, 4: Mediates capacitation of sperm
  • ZP2: Mediates sperm penetration of zona pellucida
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14
Q

What is a possible function of sperm immobilisation at the tubal-uterine junction?

A
  • Once a sperm is activated, it has a very short lifespan.
  • Sperm immobilisation in the isthmus, as this establishes a pool of sperm that can be released gradually over time, extending time window for fertilisation.
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15
Q

How does the sperm penetrate the zona pellucida?

A
  1. Proteolytic activity of acrosin
  2. Physical movement of sperm
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16
Q

What is the process of sperm binding to oocyte membrane?

A
  1. Once inside the perivitelline space, microvilli on the surface of the oocyte envelop the head of the sperm, increasing SA for mediating fusion.
  2. The sperm also binds to the PM (oolemma) of the oocyte via the middle (equatorial) and posterior half (post-acrosmoal) of its head. This binding is thought to be mediated by integrin-like molecules in oolemma and ADAM on the sperm PM.
17
Q

What is the proposed process of sperm/oocyte fusion?

A
  1. Izumo1 on the sperm binds to Juno (Izumo1 receptor) on the oolemma
  2. Accumulation of CD9 in the binding area of the sperm
  3. Fusion may be a result of action of fusigenic proteins and is Ca2+-dependent
18
Q

What is the significance of the position of sperm fusion with the oocyte?

A
  • It is usually on opposite pole to the oocyte pronucleus (in mice)
  • Defines the plane of division for zygote (first cleavage division along pole defined by the site of fusion)
19
Q

What are the events that occur during activation of the ovum?

A
  1. After fusion of sperm, there is a rise in [Ca2+]i (mostly from intracellular stores) in a wave that sweeps across the oocyte lasting for 2-3 minutes, originating from the point of sperm entry.
  2. After the initial wave, there is a more uniform rise in [Ca2+]i across the whole oocyte every 3-15 minutes for the next 4-5 hours, each spike lasting 1-2 minutes.
20
Q

What is oocyte activation mediated by?

A

PLCζ from sperm

21
Q

What is the cortical reaction?

A

Waves of Ca2+ stimulates release of cortical granules from the oocyte. These contain many factors that mediate changes to prevent polyspermy.

22
Q

What are the changes that occur to prevent polyspermy?

A
  1. Ovastacin cleaves ZP2
  2. β-hexosaminidase B cleaves ZP3
  3. Tyrosine residues between adjacent ZPs are cross-linked, preventing the zona pellucida from being further penetrated by sperm
  4. Juno receptors on the oolemma are exocytosed to inhibit sperm fusion
23
Q

What are the events that take place during establishment of diploidy?

A
  1. Ca2+ transient causes phosphorylation of a number of different proteins in the zygote that contribute towards re-entry into meiosis (from stalled metaphase II).
  2. The zygote is triploid at this point, with diploid from oocyte and the addition of the haploid chromosomes from sperm.
  3. In order to re-establish diploidy, one set of maternal chromosomes in the zygote is extruded into the 2nd polar body.
24
Q

What is syngamy?

A

Process of coming together of the maternal and paternal chromosomes

25
Q

What is the sequence of events that occur to achieve syngamy?

A
  1. Once fusion has occurred, the sperm nuclear membrane breaks down and sperm chromatin begins to swell.
  2. Strands of chromatin are released into the zygotic cytoplasm.
  3. Protamines in sperm chromatin are replaced by histones.
  4. Pronuclei form around both maternal and paternal chromosomes (paternal slightly bigger).
  5. DNA is duplicated within the pronuclei to form chromosomes containing 2 sister chromatids.
  6. Pronuclei break down and maternal/paternal chromosomes line up in equator (mediated by metaphase spindle) ready for first mitotic division.
26
Q

What are the contributions of the mother to the zygote?

A
  1. PM
  2. Haploid DNA
  3. Ribosomes
  4. Mitochondria
  5. Golgi apparatus
  6. Cytoplasm
  7. Centrosome
27
Q

What are the contributions of the father to the zygote?

A
  1. Haploid DNA
  2. Centriole
  3. Non- coding RNA
  4. Proteins and enzymes mediating oocyte activation and meiosis (e.g. PLCζ)
28
Q

What are the fates of components of the sperm not inherited by zygote?

A

Targeted for destruction by proteasome

29
Q

What is parthenognesis?

A

Development of a zygote that only contains maternal DNA

30
Q

Why does parthenogenesis lead to development failure in humans?

A

No centriole from sperm

31
Q

What is androgenesis?

A

Development of zygote that contains only paternal DNA

32
Q

What is parthenogenesis caused by?

A

Loss of male pronucleus and failure for second polar body to be expelled.

33
Q

What is androgenesis caused by?

A

Loss of female pronucleus and duplication of male nucleus.

34
Q

What are the clinical consequences of partheno-/androgenesis?

A
  • Androgenesis: Hydatidiform mole
  • Parthenogenesis: Ovarian teratoma

Human Reproduction (21 decks)

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