Fertilisation Flashcards
What are the events that occur during the process of fertilisation?
- Capacitation & activation of sperm
- Penetration and fusion
- Activation of ovum and establishment of diploidy
- Syngamy
What are the overall types of changes that occur during sperm capacitation?
- Change of motility pattern: Beating of flagellum changes from gentle wave-like motion to higher amplitude whiplash motion.
- Change in surface membrane: Allowing sperm to respond to chemical factors released by the oocyte as well as fuse with it.
What are the changes that occur in a sperm during capacitation?
- Loss of membrane cholesterol: Binds to albumin in uterine secretions.
- 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.
- Increased fusogenicity with oocyte: Loss of cholesterol causes increased PM fluidity, resulting in instability.
- Biochemical changes: Removal of glycoproteins increase Ca2+ entry into the sperm, causing ↑ [cAMP]i and subsequently ↑ PKA activity.
- Increased motility: Caused by opening of Ca2+ channels.
Where does spermatozoal capacitation take place?
In uterus and tubal-uterine junction
What properties of the uterus allow for it to facilitate capacitation?
- Secretion of proteolytic enzyme
- Providing cholesterol-binding sinks
- High ionic strength
How can capacitation be reversed?
When capacitated sperm is added back to semen
What are the changes that occur during sperm activation?
- Change in spermatozoa tail motion
- Acrosome reaction
What changes to sperm tail motion occur during capacitation?
Wave-like movement → Whiplash movement (hyperactivity)
What mediates changes in sperm tail motion during activation?
Ca2+ influx causing activation of PKC
What is the acrosome reaction?
Fusion of outer acrosome membrane and sperm PM
What are the changes that occur during the acrosome reaction?
- Exocytosis of hyaluronidase: Facilitates penetration through cumulus cells around the oocyte.
- Exocytosis of β-hexosaminidase: Breaks down ZP3 and prevents further binding of sperm.
- Exposure of inner acrosomal membrane: Contains acrosin that facilitates penetration through zona pellucida.
How many types of zona pellucida proteins (ZPs) are there?
4 (ZP1-4)
What are the functions of the different ZP proteins?
- ZP1, 3, 4: Mediates capacitation of sperm
- ZP2: Mediates sperm penetration of zona pellucida
What is a possible function of sperm immobilisation at the tubal-uterine junction?
- 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.
How does the sperm penetrate the zona pellucida?
- Proteolytic activity of acrosin
- Physical movement of sperm
What is the process of sperm binding to oocyte membrane?
- Once inside the perivitelline space, microvilli on the surface of the oocyte envelop the head of the sperm, increasing SA for mediating fusion.
- 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.
What is the proposed process of sperm/oocyte fusion?
- Izumo1 on the sperm binds to Juno (Izumo1 receptor) on the oolemma
- Accumulation of CD9 in the binding area of the sperm
- Fusion may be a result of action of fusigenic proteins and is Ca2+-dependent
What is the significance of the position of sperm fusion with the oocyte?
- 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)
What are the events that occur during activation of the ovum?
- 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.
- 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.
What is oocyte activation mediated by?
PLCζ from sperm
What is the cortical reaction?
Waves of Ca2+ stimulates release of cortical granules from the oocyte. These contain many factors that mediate changes to prevent polyspermy.
What are the changes that occur to prevent polyspermy?
- Ovastacin cleaves ZP2
- β-hexosaminidase B cleaves ZP3
- Tyrosine residues between adjacent ZPs are cross-linked, preventing the zona pellucida from being further penetrated by sperm
- Juno receptors on the oolemma are exocytosed to inhibit sperm fusion
What are the events that take place during establishment of diploidy?
- Ca2+ transient causes phosphorylation of a number of different proteins in the zygote that contribute towards re-entry into meiosis (from stalled metaphase II).
- The zygote is triploid at this point, with diploid from oocyte and the addition of the haploid chromosomes from sperm.
- In order to re-establish diploidy, one set of maternal chromosomes in the zygote is extruded into the 2nd polar body.
What is syngamy?
Process of coming together of the maternal and paternal chromosomes
