Fertilisation Flashcards
Fertilisation and development in sea urchins?
Good for research as zygotes all at same stage in development.
Chemotaxis of sperm to egg in response to 14 amino acid resact peptide.
Egg carbohydrate signals initiate Ca2+ mediated exocytosis of acrosomal (head of the sperm) vesicle.
Acrosomal vesicle releases enzymes needed to penetrate the extracellular envelope.
Stimulates actin-dependent formation of acrosomal process - it allows spermatozoa to penetrate the zona pellucida and fuse with the oocyte membrane.
Spermatozoa unable to undergo the acrosome reaction will not fertilize intact oocytes.
Species specific recognition through bindin protein on acrosomal process.
Fusion between sperm and egg membranes.
Fertilisation and development in Fucus is almost the same as the sea urchin (extremely conserved).
Fertilisation in mammals?
Only some mitochondria and the nucleus enter the egg.
Fast block to polyspermy via membrane depolarisation.
Calcium waves in Medaka eggs triggered by fertilisation or Ca2+-ionophores.
Calcium shows what happens after a sperm enters.
Letting some Ca2+ in triggers a Ca2+ wave.
Only one sperm triggers Ca2+ wave and stops other sperm from getting through.
What mechanism triggers the fertilisation Ca2+ wave?
Tyrosine kinase activation of phosphatidyl inositol signalling.
Cleaves phospholipids (PIP2 hydrolysis) and forms inositol trisphosphate (InsP3).
Positive response of calcium wave?
Activation of a cluster of channels generates a puff (epithelial) or spark (muscle) of calcium.
Positive response, enhances sensitivity to calcium so more likely to be triggered again.
Elevated cytosolic calcium can initiate further calcium release from either channel (CICR).
Can get calcium to a critical point which causes the calcium wave. This can trigger many things, but then stops.
High levels of calcium could be damaging – cytotoxic.
Egg activation in Drosophila requires mechanical pressure to activate Ca2+ waves.
Calmodulin has 4 calcium binding EF hand domains. Cooperative interactions confer a sigmoidal response – need several bindings to do anything. Stops single calcium ions from signalling a wave.
Undergoes a major conformational change upon binding Calcium ions. Calcium-Calmodulin binding is preferentially localised on the mitotic apparatus during division. Could activate a kinase – CAM Kinase II.
Activates the kinase, and prevents the inhibitory domain from being active. Activates cell cycle. Egg has to resume cell cycle from dormancy.
What is the acrosomal reaction?
In most marine inverts, the acrosome reaction has two components:
- The fusion of the acrosomal vesicle with the sperm cell membrane, an exocytosis that results in the release of the contents of the acrosomal vesicle.
- Extension of the acrosomal process.
Proteolytic enzymes and proteasomes thus released, digest a path through the jelly coat to the egg cell surface.
Once the sperm reaches the egg surface, the acrosomal process adheres to the vitelline envelope and tethers the sperm to the egg.
Recognition of the egg’s extracellular coat?
There is a species specific binding event which must occur once the sperm has penetrated the egg jelly and its acrosomal process contacts the surface of the egg.
In sea urchins, the acrosomal protein mediating this recognition is bindin.
Fusion of egg and sperm membranes?
Sperm egg fusion causes the polymerisation of actin in the egg to form a fertilisation cone.
The actin from the gametes forms a connection that widens the cytoplasmic bridge between the egg and the sperm.
The sperm nucleus and tail pass through this bridge.
How is polyspermy blocked?
Fast block via rapid membrane depolarisation.
The electric potential of the egg cell membrane is changed immediately upon the entry of the sperm. Sperm can’t fuse with membranes which are positive so this blocks it.
The shift of the membrane from negative to positive is the result of a small influx of Na+ into the egg through newly opened sodium channels.
Not known if this is triggered by binding of first sperm or fusion of the first sperm.
Slow block to polyspermy.
Sperm may not be blocked if they are already attached when the membrane is depolarised.
Sperm removal is accomplished by the cortical granule reaction. Becomes active after about a minute.
Underneath the sea urchin membrane are about 15,000 cortical granules. Upon sperm entry, cortical granules fuse with egg cell membrane and release their contents into the space between cell membrane and fibrous mat of envelope proteins.
Cortical granule serine protease cleaves the protein posts that connect the vitelline envelope proteins to the egg cell membrane: it also clips off the bindin receptors and any sperm attached to them.
A fertilisation envelope is formed. A resistant membranous layer in eggs of many animals that forms following fertilization by the thickening and separation of the vitelline membrane from the cell surface and that prevents multiple fertilization.
How do the nuclei meet?
Movement of the male pro-nucleus is mediated by motor drag on micro tubules.
