lec 5- fertilization in animals Flashcards
what is fertilization?
the process by which two haploid gametes fuse together to create a new individual with genes that come from both parents
how does sperm, like from a sea urchin, that is secreted into water find the right egg?
the egg secretes soluble molecules that serve as chemoattractants for sperm of the same species
what only allows compatible sperm to fuse with an egg?
species-specific gamete-recognition proteins
what triggers the release of digestive enzymes from the acrosomal part of the sperm and what do the digestive enzymes do?
when the sperm reaches the jelly layer, release of digestive enzymes allows the sperm to penetrate the jelly layer to the egg
what happens after the sperm enters the jelly layer?
it reaches the vitelline envelope and passes through it, allowing for the fusion of the sperm and egg plasma membrane
what on the sea urchin sperm interacts with the EBR1 receptor in the vitelline envelope of the egg to allow passage?
bindin on the sperm membrane binds to the EBR1 receptor allowing passage
is bindin species-specific?
-yes, it only binds to eggs of the same species of sea urchin
-e.g. bindin that was on sperm only binded to dejellied eggs of the same species but not those of others
what happens if sperm doesn’t have bindin?
an egg can not be fertilized, making it essential
how do you know sperm with bindin fertilized an egg?
you will see a fertilization membrane around the egg
what is the jelly layer around eggs called in humans and other mammals?
zona pellucida
what are the 5 steps to fertilization in mammals?
- binding of sperm to zona pellucida
- acrosomal reaction occurs to release digestive enzymes
- released digestive enzymes allow penetration through zona pellucida
- the protein fertilin is species-specific and binds to the a6b1 integrin on the egg membrane, allowing fusion
- sperm nucleus enters egg
what does the sperm entering the egg cause?
it initiates a signal transduction pathway which causes calcium to be released to activate the egg
what happens to calcium in the egg?
it enters at sperm-egg fusion site then travels across the egg, allowing the activation of development
can eggs be artificially activated? how so?
yes, by increasing calcium concentrations inside the egg
what blocks activation in eggs?
calcium chelators, which bind calcium and prevent it from interacting with other molecules in the cell
what is polyspermy?
when more than one sperm binds to an egg
what are the two steps to blocking polyspermy?
- fast block, an immediate form of signalling that tells that a sperm has fused with an egg right when it occurs
- slow block, provides a permanent barrier to prevent sperm from fertilizing the egg
what is the resting membrane potential value for eggs?
-70mV
what occurs during fast block in sea urchins?
-when the sperm fuses to the egg, it causes a Na+ influx resulting in a change in resting membrane potential from -70mV to +20mV
-this is a rapid depolarization that changes the membrane potential so sperm cant fuse anymore, the value gradually returns to normal
what occurs during slow block in sea urchins?
-when the egg slowly repolarizes, an impenetrable membrane reforms called the fertilization membrane which prevents sperm from entering
-fast block only last for a minute, so a second block is needed
-it occurs a minute after fast block
what is the slow block to polyspermy also known as?
cortical granule reaction
what triggers the fertilization membrane to form?
-when the sperm fuses to the egg, calcium is released which causes cortical granules to release their content
-this causes the vitelline envelope to lift off of the plasma membrane
-the contents become cross linked together forming the fertilization membrane
once the fertilization membrane forms, what do the excess granules do?
form a jelly like hyaline layer between the fertilization layer and the plasma mebrane
has fast block been observed in mammals?
not really, up for debate if it even occurs
