Once you have the sperm arrive to where the egg is, this is usually in the ampulla region of the
fallopian tubes, although it can occur elsewhere but that is the typical locale. The fertilization
process is multiple step in nature. To do this, we will take these steps one at a time as we go
through looking at this overall response. The sperm, I think, you can recognize here in the
picture. The egg is the purple shape with the couple rings around it. The rings are important
when I go through the explanation because there are the different layers of the egg and those
color layers will be retained to all the different pie chart diagrams that you see. Let's start
with the first step of fertilization. In this particular step, the sperm just needs to get close
enough to the egg. It has a special matrix around the egg that it needs to get through and it
does this with the special protein called PH-20. This PH-20 allows for the sperm to get close
enough to even touch the egg. At this point, the sperm is going to have to try to break
through these couple layers of the egg. The first one is the zona pellucida. This zone is very
important. This is the first protective layer that the sperm needs to get through. There are a
few different receptors and glycoproteins that need to have an interaction take place. So the
sperm needs to bind to ZP3 portions on the zona. At this point, it has specific receptors that
are being expressed after that capsule change that happened on the trip in. If it has these
aligned ZP3 receptors to these glycoproteins on the membrane, you'll have a binding take place.
Then after the binding takes place, enzymes are released from the cap. These enzymes allow
for some digestion of the membrane of the egg, then the sperm can move in a little bit closer
and then has a second binding protein and these are ZP2 binding proteins. As this occurs, it
allows the egg head to move just inside the zona. In this next step fertilization, you need to
get the sperm just a little bit further through into the egg. This case, there's a little bit of
swimming that happens to get it to that next point or junction. Now the sperm can fuse with the
egg membrane and this case is when the DNA is released into the ovum. There is a small
signaling cascade that happens and this involves an enzyme called phospholipase C.
Phospholipase C breaks down the enzyme or the substance called PIP2 into IP3 and DAG. What
this does is cause a calcium signaling cascade to occur. This signaling cascade is important
because what happens is that this cascade elicits responses that modify the ZP2 and ZP3
glycoproteins so that no other sperm can bind to the egg. Then what happens to the sperm? It
has done its job. The flagellum, the mitochondria, the neck region, those all will disintegrate.
So all you have is the delivered DNA and this is the reason why you don't have DNA from
mitochondria in an offspring because this only comes from the mother because the father's
mitochondria are not delivered, only the DNA that will eventually be in the nucleus.