Sea Urchin Fertilization [applies to other organisms for the most part]
1. Unfertilized (i.e., immature) egg is an oocyte with a huge nucleus (called a germinal vesicle)
2. Mature eggs have two membranes:
        a.  vitelline envelope – closes to oocyte – cannot distinguish
        b. jelly outer layer
3. Jelly Layer – contains a species-specific sperm attractant (not present until egg matures)
4. Sperm swims up a concentration gradient to the egg
5. Jelly also contains a species-specific glcosaminoglycan (GAG) that activates the acrosomal reaction of the sperm.
6. Acrosomal Reaction – upon sperm binding to the egg upon fertilization
7. Sperm cells have a “cap” on the anterior end called an acrosomal vesicle
8. GAG causes the acrosomal vesicle to fuse and release its proteolytic enzymes to “eat” away the jelly layer
9. The sperm then releases a protein extension made up of actin called an acrosomal process that will actually fuse
        with the vitelline layer (underneath the jelly layer)
10. Vitelline envelope has a binding receptor for a protein found on the sperm’s acrosomal process.  Remember receptor
        mediated behaviors in the hydra (lock-n-key)
11. The protein is aptly called bindin.
12. Only one sperm can bind to an oocyte.  Having more than one sperm binding to an oocyte is called “polyspermy”.
        a. Poly = Greek for many.
13. The egg has two ways of ensuring only one sperm cell binds to it.  We call this “Two Blocks to Polyspermy”. Recall
        from Lab experiment there were about a million sperm cells around one single egg and all trying to enter the egg cell.
14. The first block is a fast one and only lasts for a minute, which is enough time for a second slower but permanent block
        to occur.
15. Two Blocks to Polyspermy
        I. Fast Block to Polyspermy
            a. Upon 1st sperms binding, the egg’s membrane potential (big fancy words for an electrical gradient between the
                    internal environment of the egg versus the external environment.  Sodium, for example, has a +2 charge.  Thus,
                    if  more sodium is outside than inside the egg we say that there is an electrical gradient or a membrane potential.
            b. Sperm can only bind to an egg when the gradient is less the negative 10 millevolts.
            c. In the fast block, sodium ions rush into the egg causing it to become equal gradient with the external environment.
                    (i.e., a zero membrane potential)
            d. This effectively prevents other sperm cells
            e. This block only takes 1/10th a second to occur – that’s very, very fast
            f. But only lasts for a minute; it transiently keeps out other sperm cells untill a more permanent block can occur.
        II. Slow Block to Polyspermy
            a. This is a permanent block to any other sperm cells from entering the egg cell
            b. This slow block, called slow cause it takes a minute to occur, occurs when the vitelline layer actually “lifts” off
                    from the oocyte.
            c. This vitelline layer then “toughens” up and is now called a fertilization membrane.
            d. Any other sperm cannot penetrate this barrier.
16. The oocyte sends out extensions of microvilli to the sperm cell and envelops it.  We call this a fertilization cone,
           especially since it looks like a cone.