Fertilization: (Boy meets girl, sperm meets egg.) a 10 step program

Sperm swims or crawls around (in some species non-motile!)<
Sperm finds egg cell(oocyte), (sometimes by chemotaxis)<
Sperm secretes enzymes to penetrate egg jelly (acrosome)
Sperm adheres selectively to oocyte.
Sperm plasma membrane fuses with oocyte plasma membrane
Sperm nucleus enters oocyte.
Oocyte has mechanisms to block any more sperm, after the 1st.
Sperm nucleus becomes the "male pronucleus"
Oocyte finishes meiosis! (talk about leaving things to the last minute!)<
New diploid organism formed by fusion of pronuclei.

Notice that the world needs new and better contraception methods.
Preventing ANY of the steps that you are now learning about could be the basis of a new method of contraception.
If process X needs to occur in order for fertilization to be accomplishes, then anything that prevents process X
will also prevent fertilization<

Also, this makes a good exam question for me to ask you:
"Invent 10 new methods for contraception"<
Sample answers: find some chemical that prevents...

motility of sperm,
adhesion of sperm,
digestion of egg jelly,
completion of meiosis,
prematurely activates mechanisms that block second sperm
or a chemical that prevents blockage of second sperm,<
fusion of plasma membranes
etc. etc.

or crawl by a special form of amoeboid locomotion (nematodes)
(note that nematodes don't have flagella or cilia at all!)

Or in many species the sperm are immobile & just get carried passively in fluids; many arthropods (textbook doesn't know this)

When there is a sperm, the flagellum is called the axoneme.
A certain genetic abnormality in humans results from genetic abnormality of the motor protein dynein, without which flagella can't beat, and cilia won't bend. "Kartagener triad"
(a syndrome with 3 different symptoms can be called a triad)
1) Male sterility: (because sperm can't swim)
2) frequent bronchial infections: (because cilia don't beat)
3) 50% have situs inversus heart, etc. mirror image reversed<

note that the textbook exaggerates to say that the heart is on the left side of your body. really, it's almost in the middle.
But the higher pressure side, & the aorta, & the pulse, are left!

Cilia develop from cells during gastrulation, and supposedly beat in the Hensen's node, and this beating is spiral.

The 50% aspect implies there is some fallback mechanism;
this will come up again in chapter 11.

Although sperm were discovered by Leewenhoek, he thought they must be parasitic animals swimming around.
(in other body fluids, especially saliva, there are swimming parasites)

Then, much later, several scientists claimed to be able to see little tiny human bodies in each sperm, that would grow.

Acrosome (=acrosomal vesicle)
sack of enzymes, for digesting a hole through egg jelly coat.

acrosomal filament assembly of actin-like proteins
(in some species, urchins, etc. but NOT any vertebrate)<

Capacitation of mammalian sperm:
sperm cells cannot fertilize egg cells if simply placed together
the sperm needs to have been actually in the oviduct for hours
(some mysterious enzymes (?) gradually produce changes)
Preventing these changes would be a new form of contraception!<

Oocyte Differences in time of completion of meiosis
learn one example of each figure 7.5

before first metaphase nematodes
first metaphase starfish dogs
second metaphase humans, frogs
after completion of meiosis sea urchins, Cniderians

cortical granules ~ 20,000
membranes fuse with plasma membrane & release granule contents

The contents of these granules include:

Enzymes that remove adhesion sites for sperm:
Producing the "zona reaction" in mammals

The proteins that form the "fertilization membrane" in urchins

Different mechanisms to prevent any more sperm getting in
once the first sperm has fused with the oocyte membrane.

The fast block to polyspermy: depolarization of electrical voltage
Electrical voltage difference between cytoplasm & outside cell,
Very much like the nerve impulse,
except slower and longer-lasting
Depolarization causes oocyte membrane not to fuse with sperm<

The slow block(s) to polyspermy

Formation of the fertilization membrane (in urchins, etc.)

Enzymatic removal of binding proteins for sperm.
Sperm can no longer stick to egg surface.