January 22, 2007
Embryology Biology 441 Spring 2007 Albert Harris
FertilizationSperm meets egg; sperm fuses with egg; egg refuses any other sperm.
Three important fusions of plasma membranes in fertilization:
2) Fusion of sperm plasma membrane with oocyte plasma membrane
3) Fusion of oocyte plasma membrane with cortical granule membranes
It amounts to a giant lysosome; fusion of the sperm plasma membrane with the membrane surrounding the acrosome releases the contents.
Even mammal eggs have a vitelline membrane (like a jelly shell).
In addition, frogs and salamanders have several additional jelly layers secreted around their egg cells, secreted by oviduct cells.
In birds and reptiles, the egg white ("albumin") is secreted around the oocyte, by the upper end of the oviduct, and then the shell is secreted by cells of the lower end of the oviduct.
If an egg ("oocyte" = egg cell) gets fertilized by 2 sperm, that will make the embryo triploid, and therefore infertile or dead.
"Polyspermy" means fertilization by more than one sperm.
Many mechanisms have evolved to 'lock the doors' as soon as one sperm had managed to fertilize the egg: "Blocks to polyspermy"
The "fast block" to polyspermy uses a propagated decrease in electrical voltage across the plasma membrane of the oocyte.
Before being fertilized, the voltage is about 70 millivolts more positive outside the cell than inside = 70 mv more negative inside than out!
NOTICE THE PARADOX! The positive charge outside is being caused by higher concentrations of (positively-charged) potassium ions
inside the cell. Their leakage outward creates this voltage.
When the oocyte is fertilized, then ion channels open in its plasma membrane that let sodium ions leak in (& also calcium ions).
The wave of depolarization propagates across the egg surface, in only a minute or so. (much is slower than a nerve impulse)
The oocyte membrane (somehow!) won't fuse with the sperm membrane after it has depolarized.
Just under the plasma membrane of oocytes are thousands of vesicles, each with its own membrane around it. The increased calcium concentration causes these to fuse with the plasma membrane, releasing the content of the ~ 20,000 "Cortical vesicles"
The contents of these vesicles causes slow blocks to polyspermy
Different kinds of slow blocks to polyspermy:
a) Enzymes in the cortical vesicles digest away adhesion molecules on the oocyte surface, that are needed for sperm to stick to eggs.
Phenomena in humans and mammals tend to be called by special names
even when the same phenomena also occur in lower animals.
b) In sea urchins, and amphibians, enzymes in the cortical granules
digest adhesion molecules that stick the vitelline membrane to the plasma membrane of the oocyte.
This also allows frog eggs to rotate, so the heavier (yolkier) white side rotates downward. In a not-yet-fertilized batch of frog eggs, you can see many of the eggs still have their white sides upward, randomly.|
And if you flip the mass of eggs over, the cells can't rotate.
You can really impress parents, friends and future in-laws with the ability to distinguish fertilized from unfertilized eggs that frogs lay in their swimming pool or bird-bath!
c) In sea urchins, a special protein in the cortical granules precipitates onto the inner surface of the vitelline membrane, to form a sperm-proof layer called "the fertilization membrane"
Notice differences in the location of fertilization
Sea urchins: simply release sperm and oocytes into sea water
Sea squirts: release sperm into sea water; but hold oocyte in body;
Frogs, most salamanders, & most teleost fish: male and female come together, female releases eggs into the water, and then the male releases sperm onto the eggs.
Some salamanders: Males come to ponds about a week before females,
and deposit gooey mucus blobs ("spermatophores").
Many fish, & all reptiles birds and mammals: Male inserts sperm into the lower end of the female oviduct, and sperm cells meet the oocytes
"coming through the rye", as it were.
Cytoplasmic rearrangements inside oocytes (after fertilization)
(control muscle cell differentiation)
Teleost fish cytoplasm forms a lump at the animal pole
Frogs & Salamanders rotation of cortical cytoplasm relative to deeper cytoplasm, forming gray crescent.
In nematodes, the A-P axis of the embryo is controlled by where the sperm fertilizes the oocyte.|
And UNC's own Bob Goldstein discovered this!!
(Duke, eat your hearts out! We hired him and they didn't! Nye nye nye)
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