| Spermatozoa are a unique
specialization of cells in men. A good way to describe a sperm
cell is to think of it as an Apollo space shuttle, as each is
comprised of three essential components; capsule, fuel tank, and
propulsion unit. 
The main function of the sperm is to carry the 18,000 male
genes to the female’s egg, just as the Apollo capsule carries
the astronauts on their mission. These genes represent the
male's contribution to the DNA make-up of the hoped-for baby.
The capsule also has a unique surface feature in its membrane
which allows it to burrow through the shell of the woman's egg.
Using the enzymes that are built into its surface, the sperm
cell digests its way through the outer eggshell called the zona
pellucida. The sperm cell is then able to enter into the main
part of the egg, the ooplasm, achieving successful
fertilization.
The second component of the sperm cell is the energy source.
Think of this as the rocket's fuel tank, its sole purpose is to
convert nutrients into energy. Without this the propulsion
system would not have the energy to take the cell up the female
genital track.
The third and final component of the sperm cell is the
propulsion system. It is the whip-like structure of the sperm
tail. It serves the important function of propelling the sperm
cell forward with a snake-like motion. It is critical that sperm
move forward and not laterally or in circles. The failure of
sperm to move forward is described as an abnormality of sperm
motility.
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