Originally published 7 October 2003
Question: Why does it take 200 million male sperm to fertilize a single female egg?
Answer: Because they won’t stop to ask for directions.
OK, just kidding. But the truth may be just as strange — if only we knew the truth.
We don’t know much about the mystery of fertilization. All we know is that in the act of sex a few hundred million little wigglers are ejected into a women’s vagina and go racing toward the egg, which is waiting somewhere up a long dark tunnel.
In fact, about 6 inches of long dark tunnel, at the upper end of a Fallopian tube. That would be equivalent to me swimming through a dark cave about 3 miles long. A cave with lots of twists and turns and blind passages. Against a stream of toxic female secretions flowing in the opposite direction.
The journey from ejaculation to fertilization can take a hour or two. If the female partner in coitus has had other recent lovers, the sperm may face competing sperm on their way to the egg. A sexually promiscuous woman’s reproductive tract can be an arena of tiny tadpole gladiators locked in chemical combat.
It is all rather like one of those computer games where you have to blast your way through a labyrinth of fiends and hazards to reach the ultimate prize. Not easy for the eager swimmers.
Only a few thousand sperm make it as far as the Fallopian tubes. Less than a hundred may find the egg.
Microphotographs of a fat round egg in the midst of swarming suitors are, I think, among the most wonderful artifacts of human ingenuity. What is invisible to the human eye — the central event of human existence! — is revealed as a frenzy of desire. Even at the level of single cells, what we see looks a lot like lust.
The writhing sperm release an enzyme that breaks apart the egg’s outer layer of cells, the corona, revealing a second wrapping, the elegantly named zona pellucida. One sperm — the fastest? the boldest? the luckiest? — latches onto the zona pellucida and penetrates the egg.
Immediately, a chemical switch (of sorts) is thrown, closing the door to any other suitors. The single successful bearer of male chromosomes is embraced by the egg, and — glory of glories — a new human being begins its journey to selfhood.
How do the sperm know where to go in that dark cavern? How do they protect themselves from the hazards of the journey? What causes them to release an enzyme when they reach the egg? How does the egg shut the door when the first sperm has gained entry?
Communication between egg and sperm — the come-hither wink, the whispered sweet nothings — is chemical. The mystery of fertilization, like most of what goes on in the human body, is a cacophonous chatter of proteins.
One protein binds to another like a jigsaw-puzzle piece with its neighbor. This causes the receptor molecule to change its shape. Another protein now binds with the new configuration of the receptor. And so on, in a sequence of shapeshifting and binding — called a signal-transduction cascade — until the appropriate response is established.
Biologists have pretty much solved the riddle of how the genes make proteins. Now the challenge is to decipher the language of proteins, to read the book of human biology in the language in which it is written.
The hundreds of millions of sperm that begin their journey toward the egg don’t need to ask for directions. Millions of years of evolution have perfected a commerce of proteins that guides the sperm to their target.
And the egg by no means waits in solitary indifference for the first of those Cupid darts to plunge into its zona pellucida. The egg has its own part in the protein dialogue, marshaling the whole of a woman’s biology to beckon and select the best of male genes.
