Experts Say Sperm Acts Like Virus
The way a sperm gets into an egg, scientists have found, is rather like the way a virus infects a cell. It is also like the way snake venom acts on blood.
Those are two of the conclusions that have emerged from research on the ways sperm and egg interact at fertilization.
During most of their existences as separate cells, the sperm and the egg protect themselves by maintaining tough outer membranes. And yet when the two meet, each readily surrenders to the other.
As easily as two soap bubbles pop into one, the membranes of egg and sperm fuse and their contents merge, launching the embryonic development of a new organism.
Generations of researchers have studied the phenomenon of fertilization, hoping to learn enough to help couples who wish to prevent it, and those who strive against odds to achieve it.
Now scientists in California and Connecticut have discovered a key part of the mechanism. Their work raises the possibility of developing a contraceptive that probably would have no other effect on the man or the woman than to block the key molecular linkage that must occur between sperm and egg before fertilization takes place.
The findings are also said to be a step toward understanding a leading cause of infertility: inability of the sperm to form the link that gains it access to the egg.
What the new research shows is that each sperm carries a special, two-part apparatus on its surface, each part consisting of one protein molecule. One part can recognize a specific receptor molecule on the egg and bind to it. Once this happens, the second part of the apparatus is engaged to make the cells join.
Exactly how this fusion happens is not clear; but somehow the membranes of the egg and sperm are opened in a way so perfectly controlled that the open edge of one is sealed to the open edge of the other. Without spilling anything, the contents of both cells are brought together inside a single membrane.
"We are years away from any practical application," said Judith M. White, who led the research at the University of California at San Francisco and published it in the journal Nature with Carl P. Blobel, Tyra G. Wolfsberg and Christoph W. Turck of UCSF, and two colleagues at the University of Connecticut. "What's interesting right now, though," she said, "is that this mechanism appears to be similar to others in nature. This is a motif we've heard before."
For example, the protein molecule that makes sperm and egg membranes fuse appears to be similar to the protein that many viruses carry on their surfaces.
And when the DNA sequence of the second key sperm protein was fed into a computer and compared with to all other known DNA sequences, only one proved similar - a component of the venom of pit vipers.