Women Use Sugar-Coated Eggs to Trap Sperm
Credit for being the first person to see a human sperm goes to a Dutchman by the name of Antoni van Leeuwenhoek approximately 330 years ago.
Today, credit for discovering a crucial event when a sperm binds to an egg just before fertilization is described in the August 2011 issue of the journal Science. Researchers from the University of Missouri, the University of Hong Kong, Academia Sinica in Taiwan and Imperial College London reveal their findings that a specific sugar group on the zona pellucida of an egg is what is responsible for binding a sperm to an egg. The significance of this discovery is that it opens the possibilities for new methods toward contraception and overcoming infertility.
A Brief History of Sperm
Antoni van Leeuwenhoek was a self-educated man who had an obsession for magnifying lenses. His obsession likely began while working in a dry goods store where magnifying lenses were used to count the threads in bolts of cloth.
Leeuwenhoek was an ultra-obsessive personality and spent a good part of his life learning how to grind glass into lenses that were far more powerful than those used by the top scientists of his day. A historically important aspect of his obsession with lenses was to look at everything from dust particles to bodily fluids under his lenses and make notes and drawings of what he saw. His greatest discovery was when he discovered “wee beasties” (protozoans) in drops of water. Leeuwenhoek went on to discover the presence of many microscopic organisms in a wide variety of niches such as the plaque scraped from teeth.
Leeuwenhoek made thousands of microscopes over his lifetime and gained the eventual admiration of the Royal Society in London. However, Leeuwenhoek was suspicious of “educated” men and to ensure that he was credited for his discoveries he refused to sell or loan any of his microscopes. He would instead, give limited demonstrations and send hundreds of letters describing what he saw over the years to the Royal Society.
One example of where Leeuwenhoek found himself in dispute was the claim to have been the first person to identify human sperm in semen. Another Dutchman—Nicolaas Hartsoeker—who learned optics from Leeuwenhoek, claimed in 1674 to have identified spermatozoids. However, Leeuwenhoek’s reputation as a ruthless observer who even captured tears from his daughter’s eyes for study under his lenses, would likely have looked at his own semen years prior to Hartsoeker’s claim.
Hartsoeker is credited, however, for being the origin of the Spermist’s Theory of Reproduction when he made the claim that within a single sperm he saw a homunculus—a pre-formed little man—of which many believed at the time must develop into a baby when planted inside a woman’s uterus.
Modern Reproductive Understanding
It wasn’t until after 1876 that reproduction finally became understood as a contribution of nuclear genetic material from both man and woman through the sperm and egg. Oscar Hertwig, a German zoologist devoted to comparative and causal animal development, discovered that sea urchin fertilization involved the direct penetration of a sperm with an egg.
Decades later, we gained a comprehensive understanding of fertilization as a stepwise process. Basically, one sperm among many reaches an egg’s outer zona pellucida—an extracellular coat of proteins surrounding the egg. With this meeting, the sperm and egg both undergo cellular changes. The sperm undergoes enzymatic changes that allow it to burrow through the zona pellucida barrier and into the egg’s cytoplasm where the haploid sperm and female genetic material combine. While the sperm is burrowing through the zona pellucida, the egg goes through a cortical reaction likened to cement hardening into concrete that prevents any other sperm from penetrating the egg.
While it has been known for decades that the sperm recognizes and binds to proteins in the zona pellucida, the molecular details have remained obscure. This was primarily due to a scarcity of human eggs to be used for research.
The limiting factor of eggs was eventually overcome through a donation of approximately 200 eggs from failed invitro fertilization therapies. With enough eggs to work with, researchers were now able to apply sensitive mass spectrometry analysis to identify what carbohydrates (sugar groups) are attached to the proteins of the zona pellucida. What they found was an abundance of the carbohydrate sequence for sialyl-lewis-x (SLeX) on the surface of the human egg.
To test their findings, the scientists then treated a sample of sperm with free-floating molecules of SLeX before exposing the sperm to an egg. What they found was that the sperm had bound to the SLeX molecules, which prevented the sperm from binding to the egg. The SLeX molecules were essentially blocking all of the SLeX receptors on the sperm. Tests using SLeX-specific antibodies also showed confirming results that SLeX is the primary binding ligand on eggs for attracting and binding sperm.
Future research will seek to identify the protein on sperm that recognizes and binds to SLeX. The usefulness of this is that it may help to understand one of the causes of male infertility and create a new therapy that can help infertile couples wishing to conceive.
Source: Pang, PC, Chiu PCN, Lee CL, Chang LY, Panico M, Morris HR, Haslam SM, Khoo KH, Clark GF, Yeung WSB, Dell A, "Human Sperm Binding is Mediated by the Sialyl-Lewisx Oligosaccharide on the Zona Pellucida" Science 18 August 2011. DOI: 10.1126/science.1207438.