How do sperm find their goal?
Jülich researchers elucidate early events during the "call of the egg cell"
[13. März 2003]
How does a sperm find the egg cell? Scientists from Research Centre Jülich have found a new answer to this question. Using special techniques, they have been able to observe what happens in the first milliseconds after a sperm comes into contact with the attractant secreted by the egg cell. The biophysicists discovered that just one single attractant molecule is sufficient to set in motion the signalling pathway in sperm. Such high sensitivity has previously only been found for photoreceptor cells that can be excited by a single light quantum. The findings have been published in the February edition of the leading journal "Nature Cell Biology".
Only if the cellular signalling pathways are precisely understood is it possible to intervene in the process. A targeted blockade of the signalling pathway in the sperm could perhaps lead to a contraception method for men.
Egg cells release chemical "attractants" in order to attract sperm. The sperm are guided by the attractant gradient surrounding the egg cell and are thus able to track the cell down. The swimming behaviour of sperm is controlled by a chemical stimulus – chemotaxis – as is observed for simple marine animals and also for man. To date it has only been possible to observe how sperm react to the egg cell's chemical attractant after several seconds.
The question is what happens in the first milliseconds after the sperm have become aware of the attractant? The group headed by Prof U. Benjamin Kaupp and Dr Ingo Weyand at the Jülich Institute of Biological Information Processing (IBI 1) used new techniques to observe how the chemical stimulus is processed in sperm. To this end, sperm from a sea urchin species are rapidly mixed with the attractant, a short-chain protein. This peptide was chemically modified by chemists from the Research Institute of Molecular Pharmacology in Berlin in such a way that it is initially inactive. The peptide can only take effect after it has been photochemically modified by a UV flash. The scientists use this trick to determine the point in time after which the sperm are exposed to the attractant. The peptide binds to a receptor protein on the surface of the sperm, thus synthesizing a second messenger – a cyclic nucleotide – in thesperm.
The scientists discovered that the concentration of the second messenger, cyclic GMP, rises very rapidly after the sperm have been stimulated by the attractant. The second messenger cyclic GMP causes an ion channel – a microscopically small pore in the cell membrane – to open and calcium ions to flow into the interior of the sperm cells. The researchers were able to reveal the causal relationship between the rapid rise in the second messenger and the flow of calcium into the cell for the first time with the aid of fast mixing methods.
In the absence of the attractant the sperm tail beats regularly and propels the cell forward on a spiral track. Under a microscope the researchers can see how the sperm change their swimming paths when they come into contact with the attractant. After the calcium has flowed into the cell, the sperm tail beats more asymmetrically and the sperm undergo a turn. Finally, the sperm gather at the attractant source.
The researchers made another exciting discovery: a single attractant molecule is sufficient to open the ion channel in the sperm and let the calcium flow in. Such a high sensitivity has so far only been found in photoreceptor cells where one light quantum is sufficient to stimulate the photoreceptor cell. The researchers now intend to work on further elucidating the signalling pathway. Further pieces of the puzzle will help to clarify the still open question of reproduction. How do sperm cells find their goal?
Making use of sperm and egg cells from sea urchins, the researchers investigate how sperm find their goal. The picture shows a sea urchin of the species Arbacia punctulata which has just released the whitish sperm.
Photo: Forschungszentrum Jülich
Distribution of sea urchin sperm in the swimming chamber before release of the attractant (top). After the attractant in the centre of the swimming chamber has been released by a UV flash the sperm accumulate there a few seconds later (bottom).
Photo: Forschungszentrum Jülich
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