All sperm are not created equal. Some contain abnormal numbers of chromosomes which can trigger miscarriages or lead to conditions such as Down鈥檚 syndrome if they manage to fertilise an egg.
鈥淭he method produced a 20-fold decrease in the frequency of sperm with abnormal numbers of chromosomes鈥�
Healthy sperm usually outcompete abnormal sperm in the arduous race to the egg, but an increase in the use of IVF techniques such as intracytoplasmic sperm injection (ICSI), in which a sperm is injected into an egg, has improved their chances. 鈥淭he presence of a single sperm makes ICSI possible,鈥� says Ulrik Kvist at the Karolinska Institute鈥檚 Andrology Centre in Stockholm, Sweden. It鈥檚 a short cut to the egg, so better methods are needed to screen out the bad sperm from the good, he says.
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Now Myung-Geol Pang at Chung-Ang University in Gyeonggi-Do, South Korea, and his colleagues may have found one. At present, fertility clinics rely on the way sperm look to select healthy ones for ICSI, which is a particularly useful method for men with very low sperm counts. However, these men are also at greater risk of producing 鈥渁neuploid鈥� sperm with the wrong number of chromosomes: while around 10 per cent of sperm from a normal male are aneuploid, that figure is closer to 70 per cent for men with low sperm counts.
Eggs can be assessed relatively easily by staining and counting the number of chromosomes in the polar body 鈥� a by-product of egg development. In contrast, staining a sperm鈥檚 chromosomes usually damages them. To get around this problem, Pang combined fluorescence in situ hybridisation (FISH), in which fluorescent DNA labels are used to identify whether chromosomes are missing, with the hypo-osmotic swelling test (HOST), used to assess whether sperm are alive. When sperm are put into a solution that is more dilute than the fluid inside them, water moves into the sperm cell, causing it to swell up. Pang wondered whether the swelling would look different in healthy sperm and those containing abnormal numbers of chromosomes. His team analysed more than 16,000 sperm from three fertile men and six with low sperm counts, and found that it did (see Diagram).
When the method was used to select sperm from men with low sperm counts, there was a 20-fold decrease in the frequency of aneuploidy in the selected sperm compared with when no selection was used. 鈥淭his is much lower than the frequency of aneuploidy in sperm taken from healthy men,鈥� says Pang, who presented at the European Society for Human Reproduction and Embryology meeting in Lyon, France, earlier this month. He now hopes to test whether the method results in healthier embryos.
鈥淚f it works it would potentially be very beneficial,鈥� says Alan Handyside of the Bridge Fertility Centre, London.
