The swim-up technique separates bovine sperm by metabolic rates, motility and tail length

Swim-up is a sperm purification method that is being used daily in andrology labs around the world as a simple step for in vitro sperm selection. This method accumulates the most motile sperm in the upper fraction and leaves sperm with low or no motility in the lower fraction but the underlying reasons are not fully understood. In this article, we compare metabolic rate, motility and sperm tail length of bovine sperm cells of the upper and lower fraction. The metabolic assay platform reveals oxygen consumption rates and extracellular acidification rates simultaneously and thereby delivers the metabolic rates in real time. Our study confirms the upper fraction of bull sperm has improved motility compared to the cells in the lower fraction and shows higher metabolic rates. This pattern was consistent across media of two different levels of viscosity. Sperm with longer flagella are selected in the upper fraction. We conclude that the motility-based separation of the swim-up technique is based on metabolic differences. Metabolic assays could serve as additional or alternative, label-free method to evaluate sperm quality, which is likely particularly useful in cases of asthenozoospermia and teratospermia. Furthermore, metabolic measurements of sperm cells can reveal differences in metabolic pathways in different environments.