Sperm cell surface dynamics during activation and fertilization

Abstract

Before the sperm cell can reach the oocyte it needs to be activated and to undergo a series of preparative steps. The sperm surface dynamics was studied in relation to this activation process and the modifications and removal of sperm surface components havebeen investigated. Bicarbonate-induced radical dependent formation of oxysterols caused efficient free sterol depletion from responsive sperm cells. This formation of oxysterols sperm cells was shown to be functional for making sperm cells competent in fertilizing the oocyte. Bicarbonate enriched media also caused a spontaneous removal of extracellular matrix components in activated sperm cells. The removal of glycosylphophatidylinositol anchored proteins was shown to be a preparative step required for sperm cells to acquire receptivity for recognizing the zona pellucida. A novel wet mount atomic force microscopic technique showed the specific removal of extracellular matrix components in the anterior part of equatorial segment of the sperm head. This is the specific site of the sperm surface involved in binding to the oocyte and in the fertilization fusion. This removal allowed the exposure membrane particles which were strictly arranged in a 17 nanometer hexagonal pattern. During the acrosome reaction this specific area fused with the underlying outer acrosomal membrane to form 1 micrometer long and 50 nanometer in diameter membrane tubules attached to the equatorial sub segment. We hypothesize that the described capacitation-dependent sperm surface changes are critical for obtaining fertilization capacity. It is matter for future research to identify the key membrane proteins involved in the fertilization fusion structures that appear on the sperm surface.