Mouse sperm undergo acrosomal exocitosis in the upper segments of the isthmus

LA SPINA F.; ROMAROWSKI A.; LIS C. PUGA MOLINA; FALZONE T.; ALEJANDRA M. VITALE; HIROHASHI N.; BUFFONE MG.

Congreso; XVI Jornadas Anuales de la Sociedad Argentina de Biología.; 2014

Of the millions of sperm ejaculated by natural mating, only thousands reach the isthmus of the oviduct. Among them, only a few reach the ampulla at the time of fertilization. Sperm from several species must undergo acrosomal exocytosis (AE) prior to fusing with the egg. It has been long thought that AE took place upon interaction with proteins from the zona pellucida (ZP) of the egg. However, it was recently reported that fertilizing sperm undergo AE prior to the interaction with the ZP. Thus, the physiological site for AE is still unknown. In this work, we provide a detailed description of the process of sperm migration through the oviduct, and the occurrence of AE during this journey by live imaging microscopy, using double transgenic sperm having acrosomal vesicles expressing green EGFP and middle pieces expressing red Ds-Red2 (mitochondria). We observed that acrosome intact sperm were only present in the lower segments of the isthmus. However, once sperm reach the upper segments of the isthmus, around 40 ? 50 % of the sperm were acrosome reacted. In addition, over 95% of the sperm underwent AE prior to entering the ampulla. These results suggest that the physiological AE might take place in the upper segments.