Analysis of the complexity of the sperm acrosomal membrane by super-resolution stimulated emission depletion microscopy compared with transmission electron microscopy.

The acrosome is a Golgi-derived sperm cell organelle enclosed by a continuous acrosomal membrane. The acrosomal membrane complexes with surrounding matrices containing molecules necessary for fertilization; however, the complex of acrosomal membrane and associating matrices (CAMAM) has not been visualized in detail under living conditions. Here, we analyzed the CAMAM at the nanometer level using super-resolution stimulated emission depletion (STED) fluorescence microscopy and equatorin-enhanced green fluorescent protein transgenic mice. The STED images were compared with the corresponding images taken by immunoelectron microscopy. Consequently, the substructure of CAMAM could be differentiated at nanometer-scale resolution by STED microscopy without the need for sectioning. The information obtained in this study will be beneficial not only for understanding the molecular mechanism of fertilization but also for cell imaging under living conditions.