Ca2+ Entry during Acrosomal Reaction
An activated sperm elongates its acrosomal process toward an egg. A key regulator of this reaction is the level of Ca2+ ions inside the sperm head. Simultaneous observation of the Ca2+ level (movie, bottom left) and sperm shape (top left) has revealed that the formation of the acrosomal process is initiated when the Ca2+ level reaches a certain threshold, irrespective of the rate of rise of the Ca2+ level.
To visualize the Ca2+ level, the sperm was loaded with the Ca2+-sensitive fluorescent dye indo-1. Green (movie, top right) and blue (bottom right) components of the fluorescence were observed simultaneously, and the Ca2+ level was calculated from their ratio and is shown in false colors (bottom left). Ionomycin was added to induce the acrosomal reaction. At time 0 in the movie, an acrosomal process is seen to elongate toward bottom left in the phase-constrast image at top left.