Fig. 8. Model of mechanosensory induction of sperm acrosome reaction. (A) Capacitated, motile and acrosome-intact sperm approach the inert polycarbonate filter lacking zona ligands. Depending on the size of the pore, the sperm is slowed or stopped as it seeks to penetrate the filter. The continued forward motility of the sperm transduces a mechanosensory signal that leads to increased intracellular Ca²⁺ and induction of the acrosome reaction. After passage through 3 µm or smaller pores, mostly acrosome-reacted sperm are observed on the far side of the filter. (B) Capacitated, motile and acrosome-intact sperm approach and bind to the zona pellucida. This binding (or the limiting size of the matrix interstices) immobilizes the sperm plasma membrane, inhibiting further progression of the sperm. However, the continued forward motility of the sperm transduces a mechanosensory signal that leads to increased intracellular Ca²⁺ and induction of the acrosome reaction. The residual acrosomal shroud is left behind bound to the surface of the zona pellucida matrix, and only acrosome-reacted sperm enter into the perivitelline space.