Hamster sperm penetration of the zona pellucida: Kinematic analysis and mechanical implications

There have been few direct observations of penetration of the zona pellucida by spermatozoa, and no detailed description of the kinematics of this process. Such information is important in evaluating the contribution of mechanical thrust by the sperm flagellum to the mechanism of zona penetration by the sperm head. To make such observations, small numbers of hamster spermatozoa were inseminated to cumulus masses slightly compressed (150 μm) between a slide and coverglass. Observations were made with interference contrast optics and videorecorded at 60 fields/sec. A total of 63 penetrating spermatozoa were recorded, of which 21 were penetrating completely cumulus-intact zonae. Direct comparison of penetration angles for cumulus-intact and cumulus-dispersed zonae suggested that the cumulus may be important in reorientation of penetrating spermatozoa, which initially lie flat on the zona surface. The beat shape during zona penetration was more complex than the simple sinusoidal waves used previously in modeling the mechanics of sperm-zona interaction. Motility during zona penetration was bimodal, having high-amplitude, low-frequency lever strokes, alternating with low-amplitude, high-frequency propagated sinusoidal waves. The completely asymmetric lever mode and the oscillatory motions of the curved leading edge of the sperm head within the zona may afford significant mechanical advantages to spermatozoa in forcing their way through that matrix. Initial calculations of the maximum force exerted by the sperm head against the zona material during lever strokes predicted values as high as 2700 μdyn. This result is two orders of magnitude higher than that previously estimated assuming more simple flagellar motility. Although not conclusive, our observations and analysis support the concept that zona penetration is more efficient when the cumulus is present, and that this may be due, in part, to a mechanical advantage conferred upon the sperm by the cumulus material.