Structural and rheological properties conferring fertilization competence to Xenopus egg-coating envelope.

The extracellular egg-coating envelope that comprises a meshwork of filaments polymerized by glycoproteins plays a pivotal role in species-selective sperm recognition and subsequent fertilization; however, the structural and rheological properties conferring fertilization competence to the egg-coating envelope remain poorly unveiled. Here we show several nanoscale-structural and viscoelastic properties of the egg-coat using the transmission electron microscopy and the quartz crystal microbalance experiments, following clamp of the egg-coat at either fertilization-competent or -incompetent statuses by short-term pretreatment with synthetic peptides. Individual filament of approximately 4.8 nm diameter crossed one another, forming several types of intersections. Higher competence-inducing treatment changed the proportion of V-, Y-, and T-type intersections, and induced more randomly deflected angles at intersections. Incompetence-inducing treatment increased the median of a Gaussian distribution of filament lengths that had a peak of 10-20 nm under control conditions; furthermore, this treatment created bumps in the 30-40 and 50-60 nm windows. Quartz crystal microbalance study revealed that viscoelasticity of the competent VE suspension was lower than that of incompetent VE, indicating that viscoelastic property required for successful fertilization resides within a specific range. These findings indicated that the architecture of the egg-coat is capable of rapid and dynamic remodeling, which determines fertilization efficiency.