Fig. 4: The ZP surface structure altered and affected fertilization with aging.

a Classification of SEM images of ZP surface. The regions surrounded by dotted lines are highlighted at the bottom. Magnification x1500 in upper panels, and x10,000 in bottom panels. Type I surface structure has ruggedness with a network and spongy appearance containing numerous pores and fenestrations. Type II shows a more compact structure with fewer pores and smooth surfaces. Type III shows that fenestrations of the outer surface were rare or even absent. b The population of ZP surface classification based on the morphology of Fig. 4a. c, d SEM images of ZP surface in fertilized eggs and unfertilized eggs. The regions surrounded by dotted lines are highlighted at the bottom. Magnification x1500 in the upper panels, and x10,000 in the bottom panels. e The ZP surface classification of fertilized and unfertilized eggs. f, g The number of sperm scars in the oocyte hemispheres.