Electron microscopic observations of the distribution of acidic anionic residues on hamster spermatozoa and eggs before and during fertilization

Abstract

Acidic anionic residues on the surfaces of the hamster eggs and spermatozoa were investigated using a colloidal iron hydroxide (CIH)-labeling technique. Both the zona pellucida and the plasma membrane of the egg are abundant in acidic anionic residues, and no obvious changes in the densities or distributions of these residues were detected before and after fertilization. Also, the relative distribution and density of acidic anionic residues on the plasma membranes of spermatozoa are not markedly changed during sperm capacitation, indicating that the number and distribution of anionic residues, per se, are probably not directly involved in this process. Prior to fusion of sperm and egg plasma membranes, the sperm head plasma membrane does not bind CIH, but once fusion has started the fused sperm—egg membrane does bind CIH. This suggests that the sperm and egg plasma membrane components intermix during gamete fusion.