Movement of spermatozoa and changes in micropyles during fertilization in medaka eggs

In the medaka, movement of spermatozoa and changes in the egg micropyles during fertilization were observed through a video camera and recorded with a video recorder to analyse sperm movement. Movement of spermatozoa as they entered micropyles in both intact and isolated chorions was compared before and after fertilization of the eggs. The inner one third of the micropyle was completely closed and the micropylar vestibule became shallow by 5 min after sperm attachment. Spermatozoa did not increase in swimming speed in the vicinity of the micropyle and were not attracted to it. The majority of spermatozoa entering the micropyle were rotating at an average frequency of 8 Hz in the right‐hand direction. The rotation direction did not correspond to the left‐hand spiral structure of the micropylar wall, though a small amplitude of the beating sperm flagellum corresponded with the narrow micropylar canal. The frequency of sperm entry into the micropyle decreased significantly in intact eggs and isolated chorions following fertilization, independent of artificial occlusion of the micropylar canal. Moreover, in isolated chorions before fertilization, spermatozoa rarely entered the micropylar canal from its inner aperture. The present data suggest the existence of some substance in the micropyle which guides spermatozoa into the micropylar canal, although the perimicropylar depression might also play a role in the guidance of spermatozoa.