MOVEMENT OF SPERMATOZOA AND CHANGES IN MICROPYLES DURING FERTILIZATION IN MEDAKA EGGS

In the medaka, movement of spermatozoa and changes in the egg micropyl es during fertilization were observed through a video camera and recor ded with a video recorder to analyse sperm movement. Movement of sperm atozoa as they entered micropyles in both intact and isolated chorions was compared before and after fertilization of the eggs. The inner on e third of the micropyle was completely closed and the micropylar vest ibule became shallow by 5 min after sperm attachment. Spermatozoa did not increase in swimming speed in the vicinity of the micropyle and we re not attracted to it. The majority of spermatozoa entering the micro pyle were rotating at an average frequency of 8 Hz in the right-hand d irection. The rotation direction did not correspond to the left-hand s piral structure of the micropylar wall, though a small amplitude of th e beating sperm flagellum corresponded with the narrow micropylar cana l. The frequency of sperm entry into the micropyle decreased significa ntly in intact eggs and isolated chorions following fertilization, ind ependent of artificial occlusion of the micropylar canal. Moreover, in isolated chorions before fertilization, spermatozoa rarely entered th e micropylar canal from its inner aperture. The present data suggest t he existence of some substance in the micropyle which guides spermatoz oa into the micropylar canal, although the perimicropylar depression m ight also play a role in the guidance of spermatozoa.