Scanning electron microscopic observations on sperm penetration in teleostean fish

The process by which a spermatozoon penetrates through the micropyle apparatus into the egg cytoplasm in the teleost, Oryzias latipes , was examined with scanning electron microscopy. In the unfertilized egg, the egg surface beneath the inner opening of the micropylar canal did not differ distinctly from the rest of the animal pole area. A spermatozoon attached effectively to the egg 1–3 seconds after insemination. In the initial step of penetration, the spermatozoon still within the micropylar canal attached perpendicularly at its apical tip to the egg surface, then the sperm head was rapidly engulfed by the folded egg surface with its many microvilli. A large blister‐like fertilization cone with a microvillus‐free surface appeared on the egg surface surrounding the penetrating spermatozoon. Concomitantly, breakdown of the cortical alveoli was initiated in the animal pole. The spermatozoon disappeared from the outer surface of the egg before the fertilization cone completely retracted. On the basis of these observations with Oryzias eggs, two steps in the sperm penetration process could be recognized: The first step was a rapid engulfment by the folded egg surface and the second was a slow engulfment by fertilization cone synchronized approximately with the initiation of the cortical reaction. The mechanism of block to polyspermy in teleostean eggs was discussed.