Electron microscopic observations on sperm entry into eggs of the rose bitterling, Rhodeus ocellatus

Morphological studies on gametes and sperm entry into the egg of Rhodeus ocellatus were conducted using transmission and scanning electron microscopy. The spermatozoon has has a round head, a mitochondrion, and a flagellum but no acrosomal structure. The egg is enclosed in a chorion that has a micropyle at the animal pole. After a spermatozoon attaches to the well-developed microvilli of the egg surface facing the micropyle, it is covered by the microvilli and incorporated into the egg cytoplasm with membrane fusion occurring between gametes. The egg cytoplasm just beneath the micropyle swelled a little while incorporating the spermatozoon and promptly plugged the micropyle. The nuclear envelope of the spermatozoon disappeared first at the apical region of the head and then progressed posteriorly through a process of vesiculation. The nucleus of the sperm head at once initiated decondensation upon incorporation through the effects of the egg cytoplasm. The mechanism of block to polyspermy in Rhodeus eggs is discussed.