Sperm Incorporation Independent of Fertilization Cone Formation in the Danio Egg:...

The responses of the egg to insemination in a modified Fish Ringer's solution (FRS) were examined in eggs of the zebrafish (Brachydanio rerio) primarily by scanning electron microscopy. FRS is a physiological saline which temporarily inhibits parthenogenetic activation of the egg for 5-8 min. Spermatozoa were collected in a small volume of water and pipetted over eggs in FRS. Eggs inseminated in FRS typically incorporated the fertilizing sperm within 3-4 min. Inseminated cells showed an absence of a fertilization cone and no cortical granule exocytosis. The deep conical depression in the egg surface beneath the micropyle remained unaltered. Control eggs inseminated in tank water developed a large fertilization cone during sperm incorporation. Occasionally, eggs inseminated in water were observed to incorporate the entire sperm head prior to egg activation. Our results corroborate earlier findings showing that in the zebrafish, cortical granule exocytosis, fertilization cone formation and elevation of the sperm entry site are not triggered by the fertilizing sperm in experimental conditions (18, 19). Furthermore, sperm incorporation requires neither egg activation nor formation of a fertilization cone in this fish.