Fertilization Cone of Carp Eggs as Revealed by Scanning Electron Microscopy: fertilization cone/carp egg/scanning electron microscopy)

The process of formation of the fertilization cone in carp eggs was examined by scanning electron microscopy. The fertilized eggs responded to penetration of one sperm by primary and secondary steps of formation of a fertilization cone of unique morphology. In the primary step, the earliest fertilization cone was seen at the superior or anterosuperior part of a fused sperm head in inseminated eggs fixed 20 sec after immersion in fresh water. The cone reached a maximum of more than 10 μm in length and 3-4 μm in thickness by 40 sec, resulting in a transient plugging of the micropylar canal. In the secondary step, usually seen at 105-120 sec, a conformation reminiscent of a very small caldera volcano was formed, with the shortened earlier cone and part of the sperm tail at its top. By 2.5 min, the fertilization cone had become conical, and the sperm tail still extended from its top. At 3 min, the sperm tail was often not detectable, but a cytoplasmic eminence was still seen as a trace of the fertilization cone. The role of the earlier fertilization cone in blocking polyspermy is discussed.