Ultrastructural study on pronuclear development in fertilized eggs from goldfish, Carassius auratus

The formation of male and female pronuclei in physiologically monospermic fertilized eggs of the goldfish, Carassius auratus, has been investigated with transmission electron microscopy. Ultrastructural observations show that at 26°C the transformation of the sperm nucleus takes place very quickly. The sperm nuclear envelope degenerates and is replaced by a large number of smooth surface vesicles 1 min post-insemination. Concomitantly, most of the condensed sperm chromatin is dispersed and is surrounded by vesicles. Dispersion of the chromatin is followed by the fusion of vesicles and the formation of a new bilaminar pronuclear envelope. Within 5-10 min post-insemination, a spheroid male pronucleus with intranuclear annulate lamellae is produced. The formation of a female pronucleus is slightly different to that of the male pronucleus. The dispersing chromatin of the egg is divided into many groups, most of which are surrounded by multilaminar envelopes 5 min post-insemination. An ellipsoid female pronucleus with a continuous bilaminar pronuclear envelope and intranuclear annulate lamellae is formed 15 min post-insemination. Subsequently, the two pronuclei migrate towards one another. When the fully developed male and female pronuclei are located in the center of the blastodisc, each changes itself into a saccular complex 25 min post-insemination.