Fusion of Egg and Sperm Pro-Nuclei
The fusion of the sperm and egg which is the beginning of the fertilization process is completed with the fusion of the male and female pronuclei to from the zygote nucleus called synkaryon. Usually the nuclear fusion occurs between 10 to 20 minutes after attachment of the sperms to the egg. In sea urchins the completion of second meiotic division occurs after the entry of the sperm and the resulting haploid egg nucleus is known as female pronucleus. The sperm nucleus after its entry into the ovum is known as male pronucleus. After its entry in the ovum there is a breakdown in the sperm nuclear envelop, decondensation of chromatin and the formation of the pronuclear envelope.
In sea urchins at the completion of meiosis the female pronucleus is located in the central region of the egg and the male nucleus is located initially in the cortical region of the egg at the site of its entry. Before the fusion of the pronuclei, the sperm pronucleus has to migrate a considerable distance through the egg cytoplasm. In sea urchins a structure called sperm aster mediates this movement of the pronucleus. The sperm aster is a complex of long microtubules that radiate from the paired sperm centrioles. The centrioles which enter into the egg along with the sperm nucleus form the microtubule organising centre for the sperm aster. The microtubules of the sperm aster push the male pronucleus towards the centre of the egg. The astral microtubules also contact female pronucleus and pull it abruptly towards the male pronucleus. This activity continues until the two pronuclei are displaced to the centre of the egg where they fuse to form the synkaryon.