Pronucleus is the nucleus of sperm or an egg cell during the process of fertilization. The sperm cell becomes pronucleus after the sperm enters the oocyte, but before the genetic material of the sperm and egg fuse. Contrary to the sperm cell, the egg cell became pronucleus then it became haploid, and not when sperm cell enters the ovum.
The sperm dose not becomes a pronucleus until it decondenses quickly inside of female egg. When sperm reaches the egg, the sperm loses its outside membrane as well as its tail as are no longer needed by female ovum. The purpose of the cell membrane is to protect the DNA outer surrounding.
The appearance of two pronuclearis the first sign of successful fertilization as observed during in vitro fertilization and is usually observed during in vitro fertilization and usually observed after 18 hours after insemination or ICSI. The zygote is then term a two-pronuclear zygote (2PN) .Two-pronuclear zygote transitioning through 1PN or 3PN stage tends to develop into poorer quality embryo than one who remain 2PN throughout development and significant in embryo selection in in vitro fertilization.