Basic mechanisms of fertilization and parthenogenesis in pigs.

Fertilization of the egg, or oocyte, initiates the entire developmental process, but while the mechanism by which the spermatozoa triggers the oocyte to resume meiosis has been studied extensively, conclusions about this process are still elusive. Some workers have suggested that a molecule on the surface of the spermatozoon may interact with a receptor on the plasma membrane of the oocyte, thereby triggering the oocyte to resume meiosis. Other workers have focused on a factor or factors located in the cytoplasm of the spermatozoa that is deposited into the cytoplasm of the oocyte. A hallmark response to fertilization in mammals is an increase in the cytoplasmic concentration of free Ca2+ in the oocyte. Many additional studies have focused on treatments that will induce the oocyte to resume meiosis without being fertilized. The process of resumption of meiosis without a spermatozoon is generally referred to as activation or parthenogenesis. Activation of the oocyte is very important for a number of oocyte- or embryo-related technologies including intracytoplasmic sperm injection (ICSI) and cloning by nuclear transfer. This review will focus on what is known about fertilization and methods to mimic this process, with an emphasis on pigs.