Oocyte biology and challenges in developing in vitro maturation systems in the domestic dog.

The oocyte of the domestic dog is unique from that of other mammalian species studied to date. Ovulation occurs either once or twice per year, with the oocyte released at the germinal vesicle stage, and then completing nuclear and cytoplasmic maturation within the oviduct under the influence of rising circulating progesterone. In vivo meiotic maturation of the bitch oocyte is completed within 48-72 h after ovulation, which is longer than 12-36 h required for oocytes from most other mammalian species. Due to these inherently novel traits, in vitro culture systems developed for maturing oocytes of other species have been found inadequate for maturation of dog oocytes. On average, only 15-20% of ovarian oocytes achieve the metaphase II stage after 48-72 h of in vitro culture. Thus far, no offspring have been produced in the dog (or other canids) by transferring embryos derived from in vitro matured oocytes. This review addresses current knowledge about dog reproductive physiology, specifically those factors influencing in vitro developmental competence of the oocyte. This summary lays a foundation for identifying the next steps to understanding the mechanisms regulating meiotic maturation and developmental competence of the dog oocyte.