Parameters influencing the maturation of bovine oocyte: a review

During the past decades, pharmaceutical companies have been making efforts to find alternative methods to animal-based tests. Several tests have been proposed as models for in vitro testing, including oocyte and bovine embryo produced in vitro in developmental toxicity screening. Under optimal experimental conditions, the in vitro maturation rate of bovine oocytes approaches 90%, the fertilisation rate is close to 80%; however, the success in progressing to blastocyst is almost half of those, attaining only 30-40%. It is also known that after in vivo insemination of normally cycling cows, approximately 85% of the ovulated oocytes will develop into an embryo. In contrast to this, in most in vitro production practises, only 15-20% of the oocytes punctured yield transferable embryos. In fact, from oocyte to embryo development, the competency of the female gamete is mandatory to assure the repeatability and feasibility of the results. The developmental competency of an oocyte can be influenced by several factors, including external components: such as the age of the cow, its nutritional status, body condition score, genetic merit for milk yield, proper function of intracellular molecular mechanisms, as well as the season. In this review, the factors influencing the quality of bovine oocytes that could possibly influence the success of producing embryos in vitro are highlighted.