Oocyte from domestic mammals: a model for the study of in vitro maturation

Oocyte maturation represents the final step of a long differentiation proce ss that allows this very special cell to fully express ifs reproductive tas k. During maturation the oocyte nucleus, blocked at the late prophase of me iosis from the end of foetal life, resumes meiosis and progress to the meta phase II stage, Beyond these nuclear aspects, oocyte maturation also involv es cytoplasmic modifications including well known morphological progression as well as poorly understood biochemical changes that are determinant for successful fertilisation and early embryo development. In physiological con ditions, maturation occurs in the preovulatory follicle after the ovulatory surge of gonadotropins, in a complex and changing environment This complex ity leaded to the formulation of the first in vitro maturation systems invo lving tissue culture media supplemented with biological fluids. A more prec ise study of the effect of individual medium components allowed the design of more simple maturation conditions providing more reproducible results wi th less sanitary risks. Amongst maturation activating factors, the epiderma l growth factor (EGF) seems to play a key function in several species, Othe r factors such as hormones, ovarian peptides (inhibin, activin) and other g rowth factors may also be involved but the interplay between these factors remains to be clearly established. Improvement of in vitro maturation techn iques allowed to evaluate the importance of oocyte intrafollicular differen tiation before maturation on the resulting developmental competence. The in creased knowledge of the regulation of intrafollicular meiotic arrest now a llow the design of a prematuration step to allow oocytes from smaller folli cles to complete their differentiation in vitro This improvement will allow a larger use of the huge reproductive potential stored in the ovary.