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

Oocyte maturation represents the final step of a long differentiation process that allows this very special cell to fully express its reproductive task. During maturation the oocyte nucleus, blocked at the late prophase of meiosis from the end of foetal life, resumes meiosis and progress to the metaphase II stage. Beyond these nuclear aspects, oocyte maturation also involves 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 conditions, maturation occurs in the preovulatory follicle after the ovulatory surge of gonadotropins, in a complex and changing environment. This complexity leaded to the formulation of the first in vitro maturation systems involving tissue culture media supplemented with biological fluids. A more precise study of the effect of individual medium components allowed the design of more simple maturation conditions providing more reproducible results with less sanitary risks. Amongst maturation activating factors, the epidermal growth factor (EGF) seems to play a key function in several species. Other factors such as hormones, ovarian peptides (inhibin, activin) and other growth factors may also be involved but the interplay between these factors remains to be clearly established. Improvement of in vitro maturation techniques allowed to evaluate the importance of oocyte intrafollicular differentiation before maturation on the resulting developmental competence. The increased knowledge of the regulation of intrafollicular meiotic arrest now allow the design of a prematuration step to allow oocytes from smaller follicles to complete their differentiation in vitro. This improvement will allow a larger use of the huge reproductive potential stored in the ovary.