Aspects of follicular and oocyte maturation that affect the developmental potential of embryos

The ability to mature, be fertilized and finally to develop into a viable e mbryo is acquired gradually by the oocyte during progressive differentiatio n throughout folliculogenesis. This process starts with oocyte growth durin g the first steps of follicular development. As the oocyte is close to its final size, other modifications occur, less spectacular but at least as imp ortant in determining the resulting ability of the oocyte to accomplish its reproductive purpose (developmental competence). These modifications, refe rred to as 'oocyte capacitation', are probably influenced by the follicle. The proportion of developmentally competent oocytes increases with follicul ar size. However, the relationship between follicular growth and oocyte com petence is not very strict, since a given oocyte may acquire its competence at any stage of follicular growth and since some examples of functional di sjunction between follicular size and oocyte competence are described. Foll icular atresia may impair the acquisition of oocyte competence, as evidence d by the parallel study of follicular characteristics and of the developmen tal potential of their oocytes treated individually through in vitro matura tion, fertilization and development. However, when atresia ii experimentall y induced in large preovulatory follicles, oocytes remain competent, indica ting that once competence is acquired, it is no longer sensitive to atresia . Oocyte maturation represents only the end of this long and progressive pr ocess and validates the preparation of the oocyte by conferring its final d evelopmental ability. As evidenced by recent cloning experiments, the cytop lasmic aspects of oocyte maturation are crucial for the acquisition of deve lopmental competence. This cytoplasmic maturation may be activated in vitro by the use of complex media supplement (serum, follicular fluid) but the u se of defined media for maturation allowed the identification of some activ e factors (such as epidermal growth factor, growth hormone, inhibin and act ivin). The study of some differential models of oocyte competence (follicul ar size and atresia, Booroola gene, prepubertal oocytes) will provide a bet ter understanding of oocyte capacitation and maturation, and allow the impr ovement of in vitro methods for oocyte maturation, which represent the most limiting step of in vitro production of embryos in large mammals.