CONTROL OF OOCYTE GROWTH AND MATURATION BY FOLLICULAR CELLS AND MOLECULES PRESENT IN FOLLICULAR-FLUID - A REVIEW

The aim of this review is to summarize the interactions between the oo cyte and its surrounding granulosa cells which are involved in the con trol of oocyte growth or apoptosis as well as those playing a key role in the ability of the oocyte to undergo nuclear (resumption as meiosi s to reach the MII stage) or cytoplasmic maturation (ability to fertil ize and develop to the blastocyst stage). The respective roles of the oocyte and of the granulosa cells in controlling the initiation of gro wth are poorly understood. During the preantral follicular stage when most oocyte growth is achieved, a local regulation appears to be in op eration involving growth factors such as fibroblast growth factor (FGF ) or epidermal growth factor/transforming growth factor alpha (EGF/TGF alpha), together with two proteins (c-kit present on the oocyte's mem brane and its ligand KL produced by granulosa cells). Insitu technique s used to detect apoptosis demonstrate apoptotic oocytes in the reserv es of primordial follicles but seldom within preantral follicles (beca use it is too fast?). Proteins involved in cell death (bax) or cell su rvival (bcl2) are present in oocytes as well as compounds (TNF alpha, Fas) involved in the initiation of apoptosis. However, the molecular a nd cellular mechanisms triggering oocyte apoptosis are not fully clari fied. Three approaches have been used to identify compounds which are relevant to the oocyte's nuclear or cytoplasmic maturation. a) Correla tion between amounts of specific compounds in follicular fluid or with in follicle cells and the oocyte's ability to mature. b) Analysis of t he consequences of pharmacological disruption of mechanisms such as st eroidogenesis on oocyte maturation, c) Analysis of the consequences of addition of graded amounts of specific compounds on oocyte maturation in defined media. Factors playing a key role in stimulating nuclear m aturation appear to be epidermal growth factor (EGF) and the inhibin ( cattle)/activin (rodents) family, while testosterone has an inhibitory effect. Cytoplasmic maturation of the oocyte appears to be stimulated by oestradiol, EGF and inhibin. (C)Inra/Elsevier, Paris.