Introduction THE CYCLIC nature of the female reproductive process is largely dependent upon the unique capability of the ovarian follicle to change in structure and function. A finely tuned process of differentiation occurs in all constituents of the follicle during folliculogenesis. Granulosa cells (GCs) display a high degree of structural change and play a key role in the functional maturation of the entire follicle. These changes in response to endocrine and paracrine stimuli create the unique environment for oocyte growth and ovulation by virtue of their biochemical activity and intimate anatomical relationship. Moreover, the maintenance of early pregnancy depends on transformation of GCs into highly differentiated luteal cells (for review see Refs. 1–3). Our understanding of GC maturation and luteinization has substantially improved in the last 15 yr largely due to the extensive use of the cell culture approach. In vitro induction of differentiation in serum-free media allowed for identification of the molecular factors involved (for review see Ref. 4).