Graafian follicle function and luteinization in nonprimates.

The ovarian follicle has two major functions. It provides for the maturation and release of a fertilizable oocyte. It also forms the corpus luteum, which promotes and maintains implantation of the embryo. For these processes to occur, the follicle has to grow, ovulate, and luteinize. The transition of granulosa cells to luteal cells requires rapid changes in the expression of specific genes. In this transition, many genes expressed in granulosa cells are turned off, whereas the genes controlling terminal differentiation to luteal cells are induced. Of major importance are changes in the regulation of specific kinase cascades that regulate proliferation and differentiation as well as specific protease cascades that control the remodeling of the follicle during ovulation and formation of the corpus luteum. The dramatic reprogramming of gene expression in granulosa cells leading to that of luteal cells is complete within 5 to 7 hours and is irreversible.