Local role of progesterone in the ovary during the periovulatory interval.

Abstract

An intraovarian role for steroids in periovulatory events was postulated as far back as the early 1970s, when Rondell hypothesized that steroids facilitate the actions of gonadotropins on follicle rupture [1]. Subsequent research indicated that progesterone (P) may be one of the mediators of periovulatory events [2,3] and Rothchild [4] provided an elegant treatise formulating the theory that progesterone was a local luteotropin promoting the development of the corpus luteum. However, it was not until after 1988, when investigators discovered that P receptors (PR) were expressed in the luteinizing granulosa cells of the ovulatory follicle in several species (monkey [5]; human [6]; rabbit [7]; rat [8,9]; chicken [10]), that the possibility of progesterone acting in a universal manner to regulate ovarian events and ultimately cyclicity, received attention. Since then, investigators have performed a variety of in vivo and in vitro studies in a number of species to examine the role of progesterone in periovulatory events. These studies generally attempted to characterize progesterone’s role by preventing progesterone action via one of three approaches: (1) administration of a PR antagonist, (2) inhibiting progesterone synthesis, or (3) studying the PR null (PR-/-) mutant mouse. While each approach has its advantages and disadvantages, collectively they provide unequivocal evidence that progesterone is an essential mediator of ovulation in all mammalian species examined to date [2,3,11–13]. Futhermore, there are suggestions, albeit less evidence, that progesterone also performs important roles in the development and/or function of the corpus luteum, particularly in species such as the monkey and cow that have a long functional luteal phase during the ovarian cycle. With recent breakthroughs in analyses of gene expression, investigators are beginning to identify progesterone-regulated genes in the ovary. The aim of this review is to highlight recent findings that demonstrate progesterone actions, and progesterone-regulated gene activity, that are associated with and perhaps critical for ovarian events, including follicle rupture and formation of the corpus luteum.