Regulation of luteal regression: the ewe as a model

This article focuses on mechanisms that regulate functional and structural regression of the corpus luteum (CL) with special emphasis on the role of prostaglandin F2 alpha (PGF2 alpha) in mediating these events in large luteal cells in the ewe. Progesterone produced by the CL is absolutely required in all mammals for implantation and early maintenance of pregnancy. Luteal regression at the end of the nonpregnant cycle involves loss of progesterone production and tissue destruction via physiologic cell death, apoptosis. These are distinct events termed functional and structural regression, respectively. In many mammals, including ewes, luteal regression is initiated by prostaglandin F2 alpha (PGF2 alpha) of uterine origin. However, the exact mechanisms of this regulation are not well understood. Functional regression appears to be directly stimulated by PGF2 alpha via activation of its membrane receptor. Whether structural regression is also initiated directly by PGF2 alpha is not known. The ovine CL contains two morphologically and functionally distinct steroidogenic cell types, designated small and large. Receptors for PGF2 alpha are exclusively located on large cells. Thus, the signal for regression is received in those cells. These data provide strong evidence that the intracellular determinant of regression resides within the large cell.