A 40-year odyssey into the mysteries of equine luteolysis

Variation is the principal barrier to progress in unraveling the complexities of biological mechanisms. The resulting slow research progress is well illustrated in the chronology of events in elucidating the mechanism for regression of the corpus luteum (luteolysis) during the equine estrous cycle. Many of the underlying foundations of the female reproductive system in farm animals were developed during the 1930s to 1950s, despite the lack of methods for determining the concentrations of circulating hormones. In the 1960s, a uterine luteolysin was postulated on the basis of several experimental approaches that indicated that the uterus was responsible for luteolysis in several farm and laboratory species. Extensive research in many laboratories led to the acceptance that prostaglandin F_2α was the uterine luteolysin and could be used for the practical control of the life of the corpus luteum. The mare was a unique research tool in the study of uterine-induced luteolysis. The chronology of progress from 1970 to 2009 in studying the mechanisms of equine luteolysis is outlined and discussed as an example of a lengthy and ongoing struggle with nature to reveal the intricacies of a biological mechanism.