Formation and maintenance of corpora lutea in laboratory animals.

T HE past decade has witnessed a revival of interest in the mammalian corpus luteum. The availability of new techniques is in part responsible but a more important factor has been the increasing use of cows, sheep, pigs and even women as experimental species. The use of these mammals has uncovered, to a much greater degree than was expected, a diversity in the physiology of the corpus luteum, as well as in mechanisms regulating other reproductive phenomena. This has been the main impetus for the renewed attack on problems of luteal physiology. For the common laboratory species--guinea pig, hamster, mouse, rat and rabbit--a long history of histophysiologic studies is available, dating back to the pioneering investigations of the first 25 yr. of this century. In fact, we are deluged with an embarrassing wealth of information, which is gradually and unfortunately being given less attention than it deserves in the training of younger investigators. The purpose of this paper is to summarize, on a comparative basis, knowledge of the corpus luteum and the factors involved in its growth and regression in the common laboratory species. Topics to be considered by other speakers at this symposium have been deliberately excluded from this review. The five species under discussion differ from one another in the characteristics of their estrous cycle and in the conditions under which the corpora lutea are formed and function (for details and references see Eckstein and Zuckerman, 1956). The mouse, hamster and rat have cycles of 4to 6-days' duration,