Mechanisms Controlling Corpus Luteum Function in Sheep, Cows, Nonhuman Primates, and Women Especially in Relation to the Time of Luteolysis

Abstract

THE CORPUS luteum (CL) of sheep, cows, and primates consists primarily of luteinized granulosa and thecal cells (1, 2). Luteal cells synthesized and secrete progesterone, and in certain species, estrogen. There appear to be two types of luteal steroidogenic cells in many species, including the sheep (3–5), cow (6–8), rhesus monkey (9, 10), and human female (11–13) which can be distinguished on the basis of their size and morphology; these have been presumed to be derived separately from granulosa and thecal cells and have been assigned different endocrine functions in the sheep (14). By determining the function of these cells at different stages during the life of the CL it may be possible to identify factors causing luteolysis and those required for the maintenance of pregnancy. The primary function of the CL is generally considered to be the production of the steroid progesterone, which prepares the uterine endometrium for implantation and maintains early pregnancy. In order to fulfill this role, the CL must remain viable for a specific period in gestation (Table 1); thereafter [at the time of the luteoplacental shift; Csapo (23)] in most species, the placenta assumes the role of progesterone synthesis for the remainder of the pregnancy and the CL is expendable. If fertilization and implantation do not occur, ovulatory cycles resume only after luteal function ceases; this emphasizes a second role of the CL, that of blocking ovulation.