The function and regulation of the primate corpus luteum during the fertile menstrual cycle

Figure 2 summarizes the changes in endocrine function and the factors which regulate the primate corpus luteum during the fertile menstrual cycle. The classical luteotropic role of LH during the menstrual cycle is superceded by CG at or before the time of implantation. The role of local factors in modulating luteal function is an area of continued research, as some factors are deemed less important (i.e., estrogen, at least prior to luteal rescue) and other possibilities (progesterone, prostaglandins, and relaxin) arise. The role of local factors has not yet been studied in the corpus luteum following its rescue in early pregnancy. Finally, it is apparent that a different type of "shift" precedes the recognized luteal-placental shift in early pregnancy, when the corpus luteum enhances or begins new activities as progesterone secretion declines. These new or augmented activities occur despite apparent desensitization of CG-responsive cAMP-mediated pathways in luteal cells. Although the cellular events promoting these changes are not known, it seems reasonable to propose that the resulting products, including estrogen (as discussed in Dr. Moudgal's chapter) and relaxin are important in early pregnancy. Thus the term "luteal-placental shift" may be a misnomer, as other activities which promote gestation continue within the corpus luteum for a limited time.