Corpus Luteum Function in Guinea Pigs, Hamsters, Rats, Mice and Rabbits

Luteal growth, function, and regression in guinea pigs, hamsters, rats, mice and rabbits depend upon complex interrelationships of ovary, pituitary, uterus and conceptus. The relative importance of each of these differs with species. In the guinea pig, the uterus plays the dominant role: the pituitary is not necessary after the zygote reaches the uterus on Day 3 and by Day 20 the placenta or the fetoplacental unit produces sufficient steroid to replace the ovaries. In the hamster, the placenta supplies lactogen which is only partially capable of maintaining the corpora lutea (CL), and although the CL persist as morphological entities in the absence of the pituitary, pregnancy terminates unless FSH (and small amounts of LH) are supplied to sustain ovarian follicles and estrogen production. In the rat and mouse, the ovary is necessary until the last 2 days of pregnancy, but the pituitary is essential for only the first 12 days after which the placenta secretes a luteotrophic complex which sustains luteal function. In the rabbit, a species in which estrogen constitutes the ultimate luteotrophin, both ovary and pituitary are necessary through gestation. In all five species, the non-gravid uterus is luteolytic, and hysterectomy during early pregnancy prolongs the lifespan of the CL. In the pregnant animal the conceptus sustains luteal function and if removed during the later stages of pregnancy the CL regress rapidly. There is convincing evidence that a prostaglandin—probably PGF 2α —is the luteolytic factor produced by the endometrium and that the conceptus exerts its luteotrophic effect either by preventing PG action or by inhibiting its production.