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T HE effects of exogenous progesterone on the corpus luteum are quite variable. No gross or microscopic effects were apparent in the pregnant rat (Sammelwitz et al., 1956; Aldred et al., 1959) or rabbit (Ulberg, 1952). Selye (1939) reported no effect of progesterone on corpora lutea of late pregnancy in mice, but Burdick (1942) found that treatment begun one or two days post-mating caused luteal regression. Whether or not progesterone is detrimental to corpora lutea, and the extent to which the corpora lutea are damaged, appears to depend upon the time of injection in gilts (Sammelwitz and Nalbandov, 1958) and ewes (Zimbelman et al., 1959). Spies et al. (1958) found that progesterone treatment from the 10th to the 25th day after mating caused regression of corpora lutea in hysterectomized as well as in pregnant gilts, indicating that progesterone acts independently of the uterus. Estrogen treatment has a beneficial effect on corpora lutea in rats (Hohlweg, 1934; Desclin, 1935; Selye et al., 1935; Wolfe, 1935; Merckel and Nelson, 1940) and rabbits (Allen and Heckel, 1936; Heckel and Allen, 1939). Kidder et al. (1955) found that stilbestrol caused luteinization of follicles when injected on dayl l of the estrual cycle of gilts. Hammond and Day (1944) observed that corpora lutea persisted for prolonged periods in heifers implanted with stilbestrol, while Greenstein et al. (1958) reported that estradiol administered from day-2 through day-12 of the estrual cycle caused early regression of corpora lutea. The purpose of this study was to determine the effects of injected progesterone and estrogen on the progesterone content, proportion of functional luteal cells, and weight of corpora lutea in open heifers.

Effects of Injected Ovarian Hormones on the Corpus Luteum of the Estrual Cycle in Cattle