Variation in Progesterone Content of the Bovine Corpus Luteum of the Estrual Cycle

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Abstract
Corpora lutea were studied at 3, 5, 7, 9, 11, 13, 15, 17 and 19 days after estrus in virgin dairy heifers to determine changes in progesterone, 4-pregnene-20 β-0l 3-one, cholesterol, DNA, RNA and percent type I and II (Foley and Greenstein, 1958) luteal cells. For each day from 7 through 17, two corpora lutea were obtained at the same stage of two different estrual cycles from each of two heifers via supra vaginal incision. For days-3, -5 and -19, corpora lutea were obtained at slaughter either from the above heifers or from similar heifers. An analysis of variance on the values for day-7 through -17 showed a significant increase in progesterone concentration and content (P<.01), 4-pregnene-20 β-ol-3-one concentration (P<.05) and percent type I and II cells (P<.01) to day-15 followed by a significant drop at day-17. Free cholesterol concentration paralleled approximately the increase in progestogen concentration to day-15, but continued to increase at day-17 after the progestogens had dropped. The concentratration of DNA and RNA did not differ significantly among days-7 through -17, but the ratio of RNA to DNA decreased significantly (P<.05) from day-13 to day-17. Corpora lutea weights did not differ significantly from day-7 through -17, with regressive changes beginning at day-17. The percent dry weight of luteal tissue increased significantly from day-7 through -17 and appeared to be progressive throughout the estrual cycle. Significant heifer repeatabilities were found for corpus luteum weight (0.54), 4-pregnene-20 β-ol-3-one concentration (0.65) and DNA concentration (0.90). It would appear that the functional activity of the bovine corpus luteum of the estrual cycle increases to about day-15 and then declines as shown by a decrease in corpus luteum weight, progestogen concentration and content, RNA/DNA and percent type I and II luteal cells.