Effect of melengestrol acetate on the bovine ovary

Summary To investigate the relation of ovarian morphology to increased body weight gain during administration of melengestrol acetate (17-acetoxy-6-methyl-16-methylenepregna-4,6-diene-3,20-dione, or MGA, a progestagen), ovaries of 100 Holstein dairy heifers were examined for characteristics of follicular maturation, signs of lipid secretion, and the presence of steroid-producing-type cell organelles. The heifers were fed either a normal or a high-protein level ration with or without 0.30–0.45 mg daily of MGA from 2.5 months of age to the age of first estrus or to breeding age (120 cm withers height). The high-level ration effect was not consistently mediated by the special development of ovarian structures. MGA effect, like the weight-gain effect, was consistently evident after the time of first estrus. MGA suppressed estrus and ovulation, and resulted in the presence in the ovary, after regression of corpora lutea, of an increased number of large follicles. The largest follicles corresponding in structure to developing proestrous, mature estrous, or early atretic follicles contained hypertrophied theca interna cells showing lipid secretion signs and ultrastructural characteristics of steroid synthesis. The majority of the largest follicles, however, were atretic or late atretic and did not contain steroid-type cells. The development of replacement follicles was evident in ovaries with late atretic largest follicles. The weight-gain effect of MGA may therefore be associated with a steroidogenic, probably estrogenic, effect in the cow.Dedicated to Prof. W. Bargmann of Kiel.This investigation was supported by scholarships of the Ministère Français des Affaires Étrangères and the Alexander von Humboldt-Stiftung to Dr. J. Priedkalns.Drs. R. G. Zimbelman and L.S. Goyings of the Upjohn Company, Kalamazoo, Michigan, and Drs. D.E. Pritchard, R.W. Purchas, H.A. Tucker, H.D. Hafs, and L.J. Boyd of the Department of Dairy Science, Michigan State University, East Lansing, Michigan, designed and conducted a study on the effects of melengestrol acetate on Holstein heifers and provided ovaries for this investigation. The author wishes to thank Dr. J. W. Lauderdale of the Upjohn Company for aid in statistical analyses; the Department of Veterinary Anatomy, University of Minnesota, St. Paul, Minnesota, and the Laboratoire d'Histologie de l'École Nationale Vétérinaire de Lyon, for assistance in the processing of the ovarian tissues for electron microscopic examination; and the Laboratoire d'Histophysiologie du Collège de France, Paris, for providing facilities for a part of the histochemical study of the ovaries.A brief report of this investigation has been presented to the Société des Sciences Vétérinaires de Lyon (Priedkalns, 1971).