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Role of the Uterus in Corpus Luteum Function¹

Iowa State University, Ames

Abstract

The initiation of corpus luteum regression is delayed by hysterectomy of the cyclic guinea pig, pig, cow and ewe. After uterine removal, luteal regression and follicular development are not changed in the unmated opossum, brush possum, 13-lined ground squirrel, mouse, rat, hamster, rabbit, dog, ferret, cat and monkey. In the human, hysterectomy may prolong the functional life span of the corpus luteum. Luteal maintenance is extended after hysterectomy of the pseudopregnant mouse, rat, hamster and rabbit. Investigations involving partial hysterectomy indicated that a threshold amount of functional uterus is necessary for the initiation of luteal regression in the guinea pig, pig, cow and ewe.

In species showing luteal persistence following hysterectomy, either an intact uterus or a uterine graft appears to produce a luteolytic factor(s) responsible for the initiation of luteal regression. This factor(s) acts locally as well as by systemic pathways. It may be carried by blood or lymph or it may diffuse through extra-vascular tissue spaces to its site of action. Uterine or endometrial grafts induce luteal regression in the hysterectomized-pseudopregnant hamster, rat, and rabbit as well as in the cyclic guinea pig, ewe and pig. A functional endometrium, as indicated by the presence of proliferated endometrial glands, is necessary for initiating luteal regression in these species with uterine transplants.

Proof of a uterine luteolytic factor(s) is indirect and depends upon the prolongation of the life span and secretory activity of the corpus luteum after hysterectomy. Furthermore, the factor(s) has not been characterized or isolated. Distention of uterine horns by beads or other foreign objects induces premature luteal regression and shortening of the estrous cycle in the guinea Dig, ewe and cow, though cvcles are unchanged in the pig.

A local mechanism for the mediation of a luteolytic factor(s) is suggested in several species. In the guinea pig, unilateral distension causes luteal regression in the adiacpnt ovarvbut not in the opposite; hysterectomy causes maintenance in the adiacent ovary but not in the opposite. When unilateral hvsterectomy is combined with unilateral ovariectomy in cyclic guinea pigs, pigs and sheep as well as in pseudopregnant rats, luteal life span is extended if the retained horn is opposite to the side of the retained ovary. Furthermore, the corpus luteum persists for a longer period of time in unilaterally hysterectomized heifers and ewes in which the retained horn is opposite to the corpus luteum than in animals with the retained horn adjacent to the corpus luteum.

Metabolism of gonadal steroids is possibly modified after hysterectomy thereby affecting the life span of the corpus luteum. The luteo-lytic effect may result from uterine metabolism of either ovarian or luteal steroids or both. It appears that progesterone is involved in the development of the uterine lytic state in the ewe. Oxytocin induced luteolysis, in the presence of the uterus, is unique in the cow. Local utero-ovarian mechanisms probably are involved in the estrous cycle shortening of oxytocin-treated cattle.

Additional evidence indicating the important role of the uterus in corpus luteum function is found in hypophysectomized or hypophysial stalk transectioned sheep, pigs and cows. The life span of the corpus luteum is prolonged in these animals in the absence of the uterus. Administration of luteinizing hormone or unfractionated ovine or porcine pituitary maintains the secretory function of the corpora lutea in hypophysectomized-hysterec-tomized pigs, but is ineffective in similar animals with an intact uterus.

¹ This work was supported by U. S. Public Health Service Grant HD 01168-08, National Institutes of Health and by the American Cyanamid Co., Princeton, N. J. Acknowledgment is made of the interest and generous assistance given during the preparation of this review by Dr. R. M. Moor, A.R.C. Unit of Reproductive Physiology and Biochemistry, University of Cambridge. Journal Paper No. J-5702 of the Iowa Agricultural and Home Economics Experiment Station, Ames. (Project No. 1325).