Müllerian Inhibiting Substance in Sex-Reversed Dogs

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Abstract
In normal males, Mullerian Inhibiting Substance (MIS), produced by testes during an embryonic critical period, is thought to induce regression of the Mullerian duct system, including the oviducts and uterus. In XX sex-reversed dogs, an apparent contradiction has been reported: The uterus persists in the presence of testes or ovotestes. The objective of this study is to determine whether testes of XX male and ovotestes of true hermaphrodite dogs produce MIS, and to examine the anatomy of Mullerian duct derivatives of affected dogs for evideence of regression. Gonadal samples were tested for MIS activity in a bioassay. The mean MIS activity score of XX males was similar to that of normal XY males and significantly greater than that of normal XX females. The mean MIS activity score of XX true hermaphrodites was intermediate between normal XX females and XY males. Within the true hermaphrodite group, ovotestes in which the proportion of testicular tissue was .gtoreq. 1/2 had higher MIS scores than those in which the proportion of testicular tissue was < 1/2. XX males had a well-developed epididymis adjacent to each testis, but no oviducts. In true hermaphrodites, the oviduct regressed and an epididymis was present when .gtoreq. 1/2 of the adjacent ovotestis was testicular, and MIS activity in that gonad was high. A few ovotestes with intermediate levels of MIS activity had both an oviduct and an epididymis. Regression of the oviductal portion of the Mullerian duct system was positively correlated to the amount of testicular tissue and the MIS activity of the gonad, as would be predicted by Jost''s original hypothesis. However, in all sex-reversed dogs, regardless of the amount of testicular tissue of level of MIS activity, the uterus was present in its entirety and was of normal size. This finding suggests that the uterine and oviductal portions of the Mullerian duct system may differ in their sensitivity to MIS or that regression of the uterine portion is not strictly dependent upon the concentration of MIS.