Muellerian ducts (paramesonephric ducts) are small structures that exist in both male and female embryos during the early fetal development, at 35-36 days of gestation, the embryo begins to differentiate into male or female. In the female, Muellerian ducts will develop to form the uterine tubes, uterus, cervix, and the upper one-third of the vagina. In the male, Muellerian ducts are lost because the testes starts producing a hormone that causes regression of the Mullerian ducts.
In Miniature Schnauzer males which are affected by Persistent Müllerian duct syndrome (PMDS), the Müllerian ducts fail to regress during sexual differentiation, and therefore, all Müllerian duct derivatives, bilateral oviducts, a complete uterus with cervix and the cranial portion of the vagina, are also present.
Approximately 50% of affected dogs have normally descended testes and are fertile . However, the remaining 50% have unilateral or bilateral cryptorchidism, and as a result can be infertile and develop testicular tumors.