(pronounced 'mool-air-ian') Syn. paramesonephric duct. The internal female sex duct, which forms on each side of a female embryo to connect the peritoneal cavity with the outside of the embryo, starting at a point close to the ovary and forming first a fallopian tube then meeting its fellow from the other side to form the uterus, and then extending downwards to form the upper part of the vagina before finally connecting with a little dimple between the urethra (in front) and the anus (behind) to reach the exterior at the vulva. Passage of eggs from the ovaries (which ovulate into the peritoneal cavity of all vertebrate species) to the outside world through this duct is how animals, including humans, reproduce. In males the ducts do not develop because the Sertoli cells of the testes produce anti-Mullerian hormone.

A hormone produced by the Sertoli cells of the testes in a male embryo to suppress the development of the Mullerian ducts.

A uterine anomaly in which the Mullerian ducts, before birth, do not join completely, with the consequence that there is a double uterus, in which each of the two sides is smaller than a normal uterus and receives just one fallopian tube. Diagnosed by hysterosalpingogram, by hysteroscopy and laparoscopy, or by transvaginal ultrasound (preferably three-dimensional ultrasound).

A mucus-like protein produced in some circumstances by surface cells of tissues derived from the Mullerian ducts. Its function is obscure but measurement as serum CA125 antigen can be useful in diagnosing adenomyosis, endometriosis and some cancers of the ovary.

A duct on each side of the developing embryo or fetus that leads from the embryo's mesonephros, or temporary kidney. In male fetuses the duct persists as the Wolffian duct to form the rete testis, the epididymis, the vas deferens and the seminal vesicle. In female fetuses it usually disappears completely (in favor of the paramesonephric duct, better known as the Mullerian duct), though portions of mesonephric duct can remain as harmless cysts, called Gartner's duct cysts.

A uterine anomaly that comes about when the uterus forms (in the embryo) from just one Mullerian duct; the uterus will be a little smaller than normal (making a miscarriage or premature labor more likely), and will be connected by a fallopian tube to just one ovary, contributing slightly to infertility (since, on average, only half of ovulations have a chance of resulting in pregnancy). Reproduction, however, can be normal, and a woman might go through life with a unicornuate uterus and not know it. Diagnosed by hysteroscopy and laparoscopy, by hysterosalpingogram or -- particularly effectively -- by three-dimensional ultrasound. Often there is a simultaneous abnormality of the kidneys, such as one kidney instead of two, diagnosable by abdominal ultrasound or, more specifically, by a special kidney x-ray study called an intravenous pyelogram.

The womb, in which pregnancy is gestated from the time of implantation of the embryo until delivery or miscarriage; formed from the joining of the two Mullerian ducts (in the absence of anti-Mullerian hormone); composed of the main, upper part (the uterine fundus) and a lower neck, or cervix, which connects it to the upper part of the vagina; most of its wall is made of muscle tissue (the myometrium), but with an inner lining of glands (the endometrium) and, on the outer surface, a thin covering of uterine serosa. Best seen on transvaginal ultrasound, especially a three-dimensional ultrasound; its cavity is best displayed with a hysterosalpingogram (or, strictly, a hysterogram).