Development of endometriosis from embryonic duct remnants

Among 18 cases of endometriosis involving ovaries and fallopian tubes, we identified three cases with foci of endometriosis adjacent to the embryonic duct remnants in the fallopian tube. The serial sections of the blocks of tissue containing areas of interest showed segments of embryonic duct remnants with changes suggestive of gradual transformation to endometrial glands. The early changes consisted of replacement of the muscular coat surrounding the epithelium of embryonic duct remnants by the endometrial stroma with positive immunoreactivities for estrogen receptor. Subsequently there were changes of the embryonic duct remnant epithelium into endometrial epithelium with diffuse immunoreactivities for estrogen receptor. The significance of this transformation as a mechanism of development of endometriosis is discussed.