Secondary müllerian system and endometriosis.

The pathogenesis of endometriosis is controversial. The two principal histogenetic theories are (1) metastases of endometrial tissues to an ectopic location (metastatic theory) and (2) metaplastic development of endometrial tissue at the ectopic site (metaplastic theory). Studies on the development of the coelomic cavity and the müllerian duct, as well as expression of CA 125 during the fetal period and in the adult, strongly suggest that coelomic epithelium-related tissues and müllerian-derived epithelia of the adult have a shared embryologic ancestry. Many manifestations of müllerian-directed metaplasia suggest that tissues derived from the coelomic epithelium and its accompanying mesenchymal cells, referred to as the "secondary müllerian system," have the potentiality to differentiate into müllerian-directed epithelium and stroma. This metaplastic potentiality of the secondary müllerian system constitutes a basic concept in the pathogenesis of endometriosis.