The exact mechanisms by which endometriosis develops are not clearly understood. Three major theories are com-monly cited.
· Direct implantation of endometrial cells, typically by means of retrograde menstruation. This mechanism is consistent with the occurrence of pelvic endome-triosis and its predilection for the ovaries and pelvic peritoneum, as well as for sites such as an abdominal incision or episiotomy scar. Direct implantation is com-monly referred to as Sampson’s theory because of his experimental work that showed the possibility of such a mechanism.
· Vascular and lymphatic dissemination of endome-trial cells (Halban’s theory). Distant sites of endometrio-sis can be explained by this process (i.e., endometriosis in locations such as lymph nodes, the pleural cavity, and kidney).
· Coelomic metaplasia of multipotential cells in theperitoneal cavity (Meyer’s theory) states that, under cer-tain conditions, these cells can develop into functional endometrial tissue. This could even occur in response to the irritation caused by retrograde menstruation. The early development of endometriosis in some adolescents before the onset of menstruation lends credence to this theory.
It is probable that more than one theory is necessary to explain the diverse nature and locations of endometriosis. Underlying all these possibilities is a yet undiscovered immunologic factor that would explain why some women develop endometriosis whereas others with similar charac-teristics do not.