Implantation of viable endometrial cells and metaplasia of one tissue type into another are both reasonable explanations for the occurrence of endometriosis. However, neither theory can account for all aspects of the disease, which could mean that several mechanisms are involved or simply that the theories are inadequate. Both assume that endometriotic tissue consists of 'normal' cells but they fail to explain why development and progression occur only in some women. In contrast, the endometriotic disease theory  considers subtle lesions due to intermittent implantation to be a normal, physiological event. If these cells are transformed because of a genetic insult, they progress to typical, cystic and deep lesions, consisting of 'abnormal' cells. An alternative explanation is that endometriosis is a heterogeneous not a single disease and the different types, which are considered here, result from different disease processes each with their own aetiology .
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