The pathophysiology of adenomyosis is unclear and likely to be multifactorial, according to UnboundMedicine. One explanation for the condition is that the increased uterine pressure associated with pregnancy makes the junctional zone of the myometrium susceptible to invasion by endometrial cells.
Adenomyosis is the benign invasion of the endometrium into the myometrium, which results in an enlarged uterus. There are various theories for the pathogenesis of adenomyosis, as UnboundMedicine reports. The metaplasia theory suggests that the myometrial smooth muscle cells undergo metaplasia to become endometrial cells. The Mullerian remnants theory suggests that Mullerian remnants in the myometrium develop into endometrium.
The tissue remodelling theory suggests that the endometrial tissue arises from tissue remodelling due to trauma. Examples of such trauma include menstruation, childbirth and spontaneous abortion as well as uterine surgery. The multipotential perivascular theory suggests that multipotential perivascular stem cells cause vascular remodelling as well as vascular smooth muscle proliferation and hypertrophy, leading to an enlarged uterus, as UnboundMedicine explains.
The epithelial-mesenchymal transition theory suggests that increased estrogen concentrations can enhance the growth of endometrial cells, causing them to invade into the myometrium. The mast cell activation theory suggests that nerve growth factor and other mediators from mast cells cause the differentiation of the myometrium into endometrial tissue and the maintenance of the endometrial tissue within the myometrium, as UnboundMedicine details.