Histologically, they consist of well-differentiated endometrial stromal cells with a plexiform capillary network, which may invade the myometrium in a tongue-like fashion and may involve lymphatic channels (Hence, the old terminology endolymphatic
stromal myosis).
In the 1976 article by Clement and Scully, (2) an origin from the endometrial stromal cells, adenomyosis,
stromal myosis, endometriosis, or multipotential myometrial cells was postulated for these stromal tumors.