Histogenesis of morphologic variations in tumors of the uterine wall

We investigated seven uterine wall lesions, selected because of the presence of unexpected patterns or components, to determine their relationship to normal uterine constituents. These lesions included a bizarre leiomyoma, a palisading leiomyoma which resembled a schwannoma, two myxomas, irradiated myometrium, a plexiform tumor, and a leiomyoma with tubular structures. All of these lesions except for the epithelial component of the plexiform tumor showed ultrastructural characteristics of smooth muscle, demonstrating a common origin from the myometrium. Two tumors, the plexiform tumor and the leiomyoma with tubules, had the pattern of epithelial structures suggesting origin from müllerian mesencyme: the plexiform tumor resembled incompletely differentiated endometrium and the tubular structures in the leiomyoma had features of mesothelium. We conclude that these varied and often confusing morphologic patterns reflect the capacity of uterine smooth muscle and stroma to undergo a wide spectrum of alterations including differentiation toward epithelial structures. Changes of this type occur in benign and malignant uterine stromal neoplasms and do not in themselves have prognostic significance.