Smooth muscle differentiation at endometrio-myometrial junction. An ultrastructural study.

To determine whether myometrial smooth muscle is newly produced at the endometrio-myometrial junction (EMJ) of the adult uterus, we examined the ultrastructure of mesenchymal components at this site during the menstrual cycle and during early pregnancy. Cells having some features of smooth muscle were found among the usual endometrial stromal cells in every specimen examined. In the follicular phase of the menstrual cycle, such cells resembled myofibroblasts, but in the luteal phase and during early pregnancy, they had more distinct cytoplasmic filaments with dense bodies and dense plaques, and other fairly well developed characteristics of smooth muscle. The identification of smooth muscle-like cells at the EMJ in the adult uterus and the finding that their morphology changes into cells having many of the characteristics of smooth muscle cells during the luteal phase and early pregnancy, suggests that smooth muscle differentiation possibly occurs from multi-potential mesenchymal cells in the endometrial stroma.