Elastin distribution in the myometrial and vascular smooth muscle of the human uterus

Magnetic resonance imaging and transvaginal ultrasonography in women of reproductive age suggest that the myometrium consists of inner and outer layers. It was hypothesized that these structural and functional differences in the myometrium might be associated with a variation in elastin distribution. Fifty-one hysterectomy specimens representing all phases of the normal menstrual cycle were studied by immunocytochemistry, orcein staining and image analysis. Elastin was present within the outer myometrial smooth muscle, but was less widely distributed in the inner smooth muscle. Immunoreactivity and staining were observed in the myometrial arteries and arterioles and within the basal portions of endometrial arterioles. Elastin was also present in perivascular tissue, particularly near the large vessels. More extravascular (i.e. perivascular and smooth muscle) elastin was present in the outer myometrium in all cases, although no distinct layering was observed. Semi-quantitative analysis of the elastin distribution in 11 full thickness specimens demonstrated a decreasing gradient from outer to inner myometrium rather than distinct layering. Contrary to previous reports, these data suggest that the external region of the myometrium is more elastic than the inner region and that elastin is found throughout the arteriolar tree of the human uterus.