glands, stroma, and blood vessels ....
There appears to be an association between the finding of
myomas, cervical polyps, and endometriosis [11,18–20]." http://www.aagl.org/wp-content/uploads/2013/03/aagl-Practice-Guidelines-for-the-Diagnosis-and-Management-of-Endometrial-Polyps.pdf
"Polyps: polyps or other growths inside the uterus can act as an obstruction to the outflow of blood during menses. This can increase clotting. These growths can also bleed themselves." http://centerforendo.com/articles/clots.htm
"Spotting between menstrual cycles is a common symptom of this condition. The menstruation periods may also be very heavy and prolonged in duration. A women with endometrial polyps who has already been through menopause may discover unexpected spotting. Pelvic pain in the presence or absence of menstruation may also be experienced. In some incidences, especially large polyps may cause infertility." http://www.wisegeek.com/what-are-endometrial-polyps.htm