The uterus is composed of three layers: the outer perimetrium, composed of connective tissue; the thick smooth muscle and elastic tissue of the myometrium in the center; and the mucosal epithelial lining called the endometrium. Functionally, the endometrium can be divided into two zones: the functionalis which is built up and sloughed monthly during menstruation; and the basalis which consists of the epithelium and glandular elements, which remain and supply the replicative cells to generate the next month's functionalis tissue.