The uterus is defined by three distinct layers. The layers are the serosa, the myometrium, and the endometrium. The serosa is the outer connective tissue cover and is shiny and tough. It also gives strength and shape. The myometrium is the inner layer that is composed of muscle fibers that run longitudinally. These fibers act under the influence of estrogen to contract and relax (peristaltic motion) to move the sperm up the reproductive tract. The third and innermost layer, the endometrium, is composed of several parts; the stroma, glands and epithelium. The epithelium lines the innermost part of the endometrium. The glands and stroma compromise the area beneath the epithelium and are important to the proper function of the uterus.