The innermost layer of the uterus, part of which builds up and sheds each menstrual cycle, is the:

1. The uterine wall has three layers: an outer serous membrane, a middle muscular layer, and an inner layer. 2. The outer serous layer is the perimetrium, and the middle smooth-muscle layer is the myometrium. 3. The innermost layer, which responds to hormones and sheds during menstruation, is different from both. 4. This innermost layer is called the endometrium. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 27 "The Reproductive System", p.1301 §The Uterus and Cervix_