The perimetrium covers the entire posterior surface of the uterus but only part of the anterior surface. The remaining part of the anterior surface consists of connective tissue or adventitia.
Both myometrium and endometrium undergo cyclic changes each month to prepare the uterus for implantation of an embryo. These changes constitute the menstrual cycle. If an embryo implants, the cycle stops, and both layers undergo considerable growth and differentiation during pregnancy (described in the next section).
The myometrium forms a structural and functional syncytium.
The myometrium is the thickest layer of the uterine wall. It is composed of three indistinctly defined layers of smooth muscle:  The middle muscle layer contains numerous large blood vessels (venous plexuses) and lymphatics and is called the stratum vasculare. It is the thickest layer and has interlaced smooth muscle bundles oriented in a circular or spiral pattern.
 The smooth muscle bundles in the inner and outer layers are predominantly oriented parallel to the long axis of activity of the endometrium during the cycle are correlated with the maturation of the ovarian follicles (see Folder 23.3). The end of each cycle is characterized by the partial destruction and sloughing of the endometrium, accompanied by bleeding from the mucosal vessels. The discharge of tissue and blood from the vagina, which usually continues for 3 to 5 days, is referred to as menstruation or menstrual fow. The menstrual cycle is defined as beginning on the day when menstrual flow begins.
During reproductive life, the endometrium consists of two layers or zones that differ in structure and function (Fig. 23.15 and Plate 96, page 880): contraction, all three layers of the myometrium work together as a functional syncytium expelling the contents of the lumen through a narrow orifice.
In the nonpregnant uterus, the smooth muscle cells are about 50 m long. During pregnancy, the uterus undergoes enormous enlargement. The growth is primarily owing to the hypertrophy of existing smooth muscle cells, which may reach more than 500 m in length, and secondarily at tributable to the development of new fibers through the division of existing muscle cells and the differentiation of undifferentiated mesenchymal cells. The amount of connective tissue also increases. As pregnancy proceeds, the uterine wall becomes progressively thinner as it stretches because of the growth of the fetus. After parturition, the uterus returns straight artery to almost its original size. Some muscle fibers degenerate, but most return to their original size. The collagen pro duced during pregnancy to strengthen the myometrium is then enzymatically degraded by the cells that secreted it. The uterine cavity remains larger and the muscular wall remains thicker than before pregnancy.
Compared with the body of the uterus, the cervix has more connective tissue and less smooth muscle. Elastic fibers are abundant in the cervix but are found in appreciable quantities only in the outer layer of the myometrium of the body of the uterus.
The endometrium proliferates and then degenerates during a menstrual cycle.
Throughout the reproductive lifespan, the endometrium undergoes cyclic changes each month that prepare it for the implantation of the embryo and the subsequent events of embryonic and fetal development. Changes in the secretory