The uterus receives the rapidly developing morula from the uterine tube. All subsequent embryonic and fetal development occurs within the uterus, which undergoes dramatic increases in size and development. The human uterus is a hollow, pear-shaped organ located in the pelvis between the bladder and rectum. In a nulliparous woman, it weighs 30 to 40 g and measures 7.5 cm in length, 5 cm in width at its superior aspect, and is 2.5 cm thick. Its lumen, which is also flattened, is continuous with the uterine tubes and the vagina.

Anatomically, the uterus is divided into two regions:

• The body is the large upper portion of the uterus. The anterior surface is almost flat; the posterior surface is convex. The upper, rounded part of the body that expands above the attachment of the uterine tubes is termed the fundus.

• The cervix is the lower, barrel-shaped part of the uterus separated from the body by the isthmus (see Fig. 22.1). The lumen of the cervix, the ceivical canal, has a constricted opening or os at each end. The internal os communicates with the cavity of the uterus; the external os, with the vagina.

The uterine wall is composed of three layers (Fig. 22.16). From the lumen outward they are

• Endometrium, the mucosa of the uterus.

• Myometrium, the thick muscular layer. It is continuous with the muscle layer of the uterine tube and vagina. The smooth muscle fibers also extend into the ligaments connected to the uterus.

• Perimetrium, the outer serous layer or visceral peritoneal covering of the uterus. The perimetrium is continuous with the pelvic and abdominal peritoneum and consists of a mesothelium and a thin layer of loose connective tissue. Beneath the mesothelium, a layer of elastic tissue is usually prominent. 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 below).

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 the uterus.

As in most bulb-shaped hollow organs, such as the gallbladder and urinary bladder, muscular orientation is not distinctive. The muscle bundles seen in routine histologic sections appear to be randomly arrayed. During uterine 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 /jlm long. During pregnancy, the uterus undergoes enormous enlargement. The growth is primarily due to the hypertrophy of existing smooth muscle cells, which may reach more than 500 ¡xm in length, and secondarily due to

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