Figure 2216

Photomicrograph of a sagittal section of a human uterus. This section shows the three layers of the uterine wall: the endometrium, the innermost layer that lines the uterine cavity; the myometrium, the middle layer of smooth muscle; and the perimetrium, the very thin layer of peritoneum that covers the exterior surface of the uterus. The deep portion of the myometrium contains the larger blood vessels (BV) that supply the uterus. x8.

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 to almost its original size. Some muscle fibers degenerate, but most return to their original size. The collagen produced during pregnancy to strengthen the myometrium is then en-zymatically degraded by the cells that secreted it. The titer-

ine 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 activity of the endometrium during the cycle are correlated with the maturation of the ovarian follicles (see Fig. 22.14). 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 flow. 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. 22.17):

• Stratum functionale or functional layer. This layer is the thick part of the endometrium, which is sloughed off at menstruation.

• Stratum basale or basal layer. This layer is retained during menstruation and serves as the source for the regeneration of the stratum functionale.

The stratum functionale is the layer that proliferates and degenerates during the menstrual cycle

During the phases of the menstrual cycle, the endometrium varies from 1 to 6 mm in thickness. It is lined by a simple columnar epithelium with a mixture of secretory and ciliated cells. The surface epithelium invaginates into the underlying lamina propria, the endometrial stroma, forming the uterine glands. These simple tubular glands, containing fewer ciliated cells, occasionally branch in the deeper aspect of the endometrium. The endometrial stroma, which resembles mesenchyme, is highly cellular and contains abundant intercellular ground substance. As in the uterine tube, no submucosa separates the endometrium from the myometrium.

The vasculature of the endometrium also proliferates and degenerates during each menstrual cycle

The endometrium contains a unique system of blood vessels (see Fig. 22.17). The uterine artery gives off 6 to 10 arcuate arteries that anastomose in the myometrium. Branches from these arteries, the radial arteries, enter the basal layer of the endometrium where they give off small straight arteries that supply this region of the endometrium. The main branch of the radial artery continues upward and becomes highly coiled; it is therefore called the spiral arteiy. Spiral arteries give off numerous arterioles that often anastomose as they supply a rich capillary bed. The capillary bed includes thin-walled dilated segments called lacunae. Lacunae may also occur in the venous system that drains the endometrium. The straight arteries and the proximal part of the spiral arteries do not change during the menstrual cycle. The distal portion of the spiral arteries, under the influence of estrogens and

Histology Contain Myometrium
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