Key

BV, blood vessels C, cilia

CT, connective tissue

Ep, epithelium L, lumen

Lym, lymphatic vessel

Muc, mucosa Mus, muscularis PC, peg cells

The uterus is a hollow, pear-shaped organ with a thick wall and, in the nonpregnant state, a narrow cavity. The uterine wall is composed of a mucosa, referred to as the endometrium; a muscularis, referred to as a myometrium; and, externally, a serosal cover, the perimetrium. The myometrium consists of smooth muscle and connective tissue and contains the large blood vessels that give rise to the vessels that supply the endometrium.

The uterus undergoes cyclical changes that are largely manifested by changes that occur in the endometrium. If implantation of an ovum does not occur after preparation for this event, the state of readiness is not maintained, and much of the endometrium degenerates and is sloughed off, constituting the menstrual flow. The part of the endometrium that is lost is referred to as the stratum functionale; the part that is retained is called the stratum basale. The stratum basale is the deeper part of the endometrium and adjoins the myometrium.

The myometrium also undergoes changes associated with implantation of a zygote. In the nonpregnant uterus, the smooth muscle cells are about 50 fim in length; during pregnancy, they undergo enormous hypertrophy, often reaching more than 500 fxm in length. In addition, new muscle fibers develop after division of existing muscle cells and division and differentiation of undifferentiated mesenchymal cells. The connective tissue also increases to strengthen the uterine wall. Fibroblasts increase by division and secrete additional collagen and elastic fibers. After parturition, the uterus nearly returns to its normal size. Most muscle fibers return to their normal size, and some degenerate. Collagen secreted during pregnancy is digested by the very cells that secreted it, the fibroblasts. Similar, but less pronounced, proliferation and degeneration of fibroblasts and collagen occur in each menstrual cycle.

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