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Structure & Function

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The uterus is a pear-shaped muscular organ situated in the pelvis between the bladder anteriorly and the rectum posteriorly. It is partially covered by the peritoneum of the pelvic floor.

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The uterus is customarily divided into the body and the cervix. The body is lined by the endometrium, whose thickness varies at different ages and stages of the menstrual cycle. The endometrium is composed of endometrial glands and mesenchymal stromal cells, both of which are very sensitive to the action of female sex hormones. At the internal os, the endometrium becomes continuous with the endocervical canal, which is lined by columnar epithelium and contains mucous glands. The epithelium changes again at the junction of the endocervix and ectocervix, where it becomes stratified squamous epithelium.

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The uterus develops from the müllerian system. Congenital anomalies of the uterus are common (Figure 53-1) and result from abnormalities in fusion of the müllerian ducts in the embryo.

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Figure 53–1.
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Congenital anomalies of the uterus.

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The Normal Endometrial Cycle

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The normal endometrium shows cyclic changes caused by corresponding changes in ovarian hormone production. Histologic examination of the endometrium in a biopsy or curettage specimen permits evaluation of the phase of the endometrial cycle (Figure 53-2). Along with the patient's menstrual history, this can provide important information about possible causes of abnormal uterine bleeding.

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Figure 53–2.
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Endometrial changes during the menstrual cycle.

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The endometrial cycle is divided into a preovulatory proliferative phase that is the result of estrogenic stimulation (Figure 53-2) and a postovulatory secretory phase that is directed by progesterone secretion by the corpus luteum. Day 1 of the cycle is the onset of menstruation.

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In the proliferative phase, there is a rebuilding of the shed endometrium from the basal layer, and mitotic figures are present in both glandular and stromal cells. The endometrium thickens, and the glands start to become tortuous. The secretory phase begins after ovulation with luteal progesterone secretion. The first histologic evidence that the endometrium is in the secretory phase is seen 2–4 days after ovulation, when subnuclear secretory vacuoles appear in the glands. Later, the cell secretions move to the apex of the cell, with the nuclei moving back to the base. Stromal edema appears on about the seventh postovulatory day. The glands become progressively more tortuous and typically serrated in the later part of the cycle (Figure 53-3). Spiral arterioles become prominent on the ninth day after ovulation. Beginning on about the ninth day after ovulation, the stromal cells become larger, with increase in the amount and glycogen content of the cytoplasm (predecidual change). In the absence of fertilization, ...

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