The Uterus is the site of implantation and growth of an embryo. The endometrium undergoes marked changes during the hormonal cycle, resulting in distinctive histological features during the menstrual, proliferative, and secretory phases.

Examine two sections of uterus (sample 1 - trichrome stain, and sample 2 - H&E) and identify the

• Endometrium,
• Myometrium,
• Perimetrium with serosa, if present, and
• The attached connective tissue, if present.

Examine the endometrial surface and note the simple columnar epithelium, with many ciliated cells. Throughout the endometrium, identify endometrial glands and vascular sinuses in the stroma. You may be able to distinguish the basal layer and functional layer of the endometrium, but this is difficult on sample 2. In the myometrium, observe the interlacing bundles of smooth muscle fibers.

Clinical note: Endometriosis, results when sloughed endometrial tissue is refluxed up the uterine tubes and surviving cells begin to grow on the surface of the ovary, mesenteries, or other organs in the peritoneal cavity. Cycles of growth and loss of this displaced endometrial tissue occur under the influence of estrogen and progesterone, which can eventually result in inflammation, pain, and scarring of the affected organs. Untreated, endometriosis can lead to infertility for several reasons.

Clinical note: Smooth muscle cells of the uterus commonly give rise to benign tumors called leiomyomas or fibroids. They grow slowly and often attain diameters of several centimeters, but usually do not produce major medical problems.

Now let's look at the stages of endometrial development.