MH 165 Uterus

Uterus

The uterus undergoes cyclic changes during the menstrual cycle divided into proliferative, secretory and menstrual phases. This specimen is from the proliferative phase.

The uterine wall is composed of three layers:

• Endometrium - specialized mucosa that undergoes marked changes during the menstrual cycle.
  • Functional Layer (or stratum functionalis) - upper two thirds that regenerates from cells in the basal layer under the influence of estrogen.
    • Simple Columnar Epithelium - epithelial cells remaining from the bases of glands rapidly proliferate and migrate to restore the surface epithelium.
    • Endometrial Stroma - cells proliferate and secrete collagen and ground substance to restore the lamina propria.
    • Uterine Glands - surface epithelium invaginates into the stroma to form new glands.
      • The glands are straight (or slightly wavy) and have narrow lumens.
      • Very little secretion.
  • Basal Layer (or stratum basalis) - lower third of the mucosa that is retained during menstruation and regenerates the functional layer.

• Myometrium - composed of three indistinct layers of smooth muscle.
  • Inner and Outer Layers - mostly longitudinal bundles of smooth muscle.
  • Middle Layer (or stratum vasculare) - thickest layer of mostly circular or spiral bundles of smooth muscle with numerous blood vessels.
• Perimetrium - covered by an outer serous layer or visceral peritoneum. (Not found in this specimen.)

The endometrium contains a unique system of blood vessels that undergo marked changes during the menstrual cycle. The blood supply to the functional layer is restored during the proliferative phase from straight arteries in the basal layer.

• Arcuate Arteries (#1,#2,#3) - 6 to 10 branches of the uterine artery that encircle the uterus in the myometrium.
• Radial Arteries (#1,#2,#3) - branches of arcuate arteries that ascend into the endometrium and give rise to:
  • Straight Arteries (#1,#2) - supply the basal layer.
  • Spiral (or coiled) Arteries (#1,#2) - pass through the basal layer and supply the functional layer.
    • Terminal Capillaries (#1,#2,#3) - dilated (or ectatic) capillaries that arise from spiral arteries.

Because the two layers of the endometrium have separate blood supplies, the basal layer is unaffected by the loss of blood to the functional layer during menstruation.