MH 167 Uterus

Uterus

The uterus undergoes cyclic changes during the menstrual cycle which is divided into proliferative, secretory and menstrual phases. This specimen is from the menstrual 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 of the mucosa is lost during menstruation.
    • Spiral arteries intermittently close causing ischemia.
    • After several days, the spiral arteries remain constricted and the lack of oxygen leads to necrosis.
    • Spiral arteries dilate again and rupture because of the disruption of the uterine glands.
    • Disgorged blood removes the functional layer causing blood discharge (menses).
  • Basal Layer (or stratum basalis) - lower third of the mucosa is retained during menstruation and blood flow is maintained through the straight arteries.

• 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.

• Arcuate Arteries - 6 to 10 branches of the uterine artery that encircle the uterus in the myometrium.
• Radial Arteries (#1,#2) - 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.