Endometriosis Pathology

The pathology of endometriosis - technical

Key Points

  • Definition: endometrial glands and stroma outside the uterine body.
  • The pathogenesis is unknown.
  • Endometriosis of the ovary frequently forms ‘chocolate’ cysts.

Endometriosis can be defined as the presence of endometrial glands and stroma outside the uterine body. It occurs in a wide variety of different sites (Table 1), but most often affects the ovaries and other sites within the pelvis, particularly the pouch of Douglas and the posterior pelvic peritoneum. Note that involvement of the serosa of the uterus is included in the definition.

Table 1 Anatomical sites of endometriosis
Common Uncommon Rare
Ovary (80%) Cervix  Lung
Uterine ligaments Vagina Pleura
Pouch of Douglas Bladder Skeletal muscle
Fallopian tube Skin* Small bowel
*Cutaneous involvement occurs characteristically in scars, particularly those from Caesarean sections

The pathogenesis of endometriosis is unknown, but there are several hypothesized mechanisms, including regurgitation of menstrual endometrium (retrograde menstruation), metaplasia of surface epithelia, and vascular or lymphatic spread of normal endometrium. The former is consistent with the anatomical distribution, but it is difficult to explain the presence of endometriosis in distant sites on this basis. This is also true of the metaplasia theory.

Macroscopically, endometriosis forms multiple peritoneal nodules and, within the ovary, forms ‘chocolate’ cysts. The inflammation associated with the presence of endometriosis leads to the formation of adhesions which may cause secondary anatomical abnormalities, particularly of the fallopian tubes; this may lead to a reduction in fertility.