Endometriosis is a condition in which the cells that line the uterus grow outside of the uterus. The condition can interfere with a woman's fertility and ability to become pregnant. Endometriosis can also cause severe pelvic pain, especially during menstruation.

Endometriosis is a common gynecological condition. It was described in medical literature more than 300 years ago and has since been recognized as a chronic, painful, and often progressive disease in women. However, the causes of endometriosis are unknown, it is widely variable in symptoms and severity, and it is difficult to diagnose. In fact, some experts believe that endometriosis may turn out to be several disorders, not just one.

Endometrial Implants

Endometriosis. Endometriosis occurs when cells from the mucus membrane lining the uterus (endometrium) form implants that attach, grow, and function outside the uterus, generally in the pelvic region. Endometrial implants consist of both following cell types:

  • Gland cells. These cells secrete hormones and other fluids and are normally located in the uterine lining.
  • Stroma cells. These are the framework cells that build supportive tissue.

Endometrial cells contain receptors that bind to estrogen and progesterone, which promote uterine growth and thickening. During endometriosis these cells become implanted in organs and structures outside the uterus, where these hormonal activities continue to occur, causing bleeding and scarring.

Endometriosis picture
Endometriosis is the condition in which the tissue that normally lines the uterus (endometrium) grows on other areas of the body causing pain and irregular bleeding.

Endometrial implants vary widely in size, shape, and color. Over the years, they may diminish in size or disappear or they may grow.

  • Early implants are usually very small and look like clear pimples.
  • If they continue to grow they may form flat injured areas (lesions), small nodules, or cysts called endometriomas, which can range from sizes smaller than a pea to larger than a grapefruit.
  • Implants also vary in color; they may be colorless, red, or very dark brown. These so-called chocolate cysts are endometriomas filled with thick, old, dark brown blood that usually appear on the ovaries.

Location of Implants

Implants can form in many areas, most commonly in the following:

  • The peritoneum. This is the smooth surface lining that covers the entire wall of the abdomen and folds over inner organs in the pelvic area.
  • On or next to the ovaries.

Less commonly they occur in other areas:

  • Cul-de-sac, an area between the uterus and rectum
  • Connective tissue that supports the uterus (called the uterosacral ligaments)
  • Vagina
  • Fallopian tube
  • Urinary tract (in about 20% of cases, usually without causing symptoms).
  • Gastrointestinal tract (in 12 - 37% of patients)
Female reproductive anatomy picture

Click the icon to see an image of the female reproductive anatomy.

Very rarely, they appear in areas far from the pelvis, including the lungs and even the arms and thighs.

Process of Endometriosis

The process of endometriosis mimics menstruation at certain stages:

  • Each month, the exiled endometrial implants respond to the monthly cycle just as they would in the uterus. They fill with blood, thicken, break down and bleed.
  • Products of the endometrial process cannot be shed through the vagina as menstrual blood and debris do. Instead, the implants develop into collections of blood that form cysts, spots, or patches.
  • Lesions may grow or reseed as the cycle continues.

The lesions are not cancerous, but they can develop to the point that they cause obstruction or adhesions (web-like scar tissue) that attach to nearby organs, causing pain, inflammation, and sometimes infertility.

The Female Reproductive System

The primary structures in the reproductive system are as follows:

  • The uterus is a pear-shaped organ located between the bladder and lower intestine. It consists of two parts, the body and the cervix.
  • When a woman is not pregnant the body of the uterus is about the size of a fist, with its walls collapsed and flattened against each other. During pregnancy the walls of the uterus are pushed apart as the fetus grows.
  • The cervix is the lower portion of the uterus. It has a canal opening into the vagina with an opening called the os, which allows menstrual blood to flow out of the uterus into the vagina.
  • Leading off each side of the body of the uterus are two tubes known as the fallopian tubes. Near the end of each tube is an ovary.
  • Ovaries are egg-producing organs that hold between 200,000 and 400,000 follicles (from folliculus, meaning "sack" in Latin). These cellular sacks contain the materials needed to produce ripened eggs, or ova.

The inner lining of the uterus is called the endometrium, and during pregnancy it thickens and becomes enriched with blood vessels to house and support the growing fetus. If pregnancy does not occur, the endometrium is shed as part of the menstrual flow. Menstrual flow also consists of blood and mucus from the cervix and vagina.

Reproductive Hormones

The hypothalamus (an area in the brain) and the pituitary gland regulate the reproductive hormones. The pituitary gland is often referred to as the master gland because of its important role in many vital functions, many of which require hormones. In women, six key hormones serve as chemical messengers that regulate the reproductive system:

  • The hypothalamus first releases the gonadotropin-releasing hormone (GnRH).
  • This chemical, in turn, stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
  • Estrogen, progesterone, and the male hormone testosterone are secreted by the ovaries at the command of FSH and LH and complete the hormonal group necessary for reproductive health.