Endometrium – Tissue and Systems

Endometrium – Tissue and Systems

This article describes the basic anatomy and functions of the human endometrium. By understanding what the endometrium is and its normal function, the symptoms of endometrial cancer will be better understood.

The endometrium is the inner lining of the uterus. The uterus is a pear shaped organ in women that is found in the pelvis and connects to the vagina via the cervix. The uterus is a hollow muscle that has the main function of providing a safe and hospitable area for a newly fertilized egg to implant and grow into a fetus and eventually be born. The uterus is between the bladder (in front of it) and the rectum (behind it). The uterus is pear shaped but more like an upside-down pear. The narrow area of the pear is the isthmus that connects to the cervix and vagina. The body of the pear has two wing-like arms that are called fallopian tubes. These tubes allow passage of an egg from the nearby ovary. The uterus in most women is a small organ and about 3-5 inches long and 2-4 inches wide. The average uterus in an adult woman weighs about 50 grams. After pregnancy the resting state of the uterus can be about 50% to 100% bigger. During pregnancy the uterus expands greatly up to 25x its original dimensions. As a woman ages and is no longer fertile (i.e. post-menopausal) the uterus shrinks without production of the usual hormones from the ovary.

The uterus has layers like most other structures in the abdomen. The innermost layer is called the endometrium. The middle layer is composed of thick layers of muscle. This muscle has a generous blood supply with many veins and artery branches. The outer layer of the uterus has a lining similar to the colon or bladder. The blood supply to the uterus is a branch of the main artery that supplies the pelvis – the internal iliac artery. This artery runs very close to the ureter which is the drainage tube from the kidney to the bladder.

The hormones that regulate the menstrual cycle (i.e. estrogen and progesterone) also affect the growth and shedding of the endometrium. Early in the cycle the endometrium grows taller in the proliferative phase. The average menstrual cycle is about 28 days. Midway at day 14 is when ovulation occurs. After this the endometrial lining begins to secrete and become thicker. This is known as the secretory phase. If the ovulated egg does not join with a sperm and implant into the thick endometrium then the lining will disrupt at about 28 days. This disruption or shedding of lining is known as menses or the process of menstruation that occurs monthly for women. The lining of the uterus, the endometrium, has receptors that respond to stimulation from hormones such as estrogen.

The female reproductive pelvic organs (i.e. ovaries, tubes, uterus, cervix, vagina, vulva) are all capable of having cancer of different types. Cancer or tumors of each of these sites will have different cell architecture or several different possibilities.

Understanding the function and reactions to hormones is key to understanding symptoms of endometrial cancer. An abnormal growth of the endometrium is most likely to cause bleeding that will simulate a menstruation in an abnormal way. If there is bleeding apart from a normal period or after the menstrual cycle has ceased this is considered abnormal and a potential symptom of endometrial cancer. If a woman has had irregular, scarce or no periods for some length of time and then begins to have them heavier or irregularly then this could be a sign of endometrial cancer.

The endometrial lining normally grows in a cyclical way. In the same manner, the endometrium can grow in an abnormal way without being a true cancer. As the lining of the endometrium becomes taller and the cells become more dense (also known as endometrial hyperplasia) the potential for cancer to arise in this type of environment increases. This will also manifest as irregular or post-menopausal bleeding.

There are several ways to assess the health and function of the uterus. A normal menstrual cycle or absence of bleeding in the post-menopausal woman is sign of a healthy uterus. During a physical examination, a physician can check the position and fullness of the uterus to feel for any abnormalities. Another way to check the health of the uterus is with an ultrasound to measure the thickness of the lining. An ultrasound can measure the size of the uterus and the thickness of the endometrium as well as look for irregularities in the “landscape” of the endometrium. A normal endometrium is only a few millimeters and should not have much irregularity. If an older woman with new onset bleeding is examined and also has an ultrasound that shows a tall or irregular endometrium, than this is very concerning for endometrial cancer.

Another way to assess the health of the uterus and its inner lining the endometrium is to take a sample of it with a needle or similar device. Taking a sample of cells from an area of the body is also called a biopsy. A biopsy of the endometrium allows examination of a small part of the uterine lining to look for irregularities or for cancer. If cancer is seen, the type of cancer can also be determined. Further treatment involves checking to see if there is any disease outside of the uterus and usually results in surgery to remove the uterus (also known as hysterectomy).


  1. www.cancer.gov/cancertopics/types/endometrial
  2. www.nlm.nih.gov/medlineplus/ency/article/000910.htm
  3. Katz, VL; Lentz GM; Lobo RA; Gershenson DM. (2007) Katz: Comprehensive Gynecology, 5th ed. Chapters 3, 32. Philadelphia, PA: Mosby Elsevier
  4. Townsend Jr, CM; Beauchamp RD; Evers BM; Mattox KL. (2008) Townsend: Sabiston Textbook of Surgery, 18th ed. Chapter 75. New York, NY: Saunders.
  5. This article was originally published on September 3, 2012 and last revision and update was 9/4/2015.