Endometrium – The Uterine Lining Explained
The endometrium is the inner mucosal lining of the uterus. It plays a key role in the menstrual cycle and in the implantation of a fertilized egg.
Things worth knowing about "Endometrium"
The endometrium is the inner mucosal lining of the uterus. It plays a key role in the menstrual cycle and in the implantation of a fertilized egg.
What Is the Endometrium?
The endometrium (Latin: endometrium uteri) is the innermost lining of the uterus (womb). It is a specialized mucous membrane consisting of two main functional layers: the functionalis, which builds up during each menstrual cycle and is shed if pregnancy does not occur, and the basalis, a permanent regenerative layer that remains intact and regenerates the functionalis after each menstruation.
Structure and Composition
The endometrium is composed of several tissue components:
- Epithelium: A single layer of columnar cells lining the surface of the uterine cavity.
- Glands: Tubular glands embedded in the underlying connective tissue that produce secretions.
- Stroma: A fibrous connective tissue matrix surrounding the glands and blood vessels.
- Blood vessels: Spiral arteries that supply nutrients to the tissue and play a central role during menstruation.
Function in the Menstrual Cycle
The endometrium undergoes a monthly cycle of growth and shedding controlled by sex hormones:
Proliferative Phase
Under the influence of estrogen, the endometrium thickens from a thin layer to several millimeters after menstruation. The glands elongate and blood vessels branch out extensively.
Secretory Phase
After ovulation, progesterone (produced by the corpus luteum) further matures the endometrium. The glands begin secreting nutrients, and the tissue is prepared for the implantation of a fertilized egg.
Menstruation
If fertilization does not occur, levels of estrogen and progesterone decline. This causes the functionalis layer to break down and shed, resulting in menstrual bleeding. The cycle then begins anew.
Role in Pregnancy
A well-prepared endometrium is essential for a successful pregnancy. The fertilized egg (blastocyst) implants into the endometrium approximately six to ten days after fertilization -- a process known as implantation. The endometrium subsequently transforms into the decidua, which, together with the trophoblast, forms the placenta and provides nutrients to the developing embryo.
Common Disorders of the Endometrium
Several conditions can affect the endometrium:
- Endometriosis: Endometrial-like tissue grows outside the uterus, such as on the ovaries or in the abdominal cavity, often causing severe pain and infertility.
- Endometritis: Inflammation of the uterine lining, most commonly caused by bacterial infection.
- Endometrial hyperplasia: Abnormal thickening of the uterine lining, often due to excess estrogen without sufficient progesterone. It can be a precursor to uterine cancer.
- Endometrial carcinoma (uterine cancer): The most common malignancy of the female genital tract in Western countries, predominantly affecting postmenopausal women.
- Endometrial polyps: Benign overgrowths of the uterine lining that may cause irregular bleeding.
Diagnosis
Several methods are used to evaluate the endometrium:
- Transvaginal ultrasound: Imaging technique used to measure endometrial thickness.
- Hysteroscopy: Direct visualization of the uterine cavity using an endoscope.
- Endometrial biopsy: Tissue sampling for histological analysis.
- Curettage: Scraping of the uterine lining for diagnostic or therapeutic purposes.
References
- Standring S. (ed.): Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier, London 2021.
- Cunningham FG et al.: Williams Obstetrics. 26th ed. McGraw-Hill Education, New York 2022.
- World Health Organization (WHO): Reproductive Health. WHO Press, Geneva 2022. Available at: https://www.who.int
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