The uterus, a pear-shaped organ, houses two distinct but interconnected tissue layers: the endometrium and the myometrium. The endometrium forms the inner lining of the uterus, while the myometrium constitutes its muscular wall. Both tissues are fundamental to reproductive health, playing specific roles in menstruation, pregnancy, and childbirth.
The Endometrium: Structure and Cyclical Changes
The endometrium, the inner mucosal lining of the uterus, consists of two layers: the functional layer (stratum functionalis) and the basal layer (stratum basalis). The stratum basalis anchors the endometrium to the underlying myometrium and remains largely unchanged throughout the menstrual cycle. In contrast, the stratum functionalis undergoes monthly transformations in response to hormonal fluctuations.
The cyclical changes of the functional layer prepare the uterus for potential pregnancy. During the proliferative phase, influenced by estrogen, the endometrium thickens, and its glands and blood vessels grow. Following ovulation, the secretory phase begins, driven by progesterone. During this phase, uterine glands become corkscrew-shaped and fill with glycogen, and blood vessels proliferate, enriching the lining to receive a fertilized egg.
If conception does not occur, estrogen and progesterone levels decrease, leading to the shedding of the thickened functional layer. This shedding process, known as menstruation, involves the constriction of spiral arteries, causing tissue breakdown and expulsion of tissue and blood. The endometrium’s regenerative capacity allows it to rebuild from the basal layer after each menstrual period. If a fertilized egg implants, the endometrium provides the necessary environment and contributes to the formation of the placenta, which supplies the developing fetus with oxygen and nutrients.
The Myometrium: Structure and Function
The myometrium forms the thick muscular wall of the uterus, situated between the endometrium and the outer serosa. It is composed of smooth muscle cells, along with supporting tissue. The myometrium is organized into distinct layers: an inner circular layer, a middle mesh-like layer with blood vessels, and an outer longitudinal layer.
The function of the myometrium is to generate uterine contractions. During menstruation, these contractions help expel the endometrial lining from the uterus. The myometrial smooth muscle cells contain contractile proteins that facilitate contraction.
The myometrium’s ability to contract is regulated by various factors, including hormones, mechanical stretch, and ion channels. During pregnancy, the myometrium stretches to accommodate the growing fetus, as its smooth muscle cells increase in size and number. During labor and childbirth, forceful myometrial contractions, stimulated by hormones, are responsible for pushing the baby through the birth canal. After delivery, the myometrium continues to contract to expel the placenta and compress blood vessels, minimizing postpartum blood loss.
Common Conditions Affecting the Uterine Tissues
Conditions Primarily Affecting the Endometrium
Endometriosis is a condition where endometrial-like tissue grows outside the uterus, such as on the fallopian tubes, ovaries, or other abdominal organs. This displaced tissue thickens and bleeds during the menstrual cycle, causing pain. Endometrial polyps are benign growths that protrude from the endometrial lining into the uterine cavity. These polyps, often caused by estrogen overstimulation, can lead to abnormal uterine bleeding. While most polyps are benign, a small percentage can become malignant, especially if they cause bleeding or occur after menopause.
Endometrial hyperplasia involves an abnormal thickening of the endometrium, often due to estrogen exposure without sufficient progesterone. This hormonal imbalance can prevent shedding, leading to cell proliferation. There are two types: typical and atypical hyperplasia; atypical hyperplasia carries an increased risk of progressing to endometrial cancer. Endometrial cancer is a malignancy affecting the endometrium, often linked to excess estrogen. Advances in understanding this cancer now categorize it into subtypes based on gene changes.
Conditions Primarily Affecting the Myometrium
Uterine fibroids are benign muscular tumors that develop within the myometrium. These non-cancerous growths originate from uterine smooth muscle cells and can vary in size and number. Symptoms often include heavy, painful periods, and their impact depends on their location and size.
Adenomyosis is a condition where endometrial tissue grows into the muscular wall of the myometrium. During each menstrual cycle, these displaced endometrial cells thicken, break down, and bleed within the muscle wall. This can cause the surrounding muscle to swell, leading to an enlarged and tender uterus, heavy menstrual bleeding, and severe menstrual pain. While distinct from fibroids, adenomyosis shares similar symptoms, making diagnosis challenging.