The myometrium is the muscular middle layer of the uterine wall, situated between the inner lining, the endometrium, and the outer layer, the perimetrium. This layer constitutes the bulk of the uterus, accounting for approximately 90% of the uterine wall’s mass. Its primary component is smooth muscle, which gives the uterus its strength and contractility. The myometrium’s functions are central to the female reproductive cycle, including pregnancy, childbirth, and menstruation.
Structure and Composition of the Myometrium
The myometrium is composed almost entirely of smooth muscle cells, also known as myocytes. These muscle cells are not under conscious control, meaning their contractions are involuntary.
The smooth muscle fibers are organized into interwoven bundles running in multiple directions—horizontally, vertically, and diagonally. This design allows the uterus to contract with force from various directions. The structure facilitates both the powerful contractions of labor and the less intense contractions associated with the menstrual cycle.
The Myometrium’s Role in Pregnancy and Childbirth
During pregnancy, the myometrium undergoes changes to accommodate a developing fetus. The uterine muscle expands, a process driven by both hypertrophy, where individual muscle cells increase in size, and hyperplasia, an increase in the number of muscle cells. This allows the uterus to stretch from the size of a pear to a size capable of holding a full-term baby. Throughout most of the pregnancy, the myometrium remains in a relatively inactive state, which is necessary for the pregnancy to be maintained.
In the final stages of pregnancy, hormonal shifts, including a change in the estrogen-to-progesterone ratio, prepare the myometrium for labor. The hormone oxytocin plays a major part in triggering the strong, rhythmic contractions of the myometrial muscle. These coordinated and forceful contractions are what cause the cervix to dilate and help push the baby through the birth canal.
The effectiveness of these labor contractions depends on the ability of the muscle cells to communicate and contract in unison. This is facilitated by an increase in gap junctions, which are protein channels that connect adjacent muscle cells. These junctions allow electrical signals to pass quickly from cell to cell, ensuring that the contractions are synchronized and powerful enough to facilitate birth.
The Myometrium’s Function During the Menstrual Cycle
The myometrium’s contractions are responsible for helping to expel the endometrial lining if a fertilized egg has not implanted in the uterine wall. This shedding of the endometrium results in menstruation, or the monthly period. The contractions that occur during menstruation are generally less powerful than those experienced during labor.
These uterine muscle contractions are the direct cause of menstrual cramps, a condition known medically as dysmenorrhea. The cramping sensations are triggered by chemicals called prostaglandins. Prostaglandins are released to stimulate the myometrium to contract and shed the uterine lining, which can sometimes lead to pain when the contractions are strong.
Disorders of the Myometrium
Several common medical conditions can arise from the myometrial tissue. Two of the most prevalent are uterine fibroids and adenomyosis. Both conditions originate within the muscular wall of the uterus and can affect women during their reproductive years.
Uterine fibroids are benign, or non-cancerous, tumors that grow from the smooth muscle cells of the myometrium. These growths can vary significantly in size, number, and location within the uterine wall. While many fibroids cause no symptoms, they can sometimes lead to issues such as heavy menstrual bleeding, pelvic pain, and pressure on the bladder or rectum.
Adenomyosis is a condition where the tissue that normally lines the uterus, the endometrium, begins to grow into the muscular myometrium. This displaced endometrial tissue continues to behave as it normally would during the menstrual cycle—thickening, breaking down, and bleeding. This process can cause the uterus to become enlarged and may result in symptoms like severe menstrual cramps and heavy, prolonged menstrual bleeding.