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Myometrium – Structure, Function and Disorders

The myometrium is the middle, muscular layer of the uterine wall. It enables contractions during childbirth and plays a central role in the female reproductive system.

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Things worth knowing about "Myometrium"

The myometrium is the middle, muscular layer of the uterine wall. It enables contractions during childbirth and plays a central role in the female reproductive system.

What is the Myometrium?

The myometrium is the middle and thickest layer of the uterine wall. It is composed primarily of smooth muscle cells arranged in multiple layers and is essential for normal uterine function. The uterine wall consists of three distinct layers: the inner endometrium (uterine lining), the middle myometrium, and the outer perimetrium (serosal covering).

Structure and Anatomy

The myometrium is organized into three muscular layers:

Inner longitudinal layer: Located directly beneath the endometrium, containing longitudinally oriented muscle fibers.
Middle circular layer: The thickest layer, featuring circularly arranged muscle fibers along with an abundant network of blood vessels and lymphatics.
Outer longitudinal layer: Adjacent to the perimetrium, also containing longitudinal muscle fibers.

The smooth muscle cells of the myometrium are connected by gap junctions, which allow coordinated, simultaneous contractions across the entire muscle.

Functions of the Myometrium

The myometrium serves several vital functions in the female body:

Labor contractions: Powerful, rhythmic contractions of the myometrium facilitate cervical dilation and expulsion of the baby during labor.
Menstruation: Mild contractions help shed the endometrial lining during menstruation.
Implantation support: The myometrium supports embryo implantation through coordinated contractions and structural stability.
Fetal protection: During pregnancy, the myometrium undergoes significant hypertrophy to accommodate and protect the growing fetus.

Hormonal Regulation

The activity of the myometrium is largely controlled by hormones:

Estrogen: Promotes muscle cell growth and increases myometrial excitability.
Progesterone: Inhibits contractions during pregnancy, maintaining uterine quiescence to prevent preterm labor.
Oxytocin: A hormone released from the hypothalamus that stimulates myometrial contractions near the end of pregnancy and during labor.
Prostaglandins: Local signaling molecules that promote contractions and contribute to menstrual pain (dysmenorrhea).

Diseases of the Myometrium

Uterine Fibroids (Leiomyomas)

Uterine fibroids are benign tumors arising from the smooth muscle cells of the myometrium. They are among the most common benign tumors in women of reproductive age. Symptoms may include heavy menstrual bleeding, pelvic pain, and pressure on adjacent organs.

Adenomyosis

In adenomyosis, endometrial tissue grows into the myometrium. This can cause heavy menstrual bleeding, dysmenorrhea (painful periods), and an enlarged uterus.

Myometritis

Myometritis is an inflammation of the myometrium, often occurring in association with endometritis (inflammation of the uterine lining) and typically caused by bacterial infections.

Leiomyosarcoma

Although rare, malignant tumors can arise from the smooth muscle cells of the myometrium. These are called leiomyosarcomas and must be distinguished from benign fibroids. They require specialized oncological treatment.

Diagnosis of Myometrial Conditions

Several diagnostic methods are available to assess the myometrium:

Ultrasound (sonography): The most common method for evaluating myometrial changes, particularly transvaginal ultrasound.
MRI (magnetic resonance imaging): Provides detailed visualization of myometrial structure and is especially useful in differentiating fibroids from adenomyosis.
Hysteroscopy: Direct visualization of the uterine cavity using an endoscope.
Biopsy: Tissue sampling for histological examination when malignancy is suspected.

References

Cunningham FG et al. - Williams Obstetrics. 25th Edition. McGraw-Hill Education, 2018.
Standring S (ed.) - Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd Edition. Elsevier, 2020.
World Health Organization (WHO) - Reproductive Health and Research: Uterine Fibroids. Available at: https://www.who.int (accessed 2024).

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