Uterine fibroids (leiomyomas) are growths that originate from the muscle tissue of the uterus. They are common, affecting many individuals during their reproductive years, though many never experience symptoms. A fibroid is a non-cancerous mass, and its internal appearance is a direct reflection of its dense composition. Understanding what the inside of a fibroid looks like requires examining its structure from a macroscopic view, visible to the naked eye, down to its microscopic cellular makeup.
Context: What Uterine Fibroids Are
Uterine fibroids are benign tumors that arise from the myometrium, the smooth muscle layer of the uterus. These growths are monoclonal, meaning each fibroid originates from a single smooth muscle cell that proliferates abnormally. Fibroids are classified based on their location, such as intramural (within the uterine wall) or submucosal (protruding into the uterine cavity). They are composed of a significant amount of extracellular material, which contributes to their characteristic firmness. The presence of these growths may cause symptoms like heavy menstrual bleeding or pelvic pressure, depending on their size and position.
The Macroscopic Internal Structure
When a fibroid is cut open, its interior is typically firm, dense, and rubbery to the touch. This consistency is unlike the softer nature of the normal uterine muscle surrounding it. A healthy fibroid often appears pale white, grayish, or pinkish-white due to its relative lack of blood vessels compared to the surrounding myometrium.
The most defining macroscopic feature is the “whorled” or “basketweave” pattern visible across the cut surface. This distinctive look is created by bundles of smooth muscle fibers and connective tissue tightly interwoven and oriented in different directions. This swirling arrangement gives the fibroid its solid, compact appearance. A fibroid is generally well-defined and separated from the adjacent uterine tissue by a thin layer of compressed tissue, often referred to as a pseudocapsule.
Cellular Composition
The interior structure is defined by two main microscopic components: smooth muscle cells and the surrounding extracellular matrix. The smooth muscle cells are elongated and spindle-shaped, and are the proliferating elements of the tumor. These cells are densely packed and arranged in intersecting bundles that create the macroscopic whorled pattern.
Interspersed among these muscle cells is a substantial amount of fibrous connective tissue, mainly composed of collagen types I and III. This collagen-rich extracellular matrix is significantly more abundant in fibroids than in the normal uterine muscle. The large volume of this stiff, protein-based material gives the fibroid its remarkable density and hard quality. The ratio of smooth muscle cells to connective tissue can vary, influencing the final firmness and stiffness of the individual fibroid.
Variations in Internal Appearance
The classic pale, whorled appearance of a fibroid can change dramatically when the growth outgrows its blood supply, leading to degeneration.
Hyaline Degeneration
Hyaline degeneration is the most common form. It causes the tissue to take on a glassy, uniform, and less cellular appearance as muscle cells are replaced by a proteinaceous material. This change makes the fibroid feel less rubbery and more homogeneous.
Cystic Degeneration
Cystic degeneration is another variation, often occurring after extensive hyaline change. The tissue liquefies and forms fluid-filled spaces, giving the fibroid a softer, more watery core.
Calcification
Calcification occurs when calcium deposits harden the tissue, often in older individuals. This makes the fibroid feel gritty, rock-hard, and appear chalky white on the interior.
Red Degeneration
Red degeneration, or hemorrhagic necrosis, is less common and typically happens during pregnancy. It results from acute blood vessel rupture and internal bleeding, which gives the fibroid a reddish-brown or “carneous” appearance.