Myoid cells are a unique and highly specialized group of cells found throughout the body, possessing characteristics that bridge the gap between two major tissue types: epithelial/connective tissue and muscle. The presence of the prefix “myo-” highlights their distinctive ability to contract, a function traditionally associated with muscle cells. These cells are integral to the structure and normal operation of various organs, serving as contractile elements that facilitate movement and expulsion, or as structural supporters. Their dual nature makes them adaptable components of tissue microenvironments, enabling them to perform mechanical work while also interacting closely with surrounding cells and the extracellular matrix.
What Defines a Myoid Cell
Myoid cells are characterized fundamentally by the presence of contractile proteins within their cytoplasm, specifically actin and myosin filaments. While they share this molecular machinery with true smooth muscle cells, myoid cells are integrated into non-muscular tissues, which sets them apart functionally and structurally. The arrangement of these filaments allows the cells to generate tension and perform mechanical contraction. Their cellular identity is complex, often reflecting a mesenchymal origin with subsequent differentiation.
Myoepithelial Cells
Myoepithelial cells represent one primary category, typically found in glandular tissues. They possess both epithelial markers, such as keratins, and muscle-like markers, like alpha-smooth muscle actin (\(\alpha\)SMA). These cells are typically stellate or “basket-shaped” and are positioned between the glandular secretory cells and the surrounding basement membrane.
Peritubular Myoid Cells
Peritubular myoid cells form the second major category, wrapping around tubular structures in organs like the testes. These cells are derived from the mesenchyme and are often classified as specialized smooth muscle cells or myofibroblasts. They contain abundant \(\alpha\)SMA, allowing them to contract rhythmically or to maintain a state of sustained tension, linking directly to their structural and transport functions.
Diverse Roles Across Body Systems
The contractile nature of myoid cells is harnessed across multiple organ systems to facilitate essential physiological processes, most prominently glandular secretion and fluid transport. Myoepithelial cells in the mammary glands form a basket-like network around the milk-producing alveoli and ducts. Upon hormonal stimulation, such as the release of oxytocin, these cells contract powerfully to squeeze the milk from the alveoli into the ducts, enabling the milk ejection reflex.
Similar mechanical expulsion roles are observed in other exocrine glands. In salivary glands, their contraction helps propel saliva through the ducts and into the oral cavity. They play an analogous role in sweat glands, assisting in the movement of sweat to the skin’s surface.
Peritubular myoid cells surround the seminiferous tubules in the testes where sperm development occurs. These cells generate regular, wave-like contractions that are necessary for moving non-motile spermatozoa and testicular fluid out of the tubules towards the epididymis. Beyond their contractile role, peritubular cells also provide structural support by synthesizing and maintaining the basement membrane of the tubule. Furthermore, they regulate spermatogenesis by communicating with Sertoli cells through the secretion of growth factors.
Myoid-like characteristics are also adopted by hepatic stellate cells (HSCs) in the liver. In a healthy liver, HSCs are quiescent, storing Vitamin A in lipid droplets and maintaining the normal architecture of the tissue. However, when the liver is injured, these cells become “activated,” transforming into a myofibroblast-like phenotype that expresses \(\alpha\)SMA and becomes contractile. This transformation is central to the progression of liver pathology.
Myoid Cells and Tissue Dynamics
The capacity of myoid cells to transform and interact with the extracellular matrix makes them major players in tissue response to chronic injury and disease progression. A prime example is their involvement in fibrosis, the formation of excessive scar tissue that impairs organ function. Following chronic injury in organs like the liver, the activated hepatic stellate cells differentiate into myofibroblasts that begin to overproduce and deposit extracellular matrix components, notably collagen.
The resulting accumulation of this matrix, driven by the fibrogenic and contractile actions of these myoid-like cells, leads to tissue stiffening and the eventual development of cirrhosis in the liver. This pathological differentiation is triggered by inflammatory signals, such as transforming growth factor-beta (TGF-\(\beta\)), and mechanical cues from the stiffening tissue itself. These activated cells not only form the scar tissue but also contract, which further distorts the organ’s structure and compromises blood flow.
In the context of cancer, myoepithelial cells frequently act as a protective shield against tumor spread. In early-stage breast cancer, for example, the presence of an intact myoepithelial layer surrounding the abnormal ductal cells defines the lesion as non-invasive ductal carcinoma in situ. These cells actively suppress tumor invasion by secreting anti-angiogenic factors and protease inhibitors, such as Maspin, which inhibit the cancer cells’ ability to degrade the surrounding tissue.
The loss or disruption of this myoepithelial barrier is the defining step that allows the tumor to transition into invasive carcinoma, where cancer cells escape into the surrounding stroma to begin the process of metastasis. Thus, myoid cells demonstrate a duality, being essential for normal function and structure but also serving as central contributors to organ failure and disease progression when their regulatory mechanisms are compromised.