Myoepithelial cells are a distinct type of cell found within various glands throughout the body. These cells possess a unique hybrid nature, exhibiting characteristics of both muscle cells and epithelial cells. They are observed in exocrine glands, where they play an important role in facilitating the release of glandular secretions.
Structure and Location
Myoepithelial cells display a stellate, or star-shaped, morphology, though some can be spindle-shaped. These cells are positioned around the secretory units, known as acini, and ducts of exocrine glands. They form a thin layer situated between the basal lamina, also called the basement membrane, and the luminal secretory epithelial cells.
Their presence is widespread across several glandular organs. They are found in mammary glands, aiding in milk ejection, in salivary glands for saliva expulsion, in sweat glands for sweat release, and in lacrimal glands, which produce tears. These cells originate from the ectoderm during embryonic development.
Diverse Functions
The primary function of myoepithelial cells is their contractile ability, stemming from muscle-like proteins such as actin and myosin within their cytoplasm. This property allows them to compress secretory units and ducts, expelling fluids from the gland. For instance, in mammary glands, contraction is stimulated by oxytocin, leading to milk ejection during breastfeeding. Their contraction also aids in the flow of saliva and the release of sweat.
Beyond their contractile role, myoepithelial cells perform several other functions that help maintain tissue health. They act as a barrier, preventing the invasion of other cells into surrounding tissues. These cells also contribute to the structural integrity and overall architecture of glandular tissues, supporting the ductal system. Myoepithelial cells influence epithelial differentiation and synthesize extracellular matrix components, including materials for the basement membrane. They also secrete various growth factors, which regulate the microenvironment of surrounding tissues.
Clinical Relevance
Myoepithelial cells are important in diagnostic pathology, particularly for distinguishing between benign and malignant lesions in glandular tissues. Their presence in a tissue sample, especially in breast biopsies, often indicates a benign process, as they form a protective layer that is typically intact in non-cancerous conditions. Conversely, the absence or disruption of this myoepithelial cell layer can be a hallmark of invasive cancers, such as invasive breast cancer.
These cells also exhibit tumor suppressor properties in various contexts. They can exert an anti-invasive effect by promoting epithelial differentiation and inhibiting angiogenesis, the formation of new blood vessels that tumors rely on for growth. Research is exploring how understanding myoepithelial cell behavior might lead to new therapeutic strategies. Targeting these cells could involve restoring their tumor-suppressing functions or manipulating their environment to inhibit cancer progression.