Myoepithelial cells are a distinct cell type found within various glandular tissues throughout the body, including the breast. These cells possess characteristics of both muscle and epithelial cells, contributing to the functions of the glands they inhabit. In the breast, their presence is fundamental to normal physiological processes, playing roles that extend beyond simple contraction. Understanding these cells provides insight into the workings of breast health.
Where Myoepithelial Cells Are Found and How They Are Structured
Within the breast, myoepithelial cells are located in a thin layer positioned between the luminal epithelial cells and the basement membrane of the ducts and lobules. This strategic placement allows them to interact with both the milk-producing cells and the surrounding connective tissue. They form a continuous layer along the ducts and a more basket-like, discontinuous network around the terminal duct lobular units, which are the milk-producing units.
Myoepithelial cells exhibit a morphology often described as spindle-shaped in ducts and stellate (star-shaped) in lobules, with multiple processes radiating from their cell bodies. A defining characteristic is the presence of contractile proteins within their cytoplasm, specifically alpha-smooth muscle actin (αSMA) and myosin. These proteins enable myoepithelial cells to contract and contribute to glandular function. They also express epithelial keratins and form desmosomal attachments with adjacent luminal and other myoepithelial cells, highlighting their dual nature.
Essential Functions in Breast Health
The contractile ability of myoepithelial cells is evident during lactation, where they play a role in milk ejection. In response to signals like the hormone oxytocin, these cells contract around the mammary ducts and alveoli, squeezing milk into the larger ducts and towards the nipple. This mechanical action supports breastfeeding.
Myoepithelial cells also form a protective barrier within the breast tissue, separating the luminal epithelial cells from the surrounding stroma. This barrier maintains the structural integrity of the breast and prevents abnormal cell spread. They contribute to the composition of the basement membrane, producing elements such as fibronectin, laminin, and collagen IV, which reinforce this layer.
Beyond their contractile and barrier roles, myoepithelial cells engage in signaling and regulation with other cells in the breast. They communicate with luminal cells and the surrounding stromal cells, influencing processes such as cell growth, differentiation, and tissue architecture. This communication occurs through the secretion of various factors and maintains mammary gland organization and function.
Myoepithelial Cells and Breast Disease
The integrity of the myoepithelial cell layer is a diagnostic indicator for pathologists evaluating breast tissue. The presence of an intact myoepithelial layer suggests a benign lesion, while its absence or disruption is a hallmark of invasive cancers. This distinction helps clinicians differentiate between benign and malignant growths.
Myoepithelial cells also act as a barrier against the invasion of cancerous cells in early-stage malignancies. In conditions like ductal carcinoma in situ (DCIS), where abnormal cells are confined within the ducts, the myoepithelial layer remains intact, preventing their spread into surrounding breast tissue. This containment underscores their role in tumor suppression in early disease stages.
However, alterations in myoepithelial cells can indicate disease progression. Myoepithelial cells surrounding DCIS lesions exhibit changes in their differentiation markers, preceding the transition to invasive disease. The disruption of this layer can lead to the release of factors that promote tumor cell proliferation and invasiveness, highlighting their role in disease progression.