The myoepithelial cell: its role in normal mammary glands and breast cancer.

Mammary gland epithelium is composed of an inner layer of secretory cells (luminal) and an outer layer of myoepithelial cells (MEC) bordering the basal lamina which separates the epithelial layer from the extracellular matrix. Mature MECs morphologically resemble smooth muscle cells; however, they exhibit features typical for epithelial cells, such as the presence of specific cytokeratin filaments. During lactation, secretory cells synthesize milk components, which are collected in alveoli and duct lumen, and transported to the nipple as a result of MEC contraction. Although the induction of MEC contraction results from oxytocin action, also other, still unknown auto/paracrine mechanisms participate in the regulation of this process. As well as milk ejection, MECs are involved in mammary gland morphogenesis in all developmental stages, modulating proliferation and differentiation of luminal cells. They take part in the formation of extracellular matrix, synthesizing its components and secreting proteinases and their inhibitors. In addition, MECs are regarded as natural cancer suppressors, stabilizing the normal structure of the mammary gland, they secrete suppressor proteins (e.g. maspin) limiting cancer growth, invasiveness, and neoangiogenesis. The majority of malignant breast cancers are derived from luminal cells, whereas neoplasms of MEC origin are the most seldom and usually benign form of breast tumours. MECs are markedly resistant to malignant transformation and they are able to suppress the transformation of neighboring luminal cells. Therefore, a deeper insight into the role of MECs in the physiology and pathology of mammary glands would allow a better understanding of cancerogenesis mechanisms and possible application of specific MEC markers in the diagnosis and therapy of breast cancer.