The mammary myoepithelial cell - Cinderella or ugly sister?

Two epithelial cell types, present in roughly equal numbers, line the entire normal duct and lobular system of the human breast. There is an inner 'luminal' or 'secretory' cell layer and an incomplete outer myoepithelial cell layer. Since the myoepithelium is incomplete, some luminal cells reach the basement membrane. The myoepithelial cells, in contrast, do not reach the luminal surface. The myoepithelial cells are attached to the basement membrane by hemidesmosomes and to the adjacent luminal cells by desmosomes. The cells have pinocytic vesicles, containing microfilaments and dense bodies resembling smooth muscle. The myofilaments are not as well developed in the cells lining the acini compared with those lining the terminal ducts and the interlobular ducts. The myoepithelial cell is, in some respects, the Cinderella of the breast. It has been largely ignored in the context of breast cancer. Although the cell's auxiliary role in lactational physiology is well recognised, this is a minor component of the process and is limited to assisting milk ejection during suckling in response of oxytocin. What, however, of the other 99% of time? Those with a more philosophical bent have found it hard to believe that the contractile function represents the be all and end all of myoepithelial cell function.Its position, interposed between stroma and lumen, places it in an ideal situation to control many aspects of luminal function. It could regulate polarity, electrolyte and fluid flow, filter and process signals of endocrine or paracrine nature, and perhaps act as an intermediary in such signalling processes by passing information both inwards and outwards in a paracrine fashion or via intra-epithelial gap-junctions [1]. One school of thought has indeed invested the myoepithelium with great significance as a paracrine inhibitor of invasion and thus an inhibitor of tumour progression [2]. Other workers have proposed that, in the absence of fully differentiated myoepithelial cel