The mammary epithelium is composed of a luminal epithelium and a basal layer containing myoepithelial cells

In adult females, the mammary epithelium is organized in two layers, a luminal epithelium and a basal layer, consisting of myoepithelial cells. Undifferentiated precursors of both luminal and myoepithelial cells are also believed to be basally localized. The mammary epithelium bilayer is surrounded by a basement membrane embedded in connective tissue. In the fully differentiated mammary gland, that is during the lactation period, luminal epithelial cells are specialized in the synthesis and secretion of milk proteins while basal myoepithelial cells display contractile properties (for reviews, see Daniel and Silberstein, 1987; Sakakura, 1991). Basal cells in the mammary epithelial bilayer differ from luminal cells in that they express a set of smooth muscle differentiation markers (Lazard et al., 1993; Deugnier et al., 1995) and in addition, similar to basal keratinocytes, specific basal cell markers such as the keratin 5 and keratin 14 (K14) pair and P-cadherin (Taylor-Papadimitriou and Lane, 1987; Daniel et al., 1995). Keratin 5/14 filaments have an important role in the cytoarchitecture of basal epithelial cells, as they are connected to hemidesmosomes, which are multicomponent adhesion complexes attaching basal cells to the underlying basement membrane (Burgeson and Christiano, 1997). The absence of K14 in null mutant mice gives rise to blistering skin disorders involving cytolysis of the basal layer of epidermis (Llyod et al., 1995). The second basal cell marker, P-cadherin, is a component of cell-cell adherens junctions, which are essential for the establishment and maintenance of epithelia (Nose and Takeichi, 1986; Takeichi, 1995). The adhesive function of cadherins depends on their association with a family of cytoplasmic regulatory molecules, the catenins, which link cadherins to the actin cytoskeleton (Aberle et al., 1996). Interestingly, inactivation of the P-cadherin gene leads to precocious development of the mammary gland, and therefore P-cadherin is believed to be involved in the control of growth and differentiation of mammary epithelium (Radice et al., 1997). The mammary myoepithelial cells, similar to basal cells in epidermis, are enriched in integrins, the major extracellular matrix receptors. In the basal layer of the mammary epithelium, histological studies have revealed the integrin subunits α1, α2, α3 and α6, β1 and β4 (Sonnenberg et al., 1035 Journal of Cell Science 112, 1035-1044 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS9889