Myoepithelial cell differentiation in the developing mammary gland: progressive acquisition of smooth muscle phenotype.

The most important portion of the mammary gland development occurs postnatally, with distinct periods of intensive morphogenesis taking place between birth and puberty and during pregnancy and lactation. To characterize the differentiation process of mammary myoepithelial cells, we have studied the expression patterns of several smooth muscle phenotypic markers, including contractile proteins, alpha-smooth muscle-actin (alpha-SM-actin), smooth muscle myosin heavy chains (SM-MHC), and calponin; components of cell-extracellular matrix adherens junctions, phosphoglucomutase-related protein (PGM), vinculin variants, integrin subunits; and laminin variant chains in the developing rat mammary gland using immunofluorescence microscopy. alpha-SM-actin- and SM-MHC-positive cells were found first in newborn animals, while calponin, PGM and alpha1 integrin subunit began to be expressed in prepubertal animals (1.5 weeks). Vinculin, beta1 and alpha3 integrin subunits were largely confined to the basal cell layer at all developmental stages examined with greater staining starting at 1.5 weeks. Meta-vinculin was identified only in myoepithelial cells of the lactating gland. gamma1 laminin chain was present in the mammary gland basement membrane throughout development, while the beta2 chain was revealed in 3-week-old animals and accumulated later in postpubertal animals (7 weeks). Similarly, beta2 laminin chain was absent from the forming alveoli basement membrane in pregnant rats and started to accumulate in the lactating gland. In addition to temporal changes, we have observed spatial differences in the distribution of the phenotypic markers. Both in pre- and in postpubertal animals, alpha-SM-actin- and SM-MHC-positive cells of the growing ductal ends contained low amounts if any of calponin, PGM, and beta2 laminin chain. We conclude that during postnatal development, mammary myoepithelial cells progressively acquire a differentiated phenotype as revealed by the expression of various smooth muscle markers. Maturation of the myoepithelial cells is accompanied by upregulation of the smooth muscle integrin expression followed by accumulation of beta2-containing laminin variant. Thus, changes in adhesion system parallel with the myoepithelial cell differentiation.