Three division-competent, structurally-distinct cell populations contribute to murine mammary epithelial renewal

Markers for the division-competent cells in mammary gland epithelium are important to the understanding of normal and neoplastic mammary epithelial growth and architectural maintenance, but have been difficult to reveal. Using the presence of condensed chromosomes as an indicator of division competence, we have described morphological markers in the nucleus and cytoplasm that reliably characterize two sets of structurally-distinct, division-competent cells in murine (i.e. mouse and rat) mammary epithelium. The markers, based, in part, on cell size, nuclear and cytoplasmic staining characteristics, and distinctive morphological features of the nucleus and cytoplasm allow classification of the division-competent cells into two categories: 'large light cells' and 'small light cells'. Based on the degree of cytoplasmic organelle differentiation, the small light cells are the least differentiated cells in the epithelium, and the large light cells appear, structurally and functionally, to be in the early stages of secretory differentiation. We demonstrate, using statistical analysis of cell counts (per unit area of epithelium) from four stages of mammary epithelial growth, that there are, in fact, three division-competent cell populations in the rat mammary epithelium, and that the large light cell is a direct precursor to terminally differentiated cells, both secretory and myoepithelial. Using our results we synthesize a morphological model of cell mitosis and the progression of epithelial differentiation in murine mammary gland from a stem cell through two progenitors.