Ck5-positive cells are precursor cells of glandular and myoepithelial cell lineages in the human breast epithelium. A new cell concept as a basis for a better understanding of proliferative breast disease?

The question of whether stem cells exist in the human breast has never been satisfactorily resolved. Using a double immunofluorescence technique for simultaneous demonstration of cytokeratin subgroups 5, 8/18/19 and the differentiation marker smooth muscle actin, we provided direct evidence of the existence of a Ck5-positive cells that differentiate to either the glandular or myoepithelial cell lineage via intermediary cells. We postulate that the Ck5-positive cells are phenotypically and behaviourally committed adult stem cells. We further provide evidence that benign proliferative breast disease lesions resemble normal breast epithelium, whereas most breast carcinomas phenotypically consist of glandularly differentiated cells. The results were corroborated biochemically by Western blotting techniques. This cell biological model provides a new tool which helps to distinguish benign and malignant breast lesions. Furthermore this model provides a basis to unravel the regulatory mechanisms that govern normal and pathological cell growth.