The culture of differentiated cells requires conditions that acknowledge the complicated cell-cell interactions that both occur in vivo and are responsible for affecting and maintaining the differentiated states of cells. In brief, one must use conditions that mimic the epithelial-mesenchymal relationship that is universal and constitutes the organizational basis for all metazoan tissues. This relationship is sustained by a set of soluble signals (autocrine, paracrine, and endocrine) and by a set of insoluble signals (the extracellular matrix). Since this is a technical and methodological article, neither the scientific evidence for the importance of the epithelial-mesenchymal relationship nor the evidence forming the basis for the culture conditions will be described. Recent reviews have discussed this background in considerable detail (1 –12
