While there is much literature on differentiation—loosely defined as the gain of a single or a series of functions—we know much less about the forces and the pathways that maintain organ morphology and function as a unit. This may be partly because it is difficult to study a tissue as a unit in vivo and there are few techniques that allow maintenance of organs in vitro long enough and in such a way as to make cell and molecular biology experiments possible. Techniques for culturing cells in three-dimensional gels (3D) as a surrogate for tissues, however, have been steadily improving (for a recent review of current models, see [1]) and the method is now used by several laboratories.

In this commentary we discuss the following: first, how our laboratory came to develop a model of the mammary gland acinus; second, what this model has told us about mechanisms that govern tissue specificity and malignancy; and third, possible directions for future studies. We summarize the evidence for the central role of ECM signaling in the maintenance of mammary function in culture and (more briefly) its role in tumorigenesis. This is followed by a discussion of the role that tissue architecture and tissue polarity (as opposed to cell polarity) may play in these processes.