Tissue architecture: the ultimate regulator of breast epithelial function

A problem in developmental biology that continues to take center stage is how higher organisms generate diverse tissues and organs given the same cellular genotype. In cell and tumor biology, the key question is not the production of form, but its preservation: how do tissues and organs maintain homeostasis, and how do cells within tissues lose or overcome these controls in cancer? Undoubtedly, mechanisms that maintain tissue specificity should share features with those employed to drive formation of the tissues. However, they are unlikely to be identical. At a simplistic level, developmental pathways may be thought of as a series of extremely rapid short-term events. Each new step depends on what came before, and the outcome is the organism itself at birth. All organs, with a few notable exceptions, such as the mammary gland and the brain, 'arrive' together and are complete when the organism is born. In mice and humans, these events occur in a mere 21 days and 9 months respectively. The stability of the differentiated state and the homeostasis of the organism, on the other hand, will last 40-110 times longer. How does the organism achieve this feat? How are tissues maintained? These questions also relate fundamentally to how tissues become malignant and, although not discussed here, to aging. 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 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. In an elegantly written and reasoned essay, Kirschner et al. coined the new science of developmental biology 'molecular vitalism'. They framed new concepts for self-organization as well as schemes for information flow in biological organization. Rao et al. reviewed and elaborated on differential-equation-based models of biochemical reaction networks and intracellular noise, with emphasis on bacteria and phage. Similarly, Hartwell et al. discussed the synergy between experiment and theory in elucidating 'modules' - collections of interacting molecules - and in unraveling how these modules collaborate to perform cellular functions such as signal transduction. We believe that many of these ideas will also be applicable to the maintenance of tissue specificity. As much as we agree with Kirschner et al. regarding the limitations of the machine analogy to biological systems, we conclude with thoughts on how we may proceed to model the complex tissue networks that govern breast tissue architecture. We suggest that our understanding of the structure and function of breast tissue would benefit from examining recent techniques for modeling large complex networks such as the World Wide Web and the Internet backbone among others