They secrete factors that contribute to cycling, or do cells take turns to divide, in alternate cycles? Future studies focused on the characterization of the specific subpopulations of cells will provide insight into their roles. Whatever they may be doing, the gene expression and metabolic programs of these cells do not appear to obscure gene expression and metabolite measurements of the population as a whole. Some of these questions require new techniques to be developed in order to address them, including Afatinib abmole methods to culture cells in microfluidic chambers that maintain cell density and communication, reporters that more accurately reproduce the actual behavior of the genes they represent, as well as tracking individual cells within the chemostat over time. The work presented here is a step towards further understanding this fascinating biological phenomenon that may provide insights into many biological processes including cell growth, cell division, metabolism, and the logic of cell-cell communication within complex, intricate microbial communities. A major paradox exists in the current understanding of how insulin-like growth factors affect the aging process. Mammalian insulin-like growth factor 1 is critical for cellular proliferation, muscle and adipose tissue differentiation, and neural development. Moreover, IGF-I enhances cell survival in the face of numerous physiologic insults that include DNA damage and loss of cell adhesion. The IGF-I paradox lies in the fact that despite the proliferation- and survival-enhancing properties attributed to IGF-I, it is a reduction in IGF-I signaling that has been shown to extend lifespan in multiple organisms including nematodes, flies, and mammals. The paradoxical effects of IGF-I on cell survival, differentiation, and lifespan suggest that there may be a tradeoff between short-term benefit and long-term survival. However, the molecular mechanisms that underlie this tradeoff remain unclear. It is possible that long-term negative consequences of IGF-I stimulation cannot be fully appreciated in the culture systems presently used to study cell growth, survival, and differentiation. In vivo, these consequences may be difficult to identify due to the pleiotrophic effects of IGF-I. In order to examine the possible consequences of increased IGF-I signaling over extended periods, a fibroblast culture model was developed that allows human fibroblasts to be maintained in a quiescent state over a period of weeks. This model takes advantage of MCDB 105 culture medium, which has been specifically formulated for survival and growth of human fibroblasts in low serum. The medium provides all essential amino acids, glucose, nutrients, and trace elements required for proliferation. When growth factors are withdrawn, fibroblast cells enter a nondividing quiescent state that is fully reversible upon the addition of growth factors or serum. Using similar conditions, human fibroblasts cultures have been maintained under serum free conditions for up to 3 weeks without impacting growth potential.