cancer progression has partially been elucidated, and several processes from growth factor secretion to evading immune response have been attributed to the stromal cells. The 1 / 18 Influence of Fibroblasts on Tumor Cell Growth and US7476724 titled “Humanized anti-cMet antibodies” respectively. There are no products in development or marketed products to declare. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials, as order EW-7197 detailed online in the guide for authors. ratio of tumor stroma has been shown to serve as an independent prognostic factor for breast cancer patients that indicates a three-fold increased risk of relapse for stroma-rich tumors. Further, stroma-related molecular signatures can be used to predict the resistance of breast cancer to neo-adjuvant chemotherapy. A desmoplastic reaction involving a variety of stromal cell types is often described as a distinct unique characteristic of PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19666110 pancreatic cancer. Similarly, stromal cells have also been implicated in cancer progression and prognosis of lung cancer. Fibroblasts 
constitute one of the most abundant cell types in the tumor stroma. In normal tissues, fibroblasts play an important role in maintaining homeostasis and in wound healing by producing an array of factors that constitute the extracellular matrix and other growth factors and cytokines that are essential for healing. The cross-talk between the tumor cells and stromal fibroblasts in the TME influences to the secretion of an array of growth factors and cytokine/chemokines that, in turn, support tumor cell growth or survival, induce neo-vascularization and generate an immuno-suppressive TME in several cancers. Currently, TAFs appear to play a key role in tumor progression, and provide significant predictive or prognostic value, as well as serve as potential therapeutic targets. To understand the mechanisms underlying the cross-talk between tumor cells and TAFs in vitro, a co-culture system in which tumor cells can interact with fibroblasts, similar to the TME in situ, is required. Conventionally, trans-well chambers are used for this purpose. Using this approach, cells are separated by a porous membrane through which soluble factors are able to diffuse freely but direct cell-cell interaction is absent. The importance of direct cell-cell contact in this context has been demonstrated by experiments showing that the collagenbased co-culture of breast cancer cells with serum-activated fibroblasts induced clonogenic growth in vitro. PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19667083 Recently, it has been shown that the direct interaction between luminal-/ basal-like breast cancer cells and fibroblasts invokes distinct phenotypic and gene expression changes that differ from trans-well co-cultures. In addition, Fujita et al., showed that pancreatic cancer cell proliferation was enhanced by directly co-culturing these cells with pancreatic stromal cells, allowing the two cell types to directly interact in the culture dishes. However, these studies were performed by culturing either one of the cell types on a flat 2D surface, which hardly represents the complex TME in vivo. It has been clearly demonstrated that the 2D culture system, although convenient for most applications, is a poor environment to study dynamic cellular interactions. Alternatively, 3D culture of cells provides an environment that preserves several phenotypic and functional characteristics of primary cells/tumors that reflects the in vivo conditions to a certain but
