( A) Changes in overall tissue composition with tumor stages in the MMTV-PyMT mouse mammary carcinoma model. Examples of stromal components that have been proposed to have a more pronounced tumor-promoting function in advanced stages than in earlier stages are fibroblasts, type I collagen and the immune cell infiltrate, as described below.Ĭhanges in stromal organization during tumor progression However, in most cases the stroma of the later stages is more supportive of tumor progression than the stroma of early stages. The importance of stage-specific changes of the stroma is not yet completely clear. As tumors develop and progress, they undergo dramatic morphological changes ( Figure 1A Egeblad et al., 2008 Lin et al., 2003), which also involves the stroma ( Figure 1B, C & D Egeblad et al., 2008 Levental et al., 2009 Lin et al., 2006 Provenzano et al., 2006). Different components of the tumor stroma similarly influence the progression of the tumor ( Table 1).

The stroma is essential for normal organ development (e.g., Cunha, 2008 Puri and Hebrok, 2010 Wiseman and Werb, 2002). Various peptide factors (e.g., growth factors, chemokines, cytokines, antibodies) and metabolites are also found in the stroma. The stromal cells include mesenchymal supporting cells (e.g., fibroblasts and adipocytes), cells of the vascular system, and cells of the immune system. The stroma can be divided into several classes: the ECM, which is composed of proteoglycans, hyaluronic acid and fibrous proteins (e.g., collagen, fibronectin and laminin), and stromal cells. Organs are composed of the cells that perform the main organ function (e.g., secrete hormones or enzymes) and the stroma (from Latin or Greek, often translated as “mat” or “bed”), the supportive framework of an organ.

Cells of the same type join together