Introduction A normal cell must develop at least two distinct critical characteristics to progress to tumorigenesis: first, it needs to become able to abnormally proliferate through alterations in the mechanisms of regulation of the cell cycle or by suppressing the pathways leading to controlled cell death (i.e. apoptosis) and, secondly, it is necessary that the cell is able to induce modifications of the environment where its growth potential can be realised. Solid invasive tumours are composed of two co-existing components: parenchymal cells (the tumour cells) and stromal cells with the surrounding extracellular matrix (ECM) (Dvorak 1986).

Growing experimental evidence indicates that tumoral stroma and its cells such as endothelial cells, fibroblasts and macrophages play an active role in tumour progression and metastasis (Bouck et al. 1996, Folkman 1996, Gasparini 1996a).

In fact, a mutual reciprocal stimulation may occur between tumour and stromal cells. Several growth factors secreted by