Biology of cancer invasion and metastasis.

Current concepts of invasion eventually leading to metastasis are discussed and exemplified by cancers of the head and neck mucosa. Invasion occurs at a number of steps, each step making an ecosystem comprising not only the neoplastic cells but also their normal counterparts, a variety of host cells and the extracellular matrix. The ecosystem concept may explain aspects of metastasis such as site-dependence and organ-specificity of cancer metastasis as well as invasiveness of normal leucocytes. Genes implicated in invasion and metastasis are actively searched for. Recently, the epithelial cell-cell adhesion molecule E-cadherin has been identified as an i- (invasion suppressor) gene product, i.e. a molecule the expression of which counterbalances i+ (invasion promotor) gene activity. Downregulation of E-cadherin in human head and neck cancers may account for their invasive and metastatic behaviour.