The term malignancy defines a complex cellular phenotype that is conventionally divided into three stages: (1) progressive multiplication in vivo-, (2) loss of coaptation resulting in the movement of cells away from the growing tumour into the surrounding tissues (invasion) ; and (3) the generation of secondary deposits elsewhere in the body (metastasis). Metastasis presupposes invasion, but progressive (and destructive) growth may occur without either. In a clinical context a tumour would not be classified as malignant unless it showed at least some evidence of invasion; but a malignant tumour may or may not metastasize. Essentially nothing is known about the genetic determinants of invasiveness. In large part this is due to the absence of any satisfactory experimental model for the study of this parameter. Systems involving the penetration of cells into avian and other embryonic membranes (Armstrong, Quigley & Sidebottom, 1982; Basson & Sidebottom, unpublished), or...

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