[An alternate model of tumoral progression]

Tumor development is usually described as the result of successive waves of genetic and epigenetic changes and of Darwinian selections that allow the progressive emergence of cells with proliferation and survival advantages. According to this model of tumoral progression, the acquisition of metastatic properties is perceived as a late stage-induced response to environmentally damaging conditions in the tumor, in particular oxygen and nutriment lack. Analyzing the consequences linked to the reactivation of embryonic programmes leads, for some cancer types, to question the linearity of this model, by showing that the dissemination of cancerous or precancerous cells may begin as early as the lesions convert from a premalignant to a malignant state. We propose to review the main findings that brought about these conclusions and to discuss what consequences they might have.