Tissue evidence of carcinogenic factors at work

  • Metaplasia: an initial change from normal cells to a different cell type (such as chronic irritation of cigarette smoke causing ciliated pseudostratified epithelium to be replaced by squamous epithelium more able to withstand the insult).

  • Dysplasia: an increasing degree of disordered growth or maturation of the tissue (often thought to precede neoplasia) such as cervical dysplasia as a result of human papillomavirus infection. Dysplasia is still a reversible process. However, once the transformation to neoplasia has been made, the process is not reversible.

  • Thus, there is a natural history from metaplasia to dysplasia to neoplasia. This is best evidenced in development of uterine cervix and respiratory tract neoplasms.