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
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.