Tumor progression

Between the two extremes of fully normal and highly malignant tissue architectures lies a broad spectrum of tissues of intermediate appearance. The different gradations of abnormality may well reflect cell populations that are evolving progressively away from normal and toward greater degree of aggressive and invasive behavior. Thus, each type of growth may represent a distinct step along this evolutionary pathway.

Some growths contain cells that deviate only minimally from those of normal tissues but may nevertheless be abnormal in that they contain excessive numbers of cells. Such growths are termed hyperplastic. In spite of their apparently deregulated proliferation, the cells forming hyperplastic growths have retained the ability to assemble into tissues that appear reasonably normal.

An equally minimal deviation from normal is seen in metaplasia, where one type of normal cell layer is displaced by cells of another type that are not normally encountered in this site within a tissue. These invaders, although present in the wrong location, often appear completely normal under the microscope. Metaplasia is most frequent in epithelial transition zones where one type of epithelium meets another.

A slightly more abnormal tissue is said to be dysplastic. Cells within a dysplasia are usually abnormal cytologically; that is, the appearance of individual cells is no longer normal. In dysplastic growths, the relative numbers of the various cell types seen in the normal tissue are no longer observed. Together, these changes in individual cells and in cell numbers have major effects on the overall tissue architecture. Dysplasia is considered to be a transitional state between completely benign growths and those that are premalignant.

Even more abnormal are the growths that are seen in epithelial tissues and termed variously adenomas, polyps, adenomatous polyps, papillomas, and in skin, warts. These are often large growths that can be readily detected with the naked eye. Under the microscope, the tissue within these adenomatous growths is seen to be dysplastic. Since adenomatous growths do not penetrate the basement membrane and invade underlying tissues, they are considered to be benign.

A further degree of abnormality is represented by growths that do invade underlying tissues. Here, for the first time, we encounter malignant tumors that have a substantial potential of threatening the life of the individual who carries them. This disparate collection of growths - both benign and malignant - are called collectively neoplasias, i.e., new types of tissue. (Some reserve the term "neoplasia" for malignant tumors.)
(Weinberg, 2007. Biology of Cancer)