Transformation of one epithelium to other epithelium
Ref Robbins 8/e p10,11; 9/e p37 Metaplasia is a reversible change in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type. In this type of cellular adaptation, a cell type sensitive to a paicular stress is replaced by another cell type better able to withstand the adverse environment. Metaplasia is thought to arise by reprogramming of stem cells to differ- entiate along a new pathway rather than a phenotypic change (transdifferentiation) of already differentiated cells. Epithelial metaplasia is exemplified by the squamous change that occurs in the respiratory epithelium of habitual cigarette smokers (Fig. 1-5). The normal ciliated columnar epithelial cells of the trachea and bronchi are focally or widely replaced by stratified squamous epithelial cells. The rugged stratified squamous epithelium may be able to survive the noxious chemicals in cigarette smoke that the more fragile specialized epithelium would not tolerate. Although the metaplastic squamous epithelium has survival advantages, impoant protective mechanisms are lost, such as mucus secretion and ciliary clearance of paiculate matter. Epithelial metaplasia is therefore a double-edged sword. Moreover, the influences that induce metaplastic change, if per- sistent, may predispose to malignant transformation of the epi- thelium. In fact, squamous metaplasia of the respiratory epithelium often coexists with lung cancers composed of malignant squamous cells. It is thought that cigarette smoking initially causes squamous metaplasia, and cancers arise later in some of these altered foci. Since vitamin A is essential for normal epithelial differentiation, its deficiency may also induce squamous metaplasia in the respiratoryepithelium. Metaplasia need not always occur in the direc- tion of columnar to squamous epithelium; in chronic gastric reflux, the normal stratified squamous epithelium of the lower esophagus may undergo metaplastic transformation to gastric or intestinal-type columnar epithelium. Metapla- sia may also occur in mesenchymal cells but in these situ- ations it is generally a reaction to some pathologic alteration and not an adaptive response to stress. For example, bone is occasionally formed in soft tissues, paicularly in foci of injury.
Dysplasia
Hyperplasia
Neoplasia
Metaplasia
General anatomy