Replacement of columnar epithelium of respiratory tract to squamous epithelium is

Metaplasia is a reversible change in which one adult cell type (epithelial/mesenchymal) is replaced by another adult cell type. Often seen in response to chronic irritation. Makes the cells able to withstand stress. Metaplasia is thought to arise by reprogramming of stem cells to differentiate along a new pathway. May result in reduced function or increased propensity for malignant transformation Two types - squamous metaplasia & columnar metaplasia. Squamous metaplasia: The normal columnar epithelial cells are focally or widely replaced by squamous epithelial cells. Eg: Chronic smokers - to survive the noxious chemicals in cigarette smoke Squamous metaplasia - columnar epithelial changes to squamous type. Eg: In chronic gastric reflux, the normal stratified squamous epithelium of lower oesophagus undergoes metaplastic transformation to gastric or intestinal type columnar epithelium. Metaplasia may also occur in mesenchymal cells but in these situations, it is generally a reaction to some pathologic alteration not an adaptive response to stress. 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 persistent may predispose to malignant transformation of the epithelium 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 respiratory Refer robbins 9/e p5