Lung - Metaplasia, Squamous
comment:Squamous metaplasia of the alveolar epithelium ( Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , and Figure 8 ) is characterized by replacement of type I and type II pneumocytes with squamous epithelium. Squamous metaplasia of the airway epithelium is characterized by replacement of bronchiolar or bronchial epithelium with squamous epithelium. Squamous epithelium is a stratified epithelium with orderly progression of cell maturation from cuboidal basal cells at the basement membrane to flattened squamous cells at the surface. The epithelium may be keratinized, and the cells may contain keratohyalin granules. Early squamous metaplasia, where only the surface cells are flattened, can be difficult to identify. Squamous metaplasia should not be confused with regeneration, since regenerating epithelial cells can assume a flattened morphology (stretching out to cover a denuded basement membrane). Generally, more than one layer of cells is present in squamous metaplasia. Squamous metaplasia can be seen as a response to chronic irritation or injury and is thought to be an adaptive change, as the squamous epithelium is more resistant to injury than is respiratory or alveolar epithelium. Consequently, squamous metaplasia is often accompanied by inflammation, fibrosis, necrosis, ulceration, and areas of epithelial hyperplasia. Though not typically considered preneoplastic, under some conditions squamous metaplasia may give rise to neoplasia (cystic keratinizing epithelioma or squamous cell carcinoma). Metaplastic epithelium in which there are atypical cells ( Figure 8 ) may be preneoplastic.
recommendation:Lung - Metaplasia, Squamous should be diagnosed whenever present and assigned a severity grade. A site modifier (i.e., alveolus, bronchiole, or bronchus) should be included in the diagnosis to indicate the location of the lesion. Associated lesions, such as inflammation, fibrosis, necrosis, or hyperplasia, should be diagnosed separately. If atypia is present, then the modifier "atypical" should be added to the diagnosis.
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