Squamous metaplasia is a frequent epithelial alteration of the tracheobronchial mucosa. Its histogenesis may be related to chronic inflammation and injury of the bronchial epithelium, which leads to replacement of the normal ciliated columnar epithelium by a squamous epithelium. Therefore, squamous metaplasia may be seen in purely inflammatory settings such as infection or bronchiectasis. Squamous metaplasia may also be a preneoplastic lesion. In this situation, squamous metaplasia may be the result of exposure to tobacco smoke, in which case it is potentially a precursor after many years to the morphologic
progression to bronchogenic carcinoma through varying degrees of dysplasia. Squamous metaplasia due to tobacco exposure may or may not progress to dysplasia and eventually carcinoma. Because of the “field effect” of tobacco smoke on the bronchial mucosa, multiple areas of squamous metaplasia may be seen in smokers, some of which may progress to dysplasia or eventually carcinoma, whereas others may not. Squamous metaplasia or varying degrees of dysplasia may be present at the margins of a carcinoma and a biopsy may be performed on it when one is not performed on the carcinoma itself. Early changes include a loss of the ciliated columnar epithelium, basal cell hyperplasia, and formation of a low columnar epithelium without cilia.

Squamous atypia of the bronchial mucosa may be the result of inflammation or injury to the bronchial mucosa, or it may be a step in the morphologic progression from normal bronchial epithelium to carcinoma due to tobacco smoke exposure. In squamous dysplasia, cellular atypia progresses through mild, moderate, and severe dysplasia toward carcinoma in situ, analogous to the progression of squamous metaplasia through increasing degrees of dysplasia to carcinoma in situ seen with cervical carcinoma. With increasing degrees of squamous dysplasia, the degree of cytologic atypia increases and the degree of thickness of the squamous metaplasia exhibiting dysplasia increases. Similar to squamous metaplasia, squamous dysplasia may be multifocal in the bronchial mucosa of smokers, with some foci potentially progressing to more severe degrees of dysplasia or carcinoma over an extended period of time. As with squamous metaplasia, squamous dysplasia due to tobacco smoke does not always progress to carcinoma, and squamous dysplasia may be seen at the periphery of a carcinoma. On occasion squamous dysplasia at the margin of a carcinoma may be sampled on endobronchial biopsy rather than the adjacent carcinoma.

In carcinoma in situ, cellular atypia has advanced from severe dysplasia to a full-thickness mucosal abnormality, with an intact basement membrane (no invasion). Grossly, bronchial mucosa may show slight granularity and papillation, or it may be indistinguishable from normal mucosa. Carcinoma in situ may overlap considerably with severe squamous dysplasia, and a range of dysplastic findings may occur in any specific case. Eventually, carcinoma in situ may invade the underlying submucosa as a superficially invasive carcinoma and then beyond as an invasive carcinoma.