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Epithelial hyperplasia

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Increase in number of cuboidal, columnar, ciliated or mucous cells without atypia. Cells maintain normal architecture of bronchioles and alveoli. The main distinctive features of regenerative hyperplasia are absence of direct link to tumor progression, and presence of inflammation and necrosis due to the inflicting toxic agent.

Epithelial hyperplasia, airways
Number of respiratory epithelial cells is increased diffusely or focally. Frequently luminal protrusions are observed, sometimes forming papillae. Mucous (goblet) cell metaplastic hyperplasia is a variant, in which the respiratory epithelium of conducting airways is replaced by mucous cells either as a single or a pseudostratified layer.
Epithelial hyperplasia, alveoli
Solitary or multiple foci of increased cellularity distal to terminal bronchioles. The background of broncho-alveolar architecture remains detectable, and epithelial cells are usually single layered. Round to oval hypertrophic type II pneumocytes with abundant eosinophilic cytoplasm line alveolar walls. In bronchiolar subvariant, also called bronchiolization of alveoli, alveolar walls are lined by cuboidal to columnar cells with features of bronchiolar differentiation, such as formation of cilia, Clara cell resemblance, and presence of mucous granules. Foci of consolidation may indicate early stages of adenoma formation. Macrophages may be present in the alveolar lumens.