Precursor lesions of the urinary bladder.

The classification of neoplastic precursor lesions in the urinary tract has evolved slowly with the gradual accumulation of clinicopathological data. Current nomenclature was codified most recently by the 2016 WHO classification, which is based on primary data with clinical outcome, consensus group statements and considerations of practical utility in routine diagnosis. This review discusses precursor lesions of urothelial, squamous and glandular lineage. For urothelial neoplasia, both flat lesions with atypia and early 'difficult-to-classify' proliferations are considered. Subtypes of squamous metaplasia, florid non-invasive squamous proliferations and frank squamous dysplasia are also addressed. Finally, rare glandular precursors of adenocarcinoma are reviewed, to include intestinal metaplasia, glandular dysplasia and villous adenoma. For each category, morphology (including differential diagnostic considerations), immunohistochemistry and any known molecular correlates are detailed. The goal is to provide a concise, practical up-to-date overview of this complex topic.