Pyloric gland type adenoma of the gallbladder with squamoid spindle cell metaplasia

This study documents a pyloric gland type adenoma of the gallbladder with prominent spindle cell metaplasia arising in a 61 year-old woman. A pedunculated polyp, 1.5 x 1.0 x 1.0 cm, was histologically diagnosed as a tubular adenoma. Most glandular structures showed positivity for a monoclonal antibody M2 (2B5) which indicates a differentiation to pylotic gland type. The spindle cell component displayed no apparent epithelial structures but stained mostly positive for pancytokeratin and cytokeratin (CK) 18, and focally for CK 5 + 6, CKs 7 and 19, whereas CKs 8, 13, 20, and non-epithelial markers could not be demonstrated. This suggests that the spindle cells were immature epithelial cells differentiating towards squamous and/or glandular cells. Even in spindle cell areas, the nuclear atypia was mild, and proliferating cells positive for MIB-1 (Ki-67) antigen were infrequently seen. This unique phenomenon, of which only three cases have been previously reported, is considered to represent benign squamoid spindle cell metaplasia.