Expression pattern of keratin subclasses in pancreatoblastoma with special emphasis on squamoid corpuscles

Expression patterns of keratins (K), both simple epithelia-type (K7, K8, K18, K19) and complex/stratified epithelia-type (K1, K4, K5/6, K10, K13, K14, K15, K16, K17), and epithelial membrane antigen (EMA) were immunohistochemically studied in six pancreatoblastomas (PBL). In all six tumors, areas with overt acinar differentiation (AA), solid areas without any specific differentiation (SO), and squamoid corpuscles (SC) were diffusely positive for K8, K18, and K19. The AA and SO in all the tumors were diffusely positive for K7, but the SC were negative or displayed only scattered reactivity for K7. In three tumors, the AA and the SC showed scattered reactivity for K5/6. No reactivity for other complex/stratified epithelia-type K was found in any of the examined tumor. All tumors were reactive for EMA with consistent predominancy in the SC. Ultrastructurally, well-developed desmosome-tonofilament complexes were only partially observed in tumor cells comprising the SC. These results implied that (i) the SC usually lack a character of complete squamous metaplasia; and (ii) the SC have a characteristic phenotype (K8/K18/K19/EMA-positive, K7-negative or scatteredly positive) that can potentially be useful to delineate the SC in PBL.