Features of a typical PSI tumor. (A) low magnification showing expanding boundary of tumor (inset enlarged in B). (B) high magnification of disarrayed tumor cells showing mitotic figures (asterisks), enlarged nuclei with prominent nucleoli (arrow; enlarged in inset), and an apoptotic body (arrowhead). (C & D) immunohistochemical staining for a keratin epithelial marker, low and high magnification, respectively. (E) immunohistochemical staining for the apoptosis marker cleaved caspase-3. (F) normal-appearing intestinal epithelium away from the tumor, showing orderly cell arrangement and small nuclei without enlarged nucleoli (compare insets of panelsF & B). Original optical magnifications 100x and 1000x.