Morphological and histological features of C26 tumor. a) Exposed C26 tumor, three weeks following subcutaneous transplant of a 0.5 mm3 tumor fragment, has grown, becoming a spheroid measuring 0.5-1 cm in diameter. The tumor mass is well-defined and vascularized. Bar 5 mm. b) Photomicrographs of H&E-stained tumor cryosections. The tumor was collected three weeks following tumor transplant for histological analysis. Top, peripheral region showing tumor encapsulation and, bottom, inner region showing anaplastic appearance and lack of a necrotic core. Inset, cell morphology and vascularization at higher magnification. Bar = 100 μm c) Mitoses (arrow) were evaluated on C26 tumor sections, based on cell morphology identified as typical of prophase to telophase, and expressed as percentage of the cells counted in 10 randomly chosen fields for each tumor. An average mitotic index of 5 ± 2% (mean ± SEM) was calculated from data derived by four independent experiments (n = 8). d) TUNEL assay on C26 tumor sections of the same samples used for the mitotic index (the arrow points to a TUNEL+ cell) yielded an apoptotic index of 9 ± 3% (mean ± SEM).