Fig. 2

Hematoxillin and eosin staining and immunohistochemistry of spontaneous feline fibrosarcoma. a A highly cellular neoplasm composed of spindle shaped to polygonal cells arranged in interlacing streams and bundles and supported by small to moderate amounts of collagenous matrix. H&E, magnification 100×. b The neoplastic cells have indistinct cell borders, small amount of fibrillar, pale eosinophilic cytoplasm, some showing slightly vacuolated cytoplasm. They have round to oval nuclei varied in size, dispersed and peripheralized chromatin with prominent magenta nucleoli (1-2). Mitotic figures are up to 8 per high power field (HPF), including bizarre mitotic figures (arrow). There are bi- or trinucleated cells as well as multinucleated giant cells (arrowhead) containing up to 9-30 and variable amounts of eosinophilic cytoplasm. Necrotic neoplastic cells undergo karyolysis and karyopycnosis are interspersed throughout the tumor. H&E, magnification 400×. c Ki-67-positive cells (brown nuclei) in feline fibrosarcoma. IHC, magnification 100× (400× insert). d PCNA-positive cells (brown nuclei) in feline fibrosarcoma. IHC, magnification 100× (400× insert)