Figure 4 (A, B) Hematoxylin and eosin staining showing trabeculae and large nests of tumor cells with a high nucleus-to-cytoplasm ratio, scant cytoplasm, nuclear hyperchromasia, and finely granular chromatin. High mitotic activity, apoptotic debris, and areas of necrosis are evident. (C) Hematoxylin and eosin staining (20×) The neoplasm is characterized by trabeculae and nests of tumor cells with a high nucleus-to-cytoplasm ratio, scant cytoplasm, nuclear hyperchromasia, and finely granular chromatin, sometimes coarse. (D) Hematoxylin and eosin staining (40×). Neoplastic cells show prominent nucleoli. Frequent mitotic figures and apoptotic debris are observed. (E) Chromogranin A immunostaining shows cytoplasmic reactivity with a perinuclear dot-like pattern. (F) Cytokeratin AE1/AE3 immunostaining shows perinuclear dot-like positivity. (G) Leukocyte common antigen immunostaining (20×), performed to differentiate from lymphoproliferative disease, shows complete negativity. (H) CD56 immunostaining shows membrane positivity diffusely across tumor cells. (I) The Ki-67 proliferative index shows very high expression in neoplastic cells (hot spots 80%), consistent with the high cell turnover of the neoplasm. (J) Synaptophysin immunostaining (40×) shows fine granular cytoplasmic positivity, indicative of neuroendocrine differentiation.