Ultrastructure of nuclear "inclusions" in pheochromocytoma and paraganglioma.

Nuclear "inclusions" similar to those seen in a variety of normal and neoplastic cells are commonly found in pheochromocytomas and paragangliomas. Rather than representing areas of nuclear or nucleolar degeneration, the "inclusions" of these tumor cells represent deep invaginations of the cytoplasm into the nucleus. The great frequency of nuclear "inclusions" in pheochromocytomas is a reflection of the high degree of nuclear angulation and lobulation commonly seen in these tumors.