Grossly, the adenoma appears as a discrete, grayish-yellow, soft mass less than 1 cm in diameter (microadenoma). Some microadenomas show little tendency to grow, but others become invasive, destroy the sella and dura, and infiltrate the surrounding structures (macroadenoma). Massive hemorrhage and infarct may also occur (pituitary apoplexy).

Histologically, the tumor cells have small round or oval nuclei, which display stippled chromatin, a characteristic of hormone-secreting tumors. Mitoses and cellular pleomorphism may occur, but are not considered as evidence of aggressiveness. The cellular pattern may be uniform or may show a papillary, glandular, perivas-cular, or trabecular arrangement (Fig. 11.32). Hormone immunohistochemical studies identify the specific hormone produced by the adenoma. Null-cell adenomas are chromophobic and are immunonegative.

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