Pituitary adenoma with rich folliculo-stellate cells and mucin-producing epithelia arising in a 2-year-old girl.

Pituitary adenoma is a rare neoplasm in childhood, with prolactin and adrenocorticotropic hormone (ACTH)-secreting adenomas predominating in this age group. Herein is reported a case of an ACTH-producing macroadenoma with an unusual histology that occurred in a 2-year-old girl. Because of the patient's age and the macroadenoma's suprasellar location and large size (up to 4 cm in diameter), radical surgery was performed under the suspicion of craniopharyngioma or germ-cell tumor. Pathologically, it was a unique pituitary adenoma composed of monotonous ACTH-producing cells, smaller folliculo-stellate cells (FSC), and mucin-producing cells. The FSC, non-hormone-secreting pituitary cells of uncertain function, were confirmed by their S-100 protein, glial fibrillary acidic protein and cytokeratin expression immunoprofiles. The abrupt transition between the prominent gland-forming mucin-producing epithelia and the FSC component suggested that the mucin-producing epithelia might be derived from the FSC. This association might represent so-called 'retrodifferentiation' of adenoma cells to the FSC and the precursor cells of Rathke's pouch.