Fine structure and origin of amyloid deposits in pituitary adenoma.

A pituitary adenoma with amyloid deposits was studied by light and electron microscopy. The amyloid material was radially oriented, deposited in the extracellular space, and often intermingled with small vesicles. It was surrounded by histiocytes with deep cytoplasmic invaginations. The histiocytes contained large autophagic vacuoles in which amyloid was also present. These features are similar to other forms of amyloidosis and suggest a histiocytic origin of the amyloid.