Walsh & Hoyt: Choristomas of the Neurohypophysis (Pituicytoma; Granular Cell Myoblastoma)

The use of the term ""choristoma"" for this tumor is probably inappropriate, as is the term ""myoblastoma."" The tumor is composed of large, round, or oval cells having a dusky, granulated, eosinophilic cytoplasm. The origin of these cells is unclear, and this has resulted in several different names being given to tumors containing them. Abrikossoff thought the cells represented immature skeletal muscle cells and therefore called the tumor a ""myoblastoma."" Although the tumor continues to be called a myoblastoma or granular cell myoblastoma, particularly when it occurs in the orbit, the myogenic theory of origin is untenable because of the absence of glycogen in the tumor cells, the occurrence of the tumor in organs where no striated muscle is present, and the electron microscopic demonstration of the absence of any continuity between tumor cells and normal striated muscle. In fact, the cells that make up these tumors appear to be identical with those that are often seen in groups in the pituitary stalk and the posterior lobe of the pituitary gland. These cell groups, unfortunately called choristomas by Sternberg, are thought by several investigators to be a variety of glial element normally present in this region, the pituicyte. Tumors containing these cells are therefore called ""pituicytomas"" or ""choristomas."" Brat et al. described nine patients with pituicytoma, all of which strongly stained for vimentin and S-100 protein. They concluded that pituicytomas are low-grade gliomas. The presence of the S-100 protein in several ""granular cell myoblastomas"" of the orbit further supports the theory that these tumors are of glial origin.