Thyroid transcription factor 1 expression in sellar tumors: a histogenetic marker?

Pituicytomas are rare low-grade gliomas of the neurohypophysis. Their morphology and variable immunophenotype have led to speculation that they arise from pituicytes. Given the role of thyroid transcription factor 1 (TTF-1) in the developing rodent infundibulum and its expression in the adult rat neurohypophysis, we speculated that TTF-1 would be a marker of human pituicytes. Using immunohistochemistry, we found that TTF-1 is strongly expressed in fetal and adult human pituicytes. A survey of sellar masses demonstrated specific TTF-1 expression in pituicytomas (n = 3), atypical pituicytomas (n = 2), and granular cell tumors (n = 4), indicating a common pituicyte lineage. TTF-1 expression in spindle cell oncocytomas (n = 8) is less easily explained but invites speculation. Our observations may have implications for the classification of these rare sellar neoplasms, all the while acknowledging the morphological diversity of pituicyte-related neoplasms.