Spontaneous Pituitary Adenomas of the Pars Intermedia in 5 Aged Horses: Histopathological, Immunohistochemical and Ultrastructural Studies

Adenomas that developed in the pars intermedia of 1 Anglo-Arab and 4 thoroughbred horses aged 19 to 29 years were examined by light microscopy, electron microscopy and immunohistochemistry. The tumors had a nodular growth pattern separated by connective tissue and often invaded the pars nervosa irregularly. The tumor cells were spindle cells with abundant basophilic cytoplasm and proliferated in a palisade arrangement in line with the blood capillaries. Histochemically, the tumor cells were stained light red with periodic acid-Schiff-Orange G stain and purple with aldehyde thyonine, thereby showing a chromatic similarity to basophilic cells of the normal pars intermedia and the pars distalis. Immunohistochemically, the tumor cells were positive for adrenocorticotropic hormone (ACTH) and also weakly positive for α-melanocyte stimulating hormone (α-MSH), β-lipotropin (β-LPH) and β-endorphin (β-END). Since these hormones are synthesized from the prohormone, proopiomelanocortin (POMC), it has been suggested that the tumor cells may have a POMC-producing structure. But the tumor cells were positive for proliferating cell nuclear antigen, and this tumor appeared to be a nonfunctional pituitary tumor because no cell atypia or exocytosis was found ultrastructually.