A trabecular adenoma of the pars intermedia of the hypophysis was seen in a 13-year-old half-bred mare that presented symptoms corresponding to Cushing's disease of man. The spindle-shaped tumor cells were for the most part ‘light’, seldom ‘dark’. Both of them were characterized by well-developed rough endoplasmic reticulum, small Golgi apparatus, and typical secretory granules with a diameter of about 200 μm. The pituitary tumor and the symptoms were accompanied by increased plasma adenocorticotrophic hormone (ACTH) and by bilateral hyperplasia of the adrenal cortex. The tumor cells of the pars intermedia were probably the source of increased ACTH, and the hyperplasia of the adrenal cortex was probably responsible for the symptoms of Cushing's disease.
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