Multiple masses in a horse's tongue resulting from an atypical perineurial cell proliferative disorder

A 5-year-old National Show horse mare presented with a soft mass on the left dorsolateral aspect of the tongue. Over the next 2 years, the mare developed numerous, similar, coalescing masses that extended along the left dorsolateral aspect to the tip of the tongue. Microscopically, the bases for these masses were slender, fusiform, mesenchymal cells that formed compact whorls around myelinated and unmyelinated nerves. These cells were labeled by antibodies directed against vimentin but not by S-100. Ultrastructurally, multiple, concentrically arranged, long, slender cell processes, with discontinuous external laminae and many pinocytotic vesicles, helped to accurately phenotype the proliferative element. Whether this unusual perineurial cell proliferative disorder is neoplastic or not remained a matter of conjecture.