C-CELL ADENOMA CONTAINING VARIOUSLY SIZED THYROID-FOLLICLES IN A HORSE

A thyroid gland tumor, showing unusual histology, was identified in a 13-year-old male Andalusian horse. Microscopically, the tumor consiste d of neoplastic proliferation of C-cell (parafollicular cell) with cyt oplasmic fine granules, containing diffusely distributed, variously si zed colloid-containing follicles. Immunohistochemically, the neoplasti c C-cell were positive for calcitonin and follicle-forming epithelial cells showed a positive reaction for thyroglobulin. Ultrastructurally, membrane-bound secretory granules up to 250 nm in diameter were found in the cytoplasm of the parafollicular cells, whereas the follicular epithelial cells had microvilli, junctional complex, and well-develope d endoplasmic reticulum.