EQUINE VETERINARY EDUCATION / AE / JANUARY 2020


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Fig 1: Gross post-mortem appearance of the equine malignant melanoma. Highly pigmented adrenal gland (red arrow); tumour thrombus, vena cava (blue arrow).


Fig 3: Gross post-mortem appearance of the malignant melanoma in the thoracic cavity. Darkly pigmented metastatic tumour masses attached to the chest wall.


Fig 2: Gross post-mortem appearance of the malignant melanoma of the right adrenal gland (after fixation).


Samples were fixed in 10% neutral buffered formalin for


microscopic examination. Tissues were processed and embedded in paraffin wax after fixation for 48–72 h, sectioned at 4–5 lm and stained with Haematoxylin & eosin (H&E). Another set of identical sections of the highly melanin- containing tissues were bleached first, followed by Mayer haematoxylin staining. Darkly pigmented samples including the adrenal gland, pleural metastases and the aortic lymph node were bleached followed by IHC using aforementioned antibodies. The IHC-stained sections counterstained with Mayer haematoxylin. Control sections of darkly pigmented samples were also immunostained. The cells in the adrenal mass consisted of nests of variable


sized, epithelioid cells containing melanin, with an interstitial, fine, fibrovascular stroma (Fig 4). These anaplastic melanocytes contained intracytoplasmic black/dark brown granules.


Bleaching of tissue sections removed the dark pigments and allowed better evaluation of cellular and nuclear details. Tumour cells exhibited variation in nuclear morphology with prominent nucleoli. There were some giant cells with large nuclei, enclosing one or more nucleoli. Few mitotic figures were seen. The nuclear and cytoplasmic appearance of the cells in the pleural metastases and the aortic lymph node was similar to those in the adrenal gland. Histological examination of the visceral organs revealed multiple small metastatic foci in the liver, spleen, kidney and lung. Melanin bleaching followed by IHC staining of samples of


the adrenal mass, pleural metastases and the aortic lymph node were strongly positive for S-100 protein and melanoma marker HMB-45. However, IHC stains for neuroendocrine markers, chromogranin A and synaptophysin, were negative.


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Fig 4: Photomicrograph showing the malignant melanoma of the right adrenal gland. Tumour cells have variable morphology. Bleached and stained by Mayer haematoxylin, 3100.


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