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Introduction: Cutaneous melanoma is the most frequent skin tumor in horses. Even though epidemiological and clinical pathological features have been examined in detail, its pathological mechanism remains unclear. In humans, neovascularization and vasculogenic mimicry of melanoma corresponds to prognosis. Objective of this study was to investigate the vascularization of equine melanoma in detail.
Materials and Methods: Samples of equine melanoma were processed for light and transmission electron microscopy. To establish melanoma cells in vitro, samples of equine melanoma were incubated on two different media. Immunohistochemical staining was performed using melanocyte (anti-S100, -MAGE, -MITF, -TYRP1) and keratinocyte (anti-pancytokeratin) markers. Moreover, melanoma cells were co-cultured with endothelial cells. Endothelial formations were identiﬁed by von Willebrand factor and anti-CD34 staining.
Results: Histology of equine melanoma showed an increased number of melanocytes in the epidermis and invasion beyond the epidermal-dermal zone. Transmission electron microscopy conﬁrmed the presence of melanosomes as marker organelles for melanocytes. Tubular formations with typical endothelial lining containing erythrocytes were observed. In addition, Periodic-acid-Schiffreaction positive structures and mosaic vessels with endothelial-like cells and melanoma cells were present suggesting vasculogenic mimicry. Melanoma cells stained positive for all melanocyte markers and displayed conﬂuency on Melanocyte Growth medium M2 after 4 weeks, but did not reach conﬂuency on Melanocyte Growth medium containing Phorbol Myristate Acetate. Both monoculture of melanoma cells and co-culture of these with endothelial cells demonstrated three-dimensional integration. Conclusion: Penetration beyond the epidermal-dermal boundary and formations corresponding to angiogenesis and vasculogenic mimicry indicate an aggressive quality of equine melanoma. Since there is no effective systemic treatment yet, strategies directed against blood vessel formation and vasculogenic mimicry may result in new approaches for therapeutic intervention in equine melanoma.