An Optimized Melanin Depigmentation Method for Histopathological and Immunohistochemical Staining of Formalin-Fixed Paraffin-Embedded Tissues

Introduction. The difficulty in diagnosis of severe melanocytic lesions is a problem to be overcome in pathological practice. Melanin bleaching is an effective approach to ameliorate melanin disturbances in severely pigmented lesions. Although various methods for improving melanin pigmentation in immunohistochemical staining have been reported, these depigmentation methods still need to be optimized and standardized. In this study, the coloring efficiency of 3,3′-diaminobenzidine (DAB) and alkaline phosphatase (AP) after melanin depigmentation was compared under the automatic immunohistochemical staining platform. Methods. The applicability of the optimized depigmentation method was validated in 10 formalin-fixed paraffin-embedded (FFPE) blocks of ocular melanoma tissues. Specimens were demelaninized with 10% hydrogen peroxide at 60°C for immunohistochemical staining (Melan-A and SOX10), and tissue chromogenic staining was performed with DAB and AP detection systems, respectively. Results. The optimized depigmentation method including immunohistochemistry (IHC) could be completed in 3 h, effectively preserving cell morphology and immunoreactivity. Among these, the color-rendering effect and contrast of AP are better than DAB. Conclusion. This optimized method can effectively remove melanin and improve the accuracy of IHC staining interpretation. AP staining has better visibility and readability without the interference of residual melanin. The comparison results showed that after melanin depigmentation, the immunohistochemical staining agent was replaced with red AP, which avoided the misjudgment caused by brown DAB when melanin depigmentation was incomplete. This improved method can be applied to future histopathological and immunohistochemical staining of melanin-deposited tissues.