Study on the Role of Histochemical Stains in Identifying Merkel Cells in Dogs

Merkel cells (MCs) are neuroendocrine cells involved with tactile sense, growth, differentiation, and homeostasis of the skin as well as in different cutaneous diseases. Specific staining techniques are required for their identification because they are not easily visible in paraffin sections stained with hematoxylin and eosin. The present study assess the histochemical features of the MCs in dogs comparing with those described for other mammals in the literature and with the use of immunohistochemistry. A systematic study of samples from MCs-rich areas from healthy dogs was carried out by use of several histologic stains, including metachromatic staining, silver stains, methylene blue, periodic acid-Schiff stain, and osmium-based staining method. MCs were detected by the Grimelius argyrophilic stain in 86.7% of the specimens. The staining was showed as dark-brown granular cytoplasmic and consistently polarized to the basal cell cytoplasm matching with the cellular distribution of the characteristic neurosecretory granules. Some modifications in the standard staining protocol, including rinsing, silver reimpregnation, and counterstain dye, enhanced the MCs identification in stratified squamous epithelium. When compared with Cytokeratin 20-immunolabeled serial sections several MCs appeared nonstained with the argyrophilic method. These differences in MC numbers between stains were statistically significant. Other histologic stains failed to identify MCs in the specimens. The results of this study indicate that Grimelius argyrophilic stain is a suitable method for demonstration of MCs in the stratified squamous epithelium of skin and mucosa. Discussion on its utility when compared with immunohistochemistry and a review of the scientific literature is also presented. Anat Rec, 302:1458-1464, 2019. (c) 2018 Wiley Periodicals, Inc.