Proliferative lesions of intra-epidermal cytokeratin CAM5.2-positive cells in canine nipples.

Non-keratinocyte cells with clear or vacuolated cytoplasm are frequently observed in the epidermis of canine nipples. Most of these cells express cytokeratin (CK) CAM5.2, a marker of luminal epithelial cells. The morphological and immunohistochemical characteristics of these clear cells were investigated. Nipple tissue from 36 dogs of both sexes was collected and labelled immunohistochemically for CAM5.2, CK7, CK14, CK18, CK20, α-smooth muscle actin, p63, melan-A, E-cadherin, epidermal growth factor receptor and oestrogen receptor (OR). The intra-epidermal CAM5.2(+) clear cells were present singly or as small clusters, mostly within the basal layer, in 22 dogs (61%). These cells also expressed CK7, CK18, E-cadherin and OR. Electron microscopy revealed that some of these cells had surface microvilli. Multifocal proliferative lesions consisting of these cells were observed in the nipples of four dogs. In these lesions, proliferating cells formed bilayered tubules with CAM5.2(+) inner and CK14/p63(+) outer cells. This is the first report describing intra-epidermal CAM5.2(+) clear cells, distinct from melanocytes and Merkel cells in dog nipples. These cells might arise from the luminal epithelium of the papillary duct.