Digital mucous cyst: Altered epidermal mucin as a clue to diagnosis.

BACKGROUND: Digital mucous cyst (DMC) is histopathologically characterized by accumulation of mucin in the dermis. Some cases of DMC also show epidermal mucin, the histopathologic appearance and staining properties of which have not been described in detail.

METHODS: A total of 24 cases of DMC were investigated by routine hematoxylin-eosin (H&E) and Alcian blue stains in addition to AE1/AE3 immunohistochemistry.

RESULTS: Nine out of the 24 cases of DMC showed epidermal mucin. As the epidermal mucin migrates upward within the epidermis, it transforms from a flocculent granular substance into one or several solid horizontal plugs with a more homogeneous appearance and incorporates cytoplasmic fragments of keratinocytes/corneocytes. The homogeneous mucin plugs stain eosinophilic or amphophilic with an H&E formulation using hematoxylin 7212 and basophilic with Gill 3 or Harris's hematoxylin. The eosinophilic staining is enhanced when the eosin solution contains phloxine.

CONCLUSIONS: The variably eosinophilic, amphophilic, or basophilic staining of epidermal mucin can be explained by its composition of basophilic mucin and eosinophilic debris from cytoplasmic fragments. The eosinophilic staining of mucin has not been reported before and can be diagnostically important because it may be mistaken for serum exudate.