The histopathological feature of the nail isthmus in an ectopic nail.

The nail unit has a unique structure. It has been recently proposed that the nail isthmus as a transitional zone between the most distal part of the nail bed and the hyponychium. A 7-year-old Japanese boy presented with an ectopic nail, an additional and independent miniature nail on the digital pulp of the right fifth finger. We studied the expression of a series of keratin in longitudinal specimens and showed the histopathological manifestation in the nail isthmus. This region in the ectopic nail is subdivided into 2 parts: a proximal and narrow part anchored to the nail plate and a distal and wide part with a semihard keratinized structure.