Basics of onychopathology
The nail apparatus is a unique cutaneous appendage that forms a functional unit with the bone of the distal phalanx and the distal interphalangeal joints with their tendons and ligaments; hence, it is also called a musculo-skeletal appendage. In addition, it is part of one of the most important sensory organs, the digital pulp. It consists of peri- and subungual connective tissue with specialized functions and at least four different epithelia: The ventral surface of the proximal nail fold, which is the source of roughly 80% of the cuticle; the matrix that produces the entire nail plate; the nailbed attaching the nail firmly to the underlying bone; and the hyponychium—pulp permitting the nail plate to separate from the nail bed. Only the cooperation of the matrix and nailbed allow a normal nail to develop.
The diagnostic gold standard of nail disorders is histopathology, which requires a biopsy that is adequate for the amount of tissue submitted as well as the correct site of biopsy. The next crucial step is the processing of the specimen in the laboratory as sections of good quality are not easy to obtain. Both H&E and PAS stains are used routinely. The pathologist has to be familiar with certain nail-specific reaction patterns that differ from those of the skin elsewhere.