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In most primates, nails are unique features consisting of compact, translucent, keratinized cells. They protect the distal dorsal area of digits and participate in multiple functions, including grip, fine motor movements, and scratching.[1] Like hair, nails are ectodermal appendages whose histology shares similarities and differences with the skin.[2] Nail abnormalities can result from local nail pathology or manifest underlying systemic disease.[3] A nail biopsy may be warranted to provide a definitive diagnosis, and understanding the macroscopic and microscopic nail histology is an essential aid to the clinicopathological diagnosis of nail diseases.

The nail matrix is responsible for nail plate growth.[15] The proximal nail matrix produces the dorsal nail plate, and the distal matrix makes the ventral nail plate, possibly with a minor contribution from the nail bed.[44] Thus, in the case of melanonychia, the depth of the pigment suggests the biopsy site. A distal matrix biopsy is sufficient if the melanin remains confined to the ventral nail plate, and a proximal matrix biopsy is required if the dorsal nail plate is involved. The distal matrix is more often engaged in longitudinal melanonychia because its melanocyte population possesses more potential activity.[12] Fingernails grow at approximately 3 mm per month, while toenails grow slightly slower at around 1 mm per month.[45] Several factors influence the rate of growth. Lowered temperatures, female sex, non-dominant hand, first and fifth digits, certain disease states, and many medications are associated with a slower growth rate.[45] A variety of changes can appear on the nail plate due to nail matrix injury. The dystrophy of the nail plate depends on the severity, duration, and location of the nail matrix insult.[46] A mild, transient, diffuse injury to the nail matrix manifests in a transverse depression across the nail plate, known as Beau’s line. Along the same spectrum, onychomadesis, a transverse separation of the nail plate, can ensue if the diffuse insult to the matrix is more severe. In comparison, a focal, prolonged abuse can produce longitudinal leukonychia.[46]