NORMAL STRUCTURE AND FUNCTION OF THE NAIL UNIT
The human nail is composed of a specialized form of keratin. All 20 nails have the same chemical makeup; the only difference is in the size of the nail. The nail unit is made up of highly specialized structures. The nail matrix is the portion of the nail unit that is responsible for production of the nail plate. The matrix lies a few millimeters behind the proximal nail fold, which ends as the cuticle (eponychium), and extends under the nail bed. Under the proximal nail bed, the nail matrix can often be appreciated as a half-circle termed the lunula. The color of the lunula is often creamy white with a hint of pink. Any damage to the nail matrix can potentially cause a temporary or permanent nail dystrophy.
The distal nail matrix is responsible for producing the ventral portion of the nail plate. The proximal nail matrix is responsible for producing the dorsal surface of the nail plate. The nail plate is made of keratin protein and is the hard portion of the nail. It is theorized to be protective to the underlying nail matrix and distal phalanx, as well as being helpful with grasping and dexterity of the fingertips. The nail plate is firmly attached to the underlying nail bed via tiny, vertically arranged interdigitations. These tiny undulations help lock the nail plate into the nail bed below. The nail plate is an avascular structure, and the underlying nail bed is highly vascular.
The nail bed is attached to the epidermis via the proximal nail fold and the cuticle, as well as the lateral nail folds on either side of the nail. Damage to the cuticle, whether by accident or during manicures or pedicures, can increase the risk of bacterial or fungal infection within the nail or the skin of the nail folds. This can lead to acute or chronic paronychia or onychomycosis. Improper trimming of the lateral aspects of the nail plate may lead to an ingrown toenail (onychocryptosis). The distal nail plate is attached to the underling epidermis by the hyponychium. Damage to this portion of the nail unit may allow for bacterial or fungal infections to take hold under or within the nail plate.
The nails grow continuously throughout a person’s lifespan. Fingernails grow on average 3 mm per month, and toenails grow a bit more slowly, on average 1 mm per month. However, these growth rates are highly variable among individuals. Both hair keratin and skin keratin types have been described to comprise the various portions of the nail unit. The hair keratin Ha1 and the skin keratins K5, K6, K16, and K17 make up the majority of the keratin types found in the adult nail. Other keratins have been identified during development of the nail.
Primary and secondary nail disorders are commonly encountered. Primary nail disorders include onychomycosis, onychocryptosis, onychoschizia (horizontal splitting), onychogryphotic nail (“ram’s horn” nail), leukonychia, median nail dystrophy, and onycholysis. These disorders are most often seen in isolation, with no underlying systemic abnormalities. Secondary nail disorders are seen in the presence of an underlying systemic disease; examples include koilonychia (caused by iron deficiency), nail plate pitting (many conditions including psoriasis and alopecia areata), pterygium formation (lichen planus), longitudinal red and white streaks and distal V-shaped nicking (Darier’s disease), clubbing (pulmonary disease), and yellow nail syndrome (pleural effusion and lymphedema). All skin examinations should include evaluation of the nails, because many systemic diseases can manifest with nail findings, and these clinical signs may be the first signs of underlying disease.