Most people limit their definition of the word dermatology to the study of skin alone.  Although skin encompasses a large portion of the field, it is imperative to remember that dermatology also includes the study of its appendages, namely the nails and hair.

The nail is a specialized structure that contains keratin. It is produced by a germinative epithelium—a specialized group of cells that continuously generates the nail plate (e.g. where nail polish is applied). Keratin is a protein that is found in hair, skin, and nails. It just so happens that the nail contains “hard” keratins while the skin and hair contain more “soft” keratins. You can appreciate this hardness by feeling your own nails. Hardness is a sign of normal, healthy nails and this area is referred to as the nail plate.

With that said, the nail plate must not be confused with the nail unit! Interestingly, what most people consider “the nail” is actually the nail plate, and the nail plate is surprisingly not considered one of the four anatomical structures that comprise the nail unit. In other words, the true anatomy of the nail lies within the nail unit, which has four different components: the nail bed, the hyponychium, the proximal nail fold, and the matrix.

Although the nail plate is not technically part of the true anatomy of the nail, it is still an important concept that should be mentioned and briefly discussed. How can this article pass on the opportunity to talk about the nail plate when this is probably considered by the majority as “the nail” ? The nail plate is made of cell with lots of keratin oriented in such a way that they are flattened in the plane of the plate. This unique orientation and its natural curvature (in addition to the keratins) are thought to provide its structural rigidity.

Nail plates are roughly rectangular and flat but demonstrate considerable variation within the normal population.  The plate is actually translucent; however, it appears pink due to a rich underlying network of small blood vessels (capillaries) that weave through the nail bed, which is firmly adherent to the nail plate above.

The white, crescent-shaped area on the proximal nail plate (farthest from the finger tip and near the cuticle) is known as the lunula.  It projects from under the proximal nail fold, and also represents the most distal portion of the matrix. This ultimately determines the shape of the free edge of the nail plate (piece of nail plate closest to the finger tip).

As the nail plate emerges from the matrix, it is bordered by three normal skin structures—two lateral nail folds (skin on each side of the nail plate; also known as nail grooves) and one single proximal nail fold (near the cuticle).  The proximal nail fold is of crucial importance as it plays a fundamental role in the formation of the nail plate.  In fact, approximately 25% of the nail plate’s total surface area is located under the proximal nail fold.

The four components that comprise the nail unit serve vital functions, which is necessary to maintain the integrity of the nail plate.  The matrix is a thick layer of cells that lie above the middle part of the distal phalanx of the digits (the tiny piece of bone in the finger tip).  It is sandwiched between the proximal nail fold and the lunula.  It is arguably the most important component as it generates the bulk of the nail plate.

The proximal nail fold is an invaginated, wedge-shaped fold of skin on the dorsum of the distal digit (where the nail plate meets skin on the back of one’s hand).  It protects much of the matrix and newly forming nail plate.  Another common word many people are familiar with and associate with the nail is the cuticle, or the eponychium. In addition to the nail plate, this structure is not considered one of the four true anatomical components of the nail unit; however, it should be mentioned since the cuticle is a part of the proximal nail fold.  It serves as a sealant and protective barrier against entry of infectious organisms into the proliferating matrix.  Disruption of the cuticle allows irritants to enter, which subsequently leads to the inflammation seen in chronic paronychia—an infection primarily caused by the yeast fungus Candida albicans.

The hyponychium is a narrow zone of cells between the nail bed and the distal nail groove beneath the free edge of the nail plate.  It plays a very similar role to that of the cuticle but in an inverted manner.  It seals the undersurface of the nail plate where it lifts off the tip of the digit. Disruption of this area results in the creation of a potential space between the nail plate and nail bed as can be appreciated in onycholysis—a separation of the nail plate from the underlying nail bed causing a proximal extension of free air.

Lastly, the nail bed begins where the distal matrix or lunula ends; it extends to the hyponychium, where the free edge of the nail separates.  Its contribution to nail plate formation is still debated.  The nail bed keratin-filled cells supposedly contribute about 20% of nail plate’s thickness and mass. Its main function is to keep the nail plate attached to the nail unit.

THE SKINNY
Writing about anatomy can be quite the challenge and this article might sometimes read too scientifically. Despite the familiarity of words like cuticle and nail plate (e.g. where some women [and men!] apply nail polish), these two structures are not considered part of true nail anatomy. This article attempts to describe each nail unit’s individual structure, where its located, and its main function. It also describes superfluous structures like the cuticle and nail plate to some degree.

REFERENCES:
Conejo-Mir, JS. Nail. In Sternberg, SS: Histology for Pathologists. New York: Raven Press. 1992.

Zaias, N: The Nail in Health and Disease, 2nd ed. Norwalk, CT: Appleton & Lange. 1990.

Forslind B. Biophysical studies of the normal nail. Acta DermVenereol 1970; 50:161-8.

Stone, M, Styles, AR, Cockerell, CJ, Histology of the normal nail unit. In Hordinsky, MK, Sawaya, ME, Scher RK: Atlas of Hair and Nails. Philadelphia: Churchill Livingstone. 2000.

Tosti, A, Piraccini, BM: Biology of nails and nail disorders. In Wolff, K, Goldsmith, LA, Katz, SI, et al: Fitzpatrick’s Dermatology in General Medicine, 7th ed. New York: McGraw Hill Medical. 2003.

Fleckman, P. Basic science of the nail unit. In Scher, RK, Daniel, CR: Nails: Therapy, Diagnosis, Surgery, 2nd ed. Philadelphia: WB Saunders. 1997.