Embryologic, Histologic, and Anatomic Aspects
A nail unit consists of a nail plate and tissues around and under it (Fig. 1.60). Situated on the dorsal aspect of the distal phalanx of every finger and toe, a nail, known also as a nail plate, is a hard, convex, rectangular, translucent structure that measures 0.5 to 0.7 mm in thickness. When viewed longitudinally, a nail can be divided into (1) a proximal portion positioned beneath the surface of the skin, (2) a portion that covers the nail bed, and (3) a distal edge that is free and, if uncut and protected from trauma, grows indefinitely. Except for its distal edge, a nail inserts into grooves in the skin that are deep proximally and more shallow laterally. The two lateral grooves are demarcated by folds of skin that overhang the nail, i.e., lateral nail folds that are continuous with the proximal nail fold that overrides the proximal groove. The so-called distal groove is not really a groove, but a slightly elevated margin that marks the distal boundary of the hyponychium, i.e., the narrow zone between the nail bed and true skin of fingers and toes.
Anatomic structure of a normal nail unit.
The proximal portion of a nail is situated in a wedge-shaped, beveled cavity that extends underneath the proximal nail fold for a distance of about 5.0 mm. The cornified layer of the proximal nail fold, the cuticle, arises from the ventral surface of the proximal nail fold and glides distally onto the surface of the nail for about 1 to 2 mm. Cornified cells that arise from the most distal aspect of the proximal nail fold also contribute to the cuticle. The cuticle constitutes a soft layer of cornified cells that seals off a potential space between the dorsal surface of the proximal nail and the ventral surface of the proximal nail fold. When cuticles are violated by a manicurist, fingers are rendered susceptible to paronychia because no longer is a seal in place to protect the space between the ventral surface of the proximal nail fold and the nail plate from intrusion of microorganisms like bacteria or yeast.
The lunula, a whitish crescent-shaped zone, is visible just distal to the proximal nail fold. It delineates the distal margin of generative epithelium of the nail matrix and is visible on thumbnails almost always, but hardly ever on the nails of the fifth finger and inconstantly on nails of the other digits. The reason for the white color of the lunula is not known.
A nail rests on tissues of the nail bed, which is epithelium that lies above a richly vascular dermis that is contiguous with the periosteum of the distal phalanx. Compression of the nail forces blood out of the underlying vessels and causes blanching of the usually pink nail bed. Distal to the nail bed is a narrow zone of skin, the hyponychium, which merges with volar skin of the tip of a digit. The hyponychium is separated from overt volar skin by a distal groove, a slightly elevated margin against which the free edge of the nail tends to abut.
Histologically, a nail unit and structures that support and are continuous with it possess five epithelial components that cornify independently of one another. Beginning with dorsal proximal skin and progressing around, behind, and under the nail to the distal edge, they are (1) the epidermis of the proximal nail fold, (2) the epithelium of the nail matrix, (3) the epithelium of the nail bed, (4) the epidermis of the hyponychium, and (5) the epidermis of volar skin of the digit.
The dorsal and ventral surfaces of the proximal nail fold are composed of all four “layers” present in normal epidermis and they cornify in an identical manner. As already stated, the cornified cells that constitute the cuticle are derived predominantly from epithelium of the ventral surface of the proximal nail fold, with a smaller contribution from the most distal aspect of epithelium of the dorsal surface of the proximal nail fold.
The generative epithelium of the nail matrix consists of germinative cells that differentiate into spinous cells and thence to orthokeratotic cells of the nail itself. No granular zone is present in matrical epithelium of a normal nail unit, testimony to the fact that keratohyaline granules are not essential to cornification at that site. In longitudinal sections, a nail matrix is visualizable as an oblique, wedge-shaped, blind end of a nail unit that extends proximally for about 5.0 mm beneath the proximal nail fold.
The epithelium of the nail bed stretches from the lunular border of the matrix to the epidermis of the hyponychium. It, too, is devoid of a granular zone. For purposes practical, mature epithelial cells of the nail bed are apposed tightly to the undersurface of the orthokeratotic nail plate; no cornified cells of the bed are apparent. Epithelial rete ridges and connective tissue papillae with which they interdigitate in the region of the normal nail bed are long, narrow, angulated, and sometimes even pointed.
The epidermis of the hyponychium is situated between the epithelium of the nail bed and that of the distal groove. The hyponychium cornifies in the same manner as volar epidermis with formation of a granular zone and a thick, compact, orthokeratotic cornified layer. Because epidermis of the hyponychium is so similar to that of epidermis of volar skin, a distinction between them may not be necessary other than for purposes of convention.
Volar epidermis at the tip of digits begins at the distal groove. It is characterized by a well-developed undulate pattern of epidermal rete ridges and dermal papillae, a prominent granular zone, and a thick, compact, orthokeratotic cornified layer.
The dermis beneath the epithelium of the nail unit is highly vascular, especially in papillae. In addition, special arteriovenous shunts, known as Sucquet-Hoyer canals and lined by endothelial cells that, in turn, are rimmed by glomus cells, are present in the dermis. They help to regulate temperature of the digits, especially in cold climes. No infundibula, folliculosebaceous-apocrine units, or eccrine units are present in a nail bed. The distance between the nail matrix and the dorsal surface of the bony distal phalanx is short. Few or no adipocytes are intervening.
The kinetics of the epithelium of a nail unit depend on the behavior of matrical cells. The proximal portion of the matrix is the main source of cornified cells that constitute the dorsal portion of the nail plate, whereas the distal portion of the matrix, demarcated by the lunula, supplies corneocytes to the ventral portion of the nail. The nail is directed outward as a continuation of the angle assumed by the matrix along guides provided by the proximal nail fold and the lateral nail grooves and folds. This explains, in part, why nails grow forward rather than upward.
Cornified cells of the nail bed are few and distinct from those of the nail plate, but both cornified products move together distally at the same pace. The growth of the nail distally is propelled by mitoses in the epithelium of the matrix, whereas whatever cornified cells are produced by epithelium of the nail bed are carried distally by movement of the nail itself. The extent of cornification of normal nail bed epithelium is so slight that it cannot be detected in sections stained conventionally. No major exchange physiologic occurs between the ventral surface of the nail and the epithelium of the nail bed on which it rests, yet the two apposing surfaces are attached firmly. The attachment is so strong that when the nail is avulsed forcibly, separation does not occur between the nail and the epithelium of the nail bed but between the epithelium of the nail bed and the subjacent dermis. This remarkable adhesion is thought to result, in large measure, from the interlocking of longitudinal parallel ridges and furrows formed at the interface between the epithelium of the nail bed and the dermis.
Nail plate, stratum corneum, and hair shaft all consist of corneocytes, but each of them has different biochemical and biophysical properties, as can be told at a glance or by touch, casually. Nail and hair differ from stratum corneum because they do not desquamate, and nail and stratum corneum differ from hair because their growth is continuous, not cyclical. Like hair follicles, however, nail units in an embryo form from a downgrowth of primitive ectoderm into mesenchyme, but without any accompanying papilla. This downgrowth is visible first in a 9-week-old embryo; subsequent formation of a nail is complete by about the 20th week of gestation. Hair and nail are products of matrical epithelium that gives rise to its own distinctive cornified product. Histologically, viable epithelium of the nail bed is nearly indistinguishable from living epithelium of the follicle at the isthmus.
Findings similar to those in epithelium of the normal nail bed are seen also in the outer sheath at the isthmus and in a follicle well advanced in catagen. The nail bed, despite its likeness histologically to the isthmus, is analogous in a way to the outer sheath at the stem (both are covered by a layer of corneocytes arranged compactly). A nail plate is comparable to a hair shaft. Just as outer sheath at the stem is compressed by a firm cornified inner sheath, so, too, a nail bed is compressed by a firm cornified nail. When the constraining influences of an inner sheath no longer are applied at the isthmus, the outer sheath there cornifies in a distinctive unfettered fashion. When a nail separates from its bed as a consequence of a variety of processes pathologic, the epithelium of the bed proceeds to cornify in an equally unrestrained manner.
Fingernails lengthen at a rate of approximately 0.1 mm daily, whereas toenails grow at about one third to one half that rate. After avulsion, fingernails return to their former length in about 6 months, whereas toenails need 12 to 18 months to regrow fully. In general, fingernails grow more rapidly on the dominant hand. In addition, the longer the digit, the more rapid the rate of nail growth. Nails grow faster in summer than in winter, just as hair does, and more rapidly in children than in adults.
A nail grows continuously, unless the matrix of it is injured in some way, either by physical trauma or by local or systemic disease. Transverse grooves in nails, known as Beau’s lines, reflect temporary malfunction of a nail matrix, such as occurs during an acute febrile illness. A longitudinal furrow or ridge in a nail implies damage to its matrix focally, and a longitudinal pigmented streak in a nail signals a proliferation of melanocytes in the matrix focally, that usually being a melanotic macule, but, episodically, a junctional melanocytic nevus or a melanoma. Pits in nails, often a sign of psoriasis, are an evidence of pinpoint damage to the proximal part of the matrix and represent loci where tiny clusters of parakeratotic cells have been lost from the surface of the nail. Smooth white spots in a nail, termed leukonychia, result from damage to the distal part of the matrix and represent parakeratotic cells in the ventral portion of the nail plate. In lichen planus, scarring of the matrix focally consequent to the effects of lymphocytes in a dense infiltrate there produces a pterygium, i.e., fusion of the epithelium of the proximal nail fold with the epithelium of the matrix and nail bed, whereas scarring of a matrix totally in that disease results in complete loss of the nail. Long-standing infiltrates of inflammatory cells that involve the proximal nail fold, as occurs in candidal paronychia, may extend to the matrix and injure it with resultant cessation of nail growth and, ultimately, shedding of a nail.
Nails have several functions. Most important, they protect the terminal phalanges. A network of myelinated sensory nerves in the dermis of the nail bed, some nerves of which terminate near Merkel cells within the epithelium of the nail bed, enables the nail unit to contribute to appreciation of fine tactile stimuli. Nails also are used as tools with which to scratch the skin, grasp minute objects, and turn pages. Last, they can be ornamented in a variety of ways designed to attract attention favorably to those who bear them.