anatomy and physiology of nail

8/3/2019 anatomy and physiology of Nail

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NAIL APPARATUS

Nails (Fig. 7.18) are homologous with the cornified layer of the general epidermis.They consist of compacted, anucleate, keratin-filled squames in two or threehorizontal layers. Ultrastructurally, the squames contain closely packed filaments

which lie transversely to the direction of proximodistal growth, and are embedded

in a dense protein matrix. Unlike the general epidermis, squames are not shed fromthe nail plate surface. A variety of mineral elements are present in nail, including

calcium. Calcium is not responsible for the hardness of nail: this is determined by

the arrangement and cohesion of the layers of squames, and their internal fibres.The water content of nail is low, but nail is 10 times more permeable to water than

the general epidermis. The softness and elasticity of the nail plate is related to its

degree of hydration.

Fig. 7.18 The organization and terminology of the structures associated with afingernail.

(By permission from Paus R, Peker S 2003 Biology of hair and nails. In: Bolognia

JL, Jorizzo JL, Rapini RP (eds) Dermatology. London: Mosby.)

The nail apparatus consists of the nail plate, proximal and lateral nail folds, nail

matrix, nail bed and hyponychium.

Nail plate

The nail plate is embedded within the proximal and lateral nail folds. It isapproximately rectangular in shape and is mostly convex in both longitudinal and

transverse axes: there is considerable inter- and intra-individual variation (Fig.

7.18). The thickness of the plate increases proximodistally from about 0.7 mm to

1.6 mm: the terminal thickness varies between individuals. The surface of the nail

plate may show fine longitudinal ridges, and its undersurface is grooved bycorresponding ridges in the nail bed. Disturbances of growth pattern or disease

may lead to transverse ridging or grooves, and minute trapped air bubbles may

produce white flecks. These defects move distally with growth of the plate.

The nail plate arises from compacted cornified epithelial cells derived from the

dorsal, intermediate and ventral nail matrices. It is densely adherent to the


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matrices on its undersurface, but becomes a free structure distal to the

onychodermal band, where it separates from the nail bed. The dorsal aspect of thenail plate originates from the more proximal regions of the germinal matrix, i.e.

dorsal and intermediate matrices, whereas the deeper, volar aspect of the plate

originates from the ventral matrix.

Nail folds

The sides of the nail plate are bordered by lateral nail folds which are continuouswith the proximal fold (Fig. 7.18). The lateral nail folds enclose the lateral free

edges of the nail plate and are bounded by the attachment of the skin to the

lateral aspect of the distal phalanx margin and the lateral nail. The proximal nailfold provides the visible proximal border to the nail apparatus. It consists of two

epidermal layers, superficial and deep, separated by a core of dermis. The

epidermis of the superficial layer lacks hair follicles and epidermal ridges: itscornified distal margin extends over the nail plate for a little distance as the

cuticle or eponychium. The deep layer merges with the nail matrix.

The eponychium is bounded by the fascial attachment of the skin to the base of

the distal phalanx, distal to the insertion of the extensor tendon, and its distal

free edge. It adheres to the dorsal aspect of the nail plate and overlies the root

of the nail.

Nail matrix

The nail matrix is the main source of the nail plate, and it can be divided into threeparts. Proximally, the dorsal matrix is defined as the volar surface (undersurface)

of the proximal nail fold. The intermediate matrix (germinal matrix) starts wherethe dorsal matrix folds back on itself and extends as far as the distal portion of

the lunule. The ventral matrix (sterile matrix) is the remainder of the nail bed: it

starts at the distal border of the lunule and ends at the hyponychium.

The matrix epithelium consists of typical basal and prickle cell layer keratinocytes,

among which are scattered melanocytes and Langerhans cells. Cornified cells of thedorsal and ventral aspects of the matrix are steadily extruded distally to form the

nail plate: the proximal 50% of the nail matrix contributes 80% of the nail plate.This process continues into the nail bed at the distal edge of the lunule, which is

formed where the distal portion of the ventral matrix underlies the nail plate.


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The lunule is pale, opaque and convex and is more prominent in the thumb than the

other digits. It is not known why the lunule is so pale compared with the moredistal translucent pink nail bed. The lack of colour may reflect the thickness of the

epidermis in the lunule and/or a paucity of capillaries in the dermis of the lunule.

Nail bed

The nail bed epidermis extends from the distal margin of the lunule to thehyponychium. The distal margin of the nail bed, at which point the nail platebecomes free of the nail bed, is called the onychodermal band. The surface of the

nail bed is ridged and grooved longitudinally, corresponding to a similar pattern on

the undersurface of the nail plate. This results in a tight interlocking of the twowhich prevents the invasion of microbes and the impaction of debris underneath

the nail. The epidermis of the nail bed is thin and lacks a stratum granulosum. It

consists of two to three layers of nucleated cells which lack keratohyalin granules,and a thin cornified layer which moves distally with the growing nail plate. It

contains an occasional sweat gland distally.

The dermis of the nail bed is anchored to the periosteum of the distal phalanx

without any intervening subcutaneous layer. It forms a distinct compartment,

which means that infections of the nail bed, or other local causes of a rise inpressure (e.g. haematoma) may cause severe pain which is only relieved by excision

of part or all of the nail plate. The dermis is richly vascularized. The blood vessels

are arranged longitudinally and display numerous glomus bodies, which areencapsulated arteriovenous anastomoses involved in the physiological control of

peripheral blood flow in relation to temperature (see Ch. 6 and Ch. 50). The dermis

is well-innervated, and contains numerous sensory nerve endings, including Merkel

endings and Meissner's corpuscles.

Nail bed cells differentiate towards the nail plate, and contribute to its thickness

ventrally.

Hyponychium

The hyponychium is the area under the free nail between the onychodermal bandproximally and the distal groove. It is an epidermal ridge which demarcates the

junction between the finger pulp and the subungual structures.

Growth of nail


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Nail growth is determined by the turnover rate of the matrix cells, which varieswith digit, age, environmental temperature and season, time of day, nutritionalstatus, trauma and various diseases. Generally, its speed is related to the length of

the digit, being fastest (approximately 0.1 mm per day) in the middle finger of the

hand, and slowest in the little finger. Fingernails grow three to four times fasterthan toenails, quicker in summer than in winter, and faster in the young than in the

old. A fingernail grows out in about 6 months, whereas a toenail is replaced, on

average, in 18 months.

Genetic keratin disorders (Irvine & McLean 1999) may lead to nail dystrophies such

as pachyonychia, where the nails become grossly thickened.

anatomy and physiology of nail

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