24th September’2021
Structure of a nail
The nail is an important skin appendage. It has several important functions. It protects the distal digit, improves fine touch sensation, and enhances aesthetic appearance. It also assists in grasping, scratching, and grooming. In animals, nails act as a tool for cutting, stabbing, or digging the ground. Toenails contribute to pedal biomechanics. Nails also have a role as a diagnostic clue to various cutaneous and systemic disorders. Nail growth continues throughout life, unlike hair. Normal nail growth is approximately 0.1 mm per day or (3 mm a month) for fingers and 0.05 mm per day for toenails. A fingernail requires 4-6 months to replace itself, while a toenail needs 12-18 months.
A nail is a unique structure, whose component parts are collectively called the nail unit. The nail unit lies immediately above the distal phalanx is composed of the nail plate, four specialized epithelia ( the proximal nail fold, the nail matrix, the nail beds, and the Hyponychium), and the lateral nail folds.
Nail plate
The nail plate is a hard translucent slightly convex, a dead keratinized structure usually pink due to a rich underlying blood supply. The small white semicircular structure at the proximal portion of the nail is the lunula which is the visible portion of the nail matrix. The surface of the nail plate is normally smooth. Longitudinal ridging and beading is a variant of normal, most commonly seen in the elderly. Aging may increase or decrease nail thickness. The nail plate is densely adherent to the nail bed. A pigmented (brown or black) band or bands may occur in more than 90% of black people.
Nail matrix
The nail matrix is a growth zone of the nail. It is a localized region (3-6 mm) beneath the proximal nail fold that produces the major part of 90% (dorsal and intermediate portion ) of the nail plate and the distal curved half moon-like edge can usually be seen through the nail plate as the white lunula. The ventral portion of the nail plate is formed by the nail bed.
Nail bed
The nail bed is the structure underlying the nail plate. The nail bed extends from the distal margin of the lunula to the hyponychium. The nail bed consists of parallel longitudinal ridges with small blood vessels at their base. Bleeding induced by trauma or vessel disease like lupus occur in depth of these grooves producing splinter hemorrhages pattern viewed through the nail plate. There is no subcutaneous tissue in the nail bed, so immediately beneath the nail bed lies the periosteum of the distal phalanx.
Hyponychium / Onchodermal band
The hyponychium is keratinized epithelial area under the free edge of the nail plate. Its proximal border is the distal limit of the nail bed (the Onchodermal band) distally it is continuous with normal volar skin. It is the transition point between the nail and the normal skin of the digit. The Onchodermal band is the point of strongest attachment between the nail and the underlying digit.
Nail folds
Proximal and lateral nail folds hold and protect the nail. The proximal nail fold overlies the nail matrix. It forms a cuticle (Eponychium) which is attached to the upper surface of the nail plate. The cuticle protects the matrix by sealing off the potential space between the nail fold and the nail plate. Loss of cuticle compromises the protective role of proximal nail fold and leaves nail matrix vulnerable to external microbes and allergens. Chronic manipulation, Manicure, inflammation, and infection can result in loss of cuticle, which can cause chronic paronychia.
Lateral nail folds are soft tissue that partially covers the nail plate from each side. They contribute to the firm adherence of the nail plate to the nail bed. Loss of volume of lateral nail folds is associated with a tendency for onycholysis. (Separation of the nail from the nail bed)
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