Structure of a nail
Nail is an important skin appendages. It has several important functions. It protect the distal digit, improve fine touch sensation and enhance aesthetic appearance. It also assist in grasping, scratching and grooming. In animals nails acts as a tool for cutting, stabbing or digging the ground. Toenails contribute to pedal biomechanics. Nails also has a role as a diagnostic clue to various cutaneous and systemic disorders. Nail growth continue throughout life unlike hairs. Normal nail growth is approximately 0.1 mm per day or (3 mm a month) for finger and 0.05 mm per day for toe nails. A finger nail require 4-6 months to replace itself, while a toenail needs 12-18 month.
Nail is a unique structure, whose component parts are collectively called the nail unit. Nail unit lies immediately above the distal phalanx is composed of nail plate, and four specialized epithelia ( the proximal nailfold, the nail matrix, the nail beds and the Hyponychium) and the lateral nail folds.
Nail plate is hard translucent slightly convex, dead keratinized structure usually pink due to rich underlying blood supply. The small white semicircular structure at proximal portion of nail is the lunula which is visible portion of nail matrix. The surface of the nail plate is normally smooth. Longitudinal ridging and beading is a variant of normal, most commonly seen in elderly. Aging may increase or decrease nail thickness. Nail plate is densely adherent to nail bed. A pigmented (brown or black) band or bands may occurs in more than 90% of black people.
Nail matrix is a growth zone of the nail. It is a localized region (3-6 mm) beneath the proximal nail fold that produce the major part 90% (dorsal and intermediate portion ) of 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 nail plate is formed by nail bed.
Nail bed is structure underlying nail plate. The nail bed extend from distal margin of lunula to hyponychium. The nail bed consists of parallel longitudinal ridges with small blood vessels at their base. Bleeding induced by trauma or vessels 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 free edge of nail plate. Its proximal border is the distal limit of of nail bed (the Onchodermal band) distally it is continuous with normal volar skin. It is the transition point between nail and normal skin of the digit. The Onchodermal band is the point of strongest attachment between the nail and the underlying digit.
Proximal and lateral nail folds hold and protect the nail. Proximal nail fold overlies the nail matrix. It form cuticle (Eponychium) which is attach to upper surface of nail plate. The cuticle protect the matrix by sealing off the potential space between nailfold and 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 cover the nail plate from each side.They contribute to firm adherence of nail plate to the nail bed. Loss of volume of lateral nail folds is associated with a tendency for onycholysis. (Separation of nail from nail bed)