Structure and Function of the Nails

Wikis > Dermatology > Structure and Function of the Nails

Nail is a plate made of hard keratin that covers the dorsal tip of the digits.

The nail is a highly specialised keratinised appendage of the skin – it is a “horny dead” product.
Thickness normally varies from 0.05 to 1mm.
Develops during ninth embryonic week

Function:
• protection of distal phalanx
helps in manual dexterity
defensive tool
• enhance tactile discrimination
aesthetic functions (finger nails)
• contribute to foot biomechanics (hallux nail provide counter pressure to the pulp of toe during propulsion)

Hardness of nail is due to low water content compared to skin (<5% of nail weight); 9% of nail weight is cystine (composed of sulphur)  assumed to contribute to hardness

Nail structures:
Nail plate:
• keratinised structure that results from maturation of the nail matrix’s epithelium
• firmly attached to nail bed, which also contributes to its formation
• proximal and lateral borders surrounded by nail folds
• separates from hyponychium at distal edge
• shape of distal phalanx influences gross shape of nail plate – usually curved in longitudinal and transverse axes
• transversely the plate has 3 sections – dorsal nail plate, intermediate nail plate and ventral nail plate. Dorsal and intermediate are produced by nail matrix; ventral is produced by nail bed

Nail matrix:
• specialised epithelial structure that produces the nail plate; lies above mid-portion of distal phalanx
• extends proximally from nail fold up to 10mm; wedge shaped on transverse section; has a dorsal (proximal) and distal (ventral) portions.
visible portion called the lunula

Lunula:
• semicircular white region at proximal end
• represents junction between nail matrix and nail bed

Nail bed:
• extends from distal margin of lunula to onychodermal band; visible through nail plate
• nail bed epithelium is very adherent to nail plate (remains attached to nail plate if the nail is avulsed); epithelium is thin (2 to 5 cell layers) – its longitudinally orientated rete pegs interdigitate with underlying dermis
• keratinisation of nail bed  forms ventral nail plate (contributes about 1/5th of thickness and mass of nail

Hyponychium:
• anatomic area between nail bed and the distal groove – where nail plate detaches
• normally covered by nail plate

Nail Folds:
- on the proximal and lateral sides of nail for support
- rim of connective tissue covered with skin

Circulation:
• derived from two digital arteries that run alongside sides of digit  branches to matrix and proximal nail fold  abundant blood supply to nail apparatus

Nerve supply:
• cutaneous sensory nerves from dorsal branches of paired digital nerves

Nail Growth:
Grows continuously throughout life; fingernails grow faster than toenails; mean of toenail growth is 1mm/month

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