Nails

Unguis (pl. ungues)

Also known as: nail, fingernail, toenail, nail plate, nail unit, lunula

Nails are plates of hard keratin produced by the nail matrix on the dorsal surface of the distal phalanges; they protect the fingertips, aid fine manipulation and are a classic window on systemic disease.

Show on body map

Overview

Fingernail growth≈3 mm/month (full regrowth ≈4–6 months)

Toenail growth≈1–1.5 mm/month (full regrowth ≈12–18 months)

The nail plate is a translucent sheet of tightly packed, hard-keratinised dead cells (rich in cystine; no stratum granulosum stage) that lies on the nail bed and is framed by the proximal and lateral nail folds. It is produced by the nail matrix, a germinative epithelium under the proximal nail fold, whose distal crescent is visible through the plate as the whitish lunula (the matrix epithelium is thick and hides the underlying capillaries). The eponychium (cuticle) is the stratum corneum extending from the proximal nail fold onto the plate, sealing the space beneath the fold; the hyponychium is the thickened epidermis beneath the free edge.

The nail bed epidermis is thin and lacks a stratum granulosum; its dermis is arranged in longitudinal ridges and is richly vascular, giving the nail its pink colour — hence the use of the nail bed to judge capillary refill, cyanosis and anaemia. Fingernails grow continuously at ≈3 mm/month (a whole nail regrows in ≈4–6 months); toenails at ≈1–1.5 mm/month (≈12–18 months). Growth is faster in summer, in the young and on the dominant hand.

Location: Dorsal surface of the distal phalanx of each finger and toe.

Parts & subdivisions

Nail plate
Hard keratin plate; root (under the proximal fold), body and free edge.
Nail matrix
Germinative epithelium beneath the proximal nail fold that forms the plate; damage causes permanent dystrophy.
Lunula
Pale crescent at the base of the plate — visible distal part of the matrix; most prominent on the thumb.
Nail bed
Epithelium and highly vascular dermis beneath the plate; longitudinal ridges anchor the plate.
Eponychium (cuticle)
Keratin extension of the proximal nail fold onto the plate; barrier against infection.
Hyponychium
Thickened epidermis under the free edge; seals the distal nail bed.
Nail folds
Proximal and lateral skin folds bordering the plate (perionychium).

Anatomy

Innervation: Dorsal branches of the proper palmar digital nerves (median nerve for the lateral 3½ digits, ulnar nerve for the medial 1½) supply the nail bed of the fingers.
Arterial supply: Dorsal branches of the proper palmar (or plantar) digital arteries form arcades beneath the nail bed and matrix.
Histology: Nail plate: compact layers of flattened, anucleate cells of hard keratin. Matrix: stratified epithelium with melanocytes. Nail bed: epidermis without stratum granulosum over longitudinally ridged dermis.
Development: Nail fields appear on dorsal digit tips at ≈week 10; nails reach the fingertips by ≈32 weeks and the toe tips by ≈36 weeks (a sign of term maturity).

Function

Protect the dorsal fingertip and toe tip
Provide counter-pressure to the pulp, enhancing fine touch and precision grip
Scratching, picking up small objects
Visible indicator of systemic disease and peripheral perfusion

Clinical significance

Finger clubbing

Loss of the normal angle (>180°) between nail plate and proximal fold, increased fluctuation of the nail bed and a positive Schamroth window test. Causes: lung cancer, bronchiectasis, cystic fibrosis, lung fibrosis, cyanotic congenital heart disease, infective endocarditis, inflammatory bowel disease, cirrhosis, thyroid acropachy.

Nail signs of systemic disease

Koilonychia (spoon nails) — iron deficiency; splinter haemorrhages — infective endocarditis or trauma; pitting and onycholysis — psoriasis; Beau lines (transverse grooves) — matrix arrest during severe illness or chemotherapy; leukonychia/Terry nails — hypoalbuminaemia, cirrhosis; Mees lines — arsenic poisoning.

Paronychia, onychomycosis and subungual melanoma

Acute paronychia is a bacterial (usually S. aureus) infection of the nail fold; chronic paronychia follows wet work and Candida. Onychomycosis (dermatophytes, especially Trichophyton rubrum) thickens and discolours toenails. A longitudinal pigmented band with pigment spreading onto the nail fold (Hutchinson sign) suggests subungual melanoma.

Test yourself

Spoon-shaped (concave) nails — koilonychia — are classically associated with:

Infective endocarditis
Iron-deficiency anaemia
Psoriasis
Chronic liver disease

Answer: Iron-deficiency anaemia

Koilonychia is a classic sign of iron deficiency. Splinter haemorrhages suggest endocarditis, pitting suggests psoriasis and white (Terry) nails suggest cirrhosis.