Nail Anatomy and Physiology
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Direct answer
The nail unit comprises the nail plate, matrix, nail bed, folds and hyponychium, and the proximal matrix produces most of the nail plate. Fingernails grow at about 3 mm per month, while toenails grow at about 1 mm per month. Because each part has a distinct source or function, damage to one site produces a predictable nail sign.
What you must remember
- Nail matrix: the germinal tissue under the proximal nail fold that makes most of the nail plate; damage causes pitting and white lines.
- Lunula: the pale half-moon at the base of the nail, which is the visible part of the matrix; it is most prominent on the thumb and often hidden on the toes.
- Nail bed: the epithelium and underlying onychodermis to which the plate adheres; onycholysis is separation of the plate from this bed.
- Eponychium and cuticle: the seal of the proximal fold, which protects the matrix from infection.
- Hyponychium: the seal under the free edge that stops debris and organisms entering the nail bed.
- Nail plate: about 0.5 mm thick and made of hard keratin formed by the matrix.
- Growth rates: fingernails regrow in about 6 months and toenails in 12 to 18 months, with growth slowing with age.
A structure-to-sign map of the nail unit
Each nail structure produces a recognisable sign when disturbed, so the table maps site to clinical result. Use it to reason from a clinical photograph back to the affected region.
| Structure | Main role | Typical sign when damaged |
|---|---|---|
| Proximal matrix | Makes most of the nail plate | Pitting and transverse Beau lines |
| Nail bed | Anchors and nourishes the plate | Onycholysis and splinter haemorrhages |
| Lunula | Visible part of the matrix | Whitening or loss in some illnesses |
| Eponychium | Seals the proximal fold | Paronychia and cuticle loss |
| Hyponychium | Seals the free edge | Fungal or debris invasion beneath the plate |
| Nail plate | Hard, protective keratin | Leuconychia, Terry nails, koilonychia |
Beau lines are transverse grooves from temporary matrix arrest, such as after an illness, and they move outward as the nail grows. Clubbing is a change in the soft tissue of the nail bed and the angle of the nail, not a plate disease, so it should not be mistaken for a matrix disorder.
Where nail anatomy questions go wrong
Most anatomy questions ask you to link a site with its sign or its growth rate.
- Trap: assigning all nail pitting to the nail bed. Correct: pitting comes from the proximal matrix, as in psoriasis and alopecia areata.
- Trap: quoting the same growth rate for fingernails and toenails. Correct: fingernails grow about 3 mm a month and toenails about 1 mm, so toenail regrowth takes much longer.
- Trap: attributing Terry nails to iron deficiency. Correct: Terry nails, with a white opaque plate and a narrow distal band, reflect hypoalbuminaemia, as in cirrhosis and nephrotic syndrome.
- Trap: treating a Beau line as a fungal infection. Correct: Beau lines are transverse grooves from temporary matrix arrest after systemic stress, and they grow out with the nail.
Frequently asked questions
Which part of the nail unit produces most of the nail plate?
The proximal nail matrix, lying under the proximal nail fold, produces most of the nail plate. Damage here results in pitting, white lines or transverse grooves, which is why psoriasis and alopecia areata cause pitting.
How fast do fingernails and toenails grow?
Fingernails grow at about 3 mm per month and toenails at about 1 mm per month. Full regrowth takes about 6 months for fingernails and 12 to 18 months for toenails, and growth slows with age.
What is the lunula and why does it matter?
The lunula is the pale half-moon at the base of the nail, representing the visible distal part of the matrix. It is most prominent on the thumb and is often hidden in the toenails.
What is the function of the hyponychium?
The hyponychium is the seal of epidermis under the free edge of the nail, which keeps debris and organisms out of the nail bed. Its breakdown, as in distal onycholysis, allows fungal and bacterial invasion of the nail bed.
What are Beau lines?
Beau lines are transverse grooves across the nail plate caused by temporary arrest of matrix growth after severe illness, trauma or chemotherapy. They move outward as the nail grows, and their position helps date the insult.