Nails

1. Overview

Nails are hard, keratinized plates located on the dorsal surface of the distal phalanges of fingers and toes. As specialized structures of the integumentary system, nails are derived from the epidermis and serve various functions including protection, enhancement of fine touch, and support in grasping and manipulating objects. Composed primarily of compacted keratinocytes, nails grow continuously throughout life and are influenced by both local and systemic health factors.

2. Location

Nails are located at the distal ends of the digits, specifically:

• Dorsal surface of fingers and toes, covering the distal phalanges.

• Embedded within a complex nail unit that includes the nail plate, nail bed, nail matrix, cuticle (eponychium), lateral nail folds, and hyponychium.

• Visible portions include the nail plate and lunula; hidden structures like the matrix lie beneath the skin at the nail base.

3. Structure

The nail apparatus consists of multiple integrated components:

• Nail plate: The hard, keratinized outer structure made of compacted dead keratinocytes.

• Nail matrix: The growth center located beneath the proximal nail fold; responsible for producing the nail plate.

• Nail bed: The underlying skin beneath the nail plate; supports and nourishes the nail as it grows.

• Lunula: The visible whitish crescent at the nail base, representing the distal portion of the matrix.

• Eponychium (cuticle): Thin skin fold that protects the matrix from pathogens and debris.

• Lateral nail folds: Skin that borders the sides of the nail, stabilizing and protecting the nail edges.

• Hyponychium: Skin under the free edge of the nail; acts as a barrier to pathogens.

4. Function

Nails serve several essential mechanical and sensory functions:

• Protection: Shield the tips of fingers and toes from trauma and injury.

• Enhanced tactile sensation: Provide counterpressure to the pulp of digits, improving touch precision.

• Assistance in grasping: Aid in picking up small objects and performing delicate tasks.

• Defense and grooming: Historically used for scratching, defense, and personal hygiene.

5. Physiological role(s)

Beyond mechanical functions, nails play roles in broader physiological and diagnostic contexts:

• Growth and regeneration: Nails grow continuously—fingernails at ~3 mm/month, toenails more slowly.

• Indicators of systemic health: Color, texture, and shape changes can reflect internal diseases, nutrient deficiencies, or systemic disorders.

• Thermoregulation (minimal): Though minor, the nail bed’s vascularization participates in heat exchange at the extremities.

• Cosmetic and social roles: Nails contribute to aesthetic appearance and social communication through grooming and cultural expression.

6. Clinical Significance

Nails are frequently involved in clinical diagnoses and can be affected by trauma, infections, autoimmune diseases, or systemic illness:

• Infections:

  • Onychomycosis: Fungal infection causing thickened, discolored, brittle nails.

  • Paronychia: Infection of the skin around the nail due to bacteria or fungi.

• Trauma:

  • Crushing injuries can lead to subungual hematomas or permanent nail deformities.

  • Repeated trauma can cause nail ridging or thickening (e.g., in runners).

• Dermatological conditions:

  • Psoriasis and lichen planus can affect nails, causing pitting, discoloration, or dystrophy.

• Systemic disease markers:

  • Clubbing: Seen in chronic hypoxia (e.g., lung disease).

  • Spoon nails (koilonychia): Associated with iron deficiency anemia.

  • Terry’s or Lindsay’s nails: Can indicate liver, kidney, or cardiovascular disease.

• Nail tumors:

  • Includes glomus tumors, subungual melanoma, and squamous cell carcinoma; may cause pain, discoloration, or nail deformity.

• Drug and toxin exposure:

  • Certain medications and heavy metal toxicity (e.g., arsenic) may cause Mees’ lines or Beau’s lines in nails.