Chapter 3: Nail Findings

NAIL ANATOMY

• Nail plate (Fig. 3-1)

  • Forms from keratinization of the nail matrix epithelium and is firmly attached to the nail bed

  • Dorsal nail plate is produced by the nail matrix

  • Ventral portion is produced by the nail bed

  • Nail thickness depends on the length of the nail matrix and nail bed

  • Pink color owing to underlying nail bed blood vessels

  • Onychocorneal band: most distal portion of firm attachment of the nail plate to the nail bed

  • Onychodermal band: pink band that lies between the onychocorneal band and the nail plate white free edge

• Proximal nail fold

  • Dorsal portion: thinner than skin of the digit, devoid of pilosebaceous units

  • Ventral portion: in continuity with the matrix, adheres to the nail plate surface, and keratinizes with a granular layer

  • Horny layer forms the cuticle and prevents the separation of the plate from the nail fold

  • Dermis contains numerous capillaries that run parallel to the surface of the skin; morphology can be altered in connective tissue diseases

• Nail matrix

  • Lies above the midportion of the distal phalanx

  • Keratinization of the proximal nail matrix cells produces the dorsal nail plate

  • Keratinization of the distal nail matrix cells produces the ventral nail plate

  • Lunula: where the distal matrix is not completely covered by the proximal nail fold but is visible through the normal nail plate as a white half-moon-shaped area

  • Cells are able to synthesize both “soft,” or skin-type, and “hard,” or hair-type, keratins—the matrix expresses keratins Ha1, K1, K10

  • Alteration in the color of lunula can be an indication of either a cutaneous or systemic disorder or a systemic drug side effect

• Nail bed

  • Extends from the distal margin of the lunula to the onychodermal band

  • Completely visible through the nail plate

  • Epithelium is adherent to the nail plate, 2 to 5 cell layers

  • Nail bed keratinization produces a thin horny layer that attaches to the ventral nail plate

  • The bed expresses keratins K6, K16, K17

  • No granular layer is present

• Hyponychium

  • Anatomic area between the nail bed and the distal groove, where the nail plate detaches from the dorsal digit

• Dermis

  • No subcutaneous tissue, no pilosebaceous units

  • Condensed connective tissue that forms a tendon-like structure connecting the matrix to the periosteum of the phalangeal bone

• Blood and nerve supply

  • Blood supply provided by the lateral digital arteries, arches supply the matrix and nail bed

  • Sensory nerves: originate from the dorsal branches of the paired digital nerves, run parallel to the digital vessels

• Nail growth

  • Fingernails: 3 mm/mo, 0.1 mm/d, take 5 to 6 months to regrow

  • Toenails: 1 mm/mo, 0.03 mm/d, take 12 to 18 months to regrow

  • After nail plate is avulsed, it takes 40 days before new fingernail will first emerge