- The nail apparatus is made up of:
- Nail plate, the horny “dead” product
- Four specialized epithelia: proximal nail fold; nail matrix; nail bed; hyponychium
- Fingernails add function to multiple uses of the hands, protect the terminal digits; add to esthetics of the fingers.
- Toenails protect the distal toes and contribute to pedal biomechanics.
- Nail apparatus disorders can be traumatic, structural, primary, manifestations of cutaneous disease (e.g., psoriasis), neoplastic, infectious, or manifestations of systemic diseases (e.g., lupus erythematosus).
Components of the Normal Nail Apparatus
Schematic drawing of normal nail.
The hard protective tool, the product of the nail apparatus. Rests on and is firmly attached to nail bed, which is attached to underlying bone. Surrounded on three sides by nail folds. Made of three horizontal layers: thin dorsal lamina, thicker intermediate lamina, ventral layer from nail bed. Hardness of nail plate due to high sulfur matrix protein. Nail plate shape relates to shape of underlying phalangeal bone. Longitudinal ridging increases with aging.
Covers proximal one-quarter of the nail plate. Has two epithelial surfaces, dorsal and ventral.
Similar to skin; thinner; devoid of pilosebaceous units. Devoid of dermatoglyphic markings.
Covers ¾ of nail plate. Closely adherent to nail plate surface; keratinizes with a granular layer.
Contains numerous capillaries that run parallel to skin surface. Can be visualized with dermoscope: permits observation of arterial and venous limbs of capillaries, which are arranged in parallel rows and appear as fine regular loops with a small space between afferent and efferent vessels. Morphology altered in collagen vascular disease.
Junction of two epithelial surfaces of PNF, projects distally onto nail surface, sealing PNF and nail. Protects structures at base of nail (germinative matrix) from irritants, allergens, bacterial/fungal pathogens. Loss of cuticle produces potential space or pocket: inflammation of this pocket results in chronic paronychia.
Usually cover lateral edges of plate.
Underlies proximal fold. Normally is white. Represents most distal region of the matrix.
Distal nail. Natural shape same as contour of distal lunula.
Proximal matrix underlies nail plate to distal border of lunula. Distal matrix is that portion distal to distal border of lunula. Produces the major part of nail plate. As in epidermis, possesses a dividing basal cell layer producing keratinocytes, which differentiate, harden, die, and contribute to nail plate—analogous to epidermal stratum ...