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INTRODUCTION

KEY POINTS

  • Nail matrix melanocytes in skin of color contain mature melanosomes that produce nail plate pigmentation.

  • Melanonychia in multiple nails reduces but does not eliminate the probability of melanoma.

  • Although melanoma of the nail unit is uncommon in skin of color, acral locations, including nails, occur disproportionately more frequently.

  • Over 90% of melanonychias arise from the distal rather than the proximal nail matrix.

  • Melanomas are more frequent in the great toe and thumb than in other digits.

  • Most melanomas arising from melanonychia striata are in situ melanomas.

The nail unit is composed of the matrix, plate, bed, proximal and lateral nail folds, and hyponychium [Figure 15-1]. The nail develops from an ingrowth of the epidermis into the dermis during gestational week 9, and the nail unit is fully developed at week 15 and then continues to grow throughout life. Because the nail unit lies immediately above the periostium of the distal phalanx, disorders of the nail and bone can affect each other. The shape of the distal phalangeal bone also determines the shape and transverse curvature of the nail. The nail functions to protect the distal phalanges and to increase tactile sensation. Toenails protect the distal toenail and aid in pedal biomechanics.

FIGURE 15-1.

Basic anatomy of the nail unit.

EMBRYOLOGY

The development of the digits, nails, and nail folds occurs early in embryonic development [Table 15-1]. Individual digits are developing by the eighth week of gestation.1 The nail develops during the ninth embryonic week from the same primitive epidermis that gives rise to hair, sweat glands, and the stratum corneum. At week 10, the primary nail field is developed. Proximal and lateral nail folds develop during weeks 13 and 14. The nail plate covers the majority of the nail bed at week 17. From week 20 to birth, the nail unit and digit grow in unison.

TABLE 15-1Nail embryology

NAIL MATRIX

Nail matrix keratinocytes divide in the basal cell layer; they keratinize in the absence of a granular layer. The site of keratinization of nail matrix onychocytes can be clearly distinguished in histologic sections as an eosinophilic area where cells show fragmentation of their nuclei and condensation of their cytoplasm. The maturation and differentiation of nail matrix keratinocytes occur along a diagonal axis that is distally oriented. Thus keratinization of the proximal nail matrix cells produces the dorsal nail plate, and keratinization of the distal nail matrix cells produces the intermediate plate [Table 15-2].

TABLE 15-2Nail matrix ...

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