Melanocytes Pattern in the Normal Nail, With Special Reference to Nail Bed Melanocytes

Abstract

Previous studies of the density of melanocytes in the normal nail bed have had conflicting results. This is unfortunate because knowing the normal values might help the difficult distinction between a benign subungual melanotic macule and an early melanoma in situ. Five specimens of normal nail unit were analyzed. On hematoxylin and eosin-stained sections the melanocytes were undetectable. We defined the melanocyte count (MC) as the number of melanocytes per 1-mm stretch of nail epithelium. The mean MC for nail matrix was 6.86 with a range of 4-14. The melanocytes were irregularly scattered in the basal and suprabasilar layer of the matrix epithelium. Abundant and uneven cytoplasmic dendrites were focally observed in the matrix. The MC for the nail bed ranged from 0 to 5 with a mean of 0.43. The melanocytes were restricted to the basal layer with thin cytoplasmic dendrites. Two cases showed a complete absence of melanocytes in the nail bed. In the ventral portion of the proximal nail fold, called the eponychium, the MC ranged between 0 and 5/mm with a mean of 2.27/mm. In conclusion, we discovered foci in normal nail beds, in which the melanocytic density can be relatively high and reach the level seen in the matrix. HMB45 is more sensitive than Microphtalmia-associated transcription factor (MITF) for the evaluation of intraepithelial melanocytic density of the nail unit. If MITF is used alone in the nail bed, its weak sensitivity may result in a false-negative interpretation and may be wrongly reassuring in the evaluation of early melanomas. On hematoxylin and eosin sections, basal and suprabasal nail keratinocytes are sometimes crowded, showing oval or elongated dark-staining nucleus and a clear cytoplasm and mimics a melanocytic proliferation. On HMB45 or Melan A staining, the morphology and the distribution of the dendrites of matrical melanocytes can mimic the dendritic pattern usually described in acral melanoma. Therefore, the interpretation of nail melanocytic atypia must be prudent.

Blue nevus of the nail: A case report and review

“…The rarity of melanocytic nevi of the nail apparatus is probably due to the lower number of melanocytes within the nail unit and due to their normal quiescence 7 . Although several groups demonstrated the complete absence of melanocytes into the nail bed, 8 Perrin et al recently confirmed the presence of melanocytes within this area and showed the presence of foci in which the number of melanocytes was similar to the density of that of the matrix (about 4‐14 cell/mm) 9 …”

Subungual Melanoma

“…Nail matrix biopsy is still essential for SUM diagnosis. Normal nail matrix has between 4 and 14 melanocytes per mm (mean 6.86 cells/mm per mm stretch of nail matrix epithelium) [7].…”

“…Dendrites are thicker and larger. Pagetoid spread is found in almost all lesions of SUM, and inflammation in the epithelial stromal interface is frequent [6,7]. In some cases of SUM with lentiginous growth of single atypical melanocytes, immunohistochemical stains with MELAN-A and HMB-45 may ease the diagnosis.…”

Photoonycholysis: new findings

“…A recent melanocyte count for the nail bed epithelium has shown that it ranged from 0 to 5 with a mean of 0.43. The melanocytes were restricted to the basal layer with thin cytoplastic dendrites . We have therefore postulated that the absence or rareness of spontaneous photoonycholysis in people with darker skin types is due to presence of melanin, providing some degree of natural photoprotection to the nail bed.…”

“…The melanocytes were restricted to the basal layer with thin cytoplastic dendrites. 32 We have therefore postulated that the absence or rareness of spontaneous photoonycholysis in people with darker skin types is due to presence of melanin, providing some degree of natural photoprotection to the nail bed. However, paradoxically the incidence of photoonycholysis has been higher in darkly pigmented individuals on benoxaprofen whose pigmentation has allowed them to get regular exposure to sunlight.…”