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

Area of Science:

• Dermatopathology
• Oncology
• Histology

Background:

• Conflicting previous studies on normal nail bed melanocyte density hinder diagnosis.
• Distinguishing benign subungual melanotic macules from early melanoma in situ is clinically challenging.
• Establishing baseline melanocyte counts is crucial for accurate pathological interpretation.

Purpose of the Study:

• To quantify melanocyte density in normal nail unit structures.
• To compare the sensitivity of HMB45 and Microphtalmia-associated transcription factor (MITF) in evaluating nail melanocytes.
• To provide reference values for normal melanocyte distribution in the nail bed, matrix, and eponychium.

Main Methods:

• Analysis of five normal nail unit specimens.
• Melanocyte count (MC) defined as melanocytes per 1-mm stretch of nail epithelium.
• Evaluation using hematoxylin and eosin, HMB45, and MITF staining.

Main Results:

• Nail matrix mean MC was 6.86 (range 4-14), with irregular distribution and dendrites.
• Nail bed mean MC was 0.43 (range 0-5), with melanocytes in the basal layer; two cases had no nail bed melanocytes.
• Eponychium mean MC was 2.27/mm (range 0-5/mm).
• HMB45 demonstrated higher sensitivity than MITF for intraepithelial melanocytic density.
• Nail keratinocytes can mimic melanocytic proliferation on H&E staining; matrical melanocyte dendrites may resemble acral melanoma patterns.

Conclusions:

• Normal nail beds can exhibit foci of relatively high melanocytic density, comparable to the nail matrix.
• MITF alone may lead to false-negative results in early melanoma detection due to weak sensitivity.
• Prudent interpretation of nail melanocytic atypia is essential, considering potential mimics on HMB45, Melan A, and H&E staining.