#dermpathJC April 2018:
Thursday, April 26th, 9pm EST
Article discussed: Melanocytes Pattern in the Normal Nail, with Special Reference to Nail Bed Melanocytes
Authors: Perrin, Christophe, MD, Michiels, Jean-F., MD, Boyer, Julien, MD, Ambrosetti, Damien, MD
American Journal of Dermatopathology, 2018;40(3):180-184.
Temporary open access available at: https://onlinelibrary.wiley.com/doi/abs/10.1111/cup.13085
Summary author: Silvija P. Gottesman, MD (@SGottesmanMD)
Journal Club Summary:
For this month’s journal club we focus on an excellent study that helps define normal melanocytic density in different parts of the nail (nail bed, proximal nail fold, and the nail matrix).
Pigmented nail lesions are divided in three defining categories: melanocytic activation AKA “melanotic macule of the nail unit (melanotic pigmentation of the matrix epithelium without any increase in the density of melanocytes), melanocytic proliferation (lentigo simplex & nevus of the nail matrix), and nail melanoma.
The melanocyte density is a helpful parameter in the distinction of melanotic macule & nail melanoma. Less than 30 melanocytes/mm favor a benign lesion, whereas more than 40 melanocytes/mm favors melanoma. Caveat: some nail melanomas can be low density, and we must rely on other histologic features.
Below is an image depicting a longitudinal section of the nail unit apparatus. Where DPNF is the dorsal proximal nail fold and the Eponychium is the ventral portion of the proximal nail fold. Beyond the proximal nail matrix is the distal nail matrix and then is the nail bed (not depicted in the image here).
Density of nail epithelium melanocytes:
– Nail eponychium (ventral proximal nail fold): between 0 and 5 melanocytes per mm, restricted to the basal cell layer.
– Nail matrix: between 4 and 14 melanocytes per mm, in the basal and suprabasal layers.
– Nail bed: between 0 and 5 melanocytes per mm, also restricted to the basal layer.
Dr Gardner (@JMGardnerMD) shared a diagnostic pearl from Dr Beth Ruben: “unlike acral nevi where pagetoid spread can be ok, pagetoid spread is a bad sign in a nail melanocytic lesion.”
HMB45 and Melan-A are more sensitive markers than tyrosinase and MITF in the detection of intraepithelial nail melanocytes. But since MITF is a nuclear marker, it may be helpful in judging the size and shape of the nuclei of nail melanocytes.
MITF (nuclear stain) in action. Small nuclei of nail melanocytes highlighted. And positive cytoplasmic staining of mastocytes in the surrounding dermis as a positive control.
This paper analyzed nail epithelium from 5 Caucasian cadavers. My understanding is that racial differences is not due to differences in the number of melanocytes, but rather the size, distribution, and number of melanosomes (all races have SAME melanocyte density). The one exception is sun damaged skin of elderly Caucasian patients, where MORE melanocytes in sun exposed skin (solar hyperplasia) is seen as compared to darker skinned patients.
For more discussion about Nail Pathology please check out Dr Gardner’s interview with Dr Beth Ruben (@BethRubenMD), a world famous dermatopathologist and nail pathologist. YouTube link: https://youtu.be/_pwNak_CzUc
Bonus: Dr Ruben’s processing techniques for nail unit tissue: make the lab aware the specimen is delicate and may also contain hard keratin. Nair (NaOH/CaOH) solution can be used prior to processing to soften the specimen. Cedarwood oil may be helpful in processing specimens as well. Soaking the block prior to cutting the tissue after processing can minimize knife trauma. Albumin can help sections stay on the slide.
Looking forward to next month’s journal club,
Silvija P. Gottesman, MD