Elsevier

Clinics in Dermatology

Volume 31, Issue 5, September–October 2013, Pages 526-539
Clinics in Dermatology

Nail pathology

https://doi.org/10.1016/j.clindermatol.2013.06.005Get rights and content

Abstract

When dealing with nails, pathologic examination is often indispensable to reach an accurate diagnosis. This requires a biopsy correctly performed by the dermatologist, a specimen correctly handled in the pathology lab, and a pathologist with good knowledge of the various nail conditions. The normal nail histology is first described in this paper. The pathologic aspects of melanocytic lesions, nonmelanocytic tumors of the nail apparatus, inflammatory nail conditions, and onychomycosis are then considered, together with their main differential diagnoses.

Introduction

Most pathologists do not like to receive nail biopsies, because they are rarely familiar with them, and the samples are frequently of bad quality. Three conditions should be fulfilled to achieve an accurate diagnosis:

  • 1.

    The nail surgeon should correctly perform the biopsy, which means the surgeon should know where to biopsy, be able to adequately perform the different types of nail biopsies, and harvest the specimen carefully. The nail surgeon should also provide the pathologist with as much information as possible regarding the clinical history, the clinical signs, the suspected diagnosis, and the type of biopsy performed. A sketch representing the nail apparatus with the location and the type of the performed biopsy is a must. There is no good nail pathologist without a good nail clinician and surgeon!

  • 2.

    The sample should be correctly handled by the pathology lab. It should be correctly orientated before being embedded in paraffin. The authors usually try to perform sections parallel to the longitudinal axis of the nail. Lateral longitudinal biopsies are cut, starting from the central part of the nail and proceeding towards the skin side. Punch biopsies of 3 to 4 mm are included as such (without having been previously cut in two). If the nail plate is present, it should be softened. Different procedures have been described.1 We used Mollifex Gurr (VWR Int Ltd), dipping the paraffin blocks in it for 2 to 12 hours, depending on the nail plate thickness. Multiple sections, special stainings (especially periodic acid-Schiff [PAS] and Fontana Masson), and immunohistostainings are often required.

  • 3.

    The pathologist should know the normal nail histology as well as the pathologic aspects of the different nail conditions.

The authors wish to acknowledge that this chapter is largely based on the nail chapter they wrote for the fourth edition of McKee's Pathology of the Skin, where more detailed information may be found.2

The dorsal part of the proximal nail fold (PNF) is similar to the skin of the digit back (Figure 1). Its ventral part is characterized by a rather thin and flattened epidermis (Figure 2). At the angle between its dorsal and its ventral part, the PNF makes a thick stratum corneum called the cuticle.

The nail matrix and nail bed are located under the nail plate and are typically devoid of stratum granulosum. The boundary between the nail matrix and the nail bed is barely visible on histologic examination. The nail matrix has a multilayered epithelium. Its keratinization process is characterized by an eosinophilic onychogenous band devoid of keratohyalin granules (Figure 2). It gives birth to the nail plate. The nail matrix epithelium is the sole site of hard keratin synthesis.3 In the midline of the nail unit, the matrix epithelium is thick with long, oblique rete ridges, distally oriented. Laterally, the matrix rete ridges are less marked. Distally, near the nail bed, the matrix epithelium is thinner, and its onychogenous band is markedly reduced. The nail bed epithelium is thin, reduced to a few cellular layers. In longitudinal sections, it appears flat while in transverse sections, marked rete ridges are observed, parallel to the longitudinal axis of the nail. The keratinization is abrupt with no granular layer. The stratum granulosum reappears only at the hyponychium, which represents the distal thickened part of the nail bed and is bordered by the distal groove and the digital pulp. The nail matrix and the nail bed dermis do not contain pilosebaceous appendages. In the distal matrix, the connective tissue is loose and edematous. In the proximal matrix and the nail bed, it is characterized by dense collagen bundles, vertically orientated, linking the nail apparatus to the periosteum. Eccrine sweat glands are absent. The dermis contains multiple glomus bodies, which represent arteriovenous anastomosis. The term “onychodermis” designates a specialized loose-nail mesenchyme observed in supernumerary digits surgically removed for polydactyly. The onychodermis is located below the matrix and nail bed and is characterized by an increased number of CD10 fibroblasts, mast cells, and mucin deposition.4

There is no true subcutaneous fat in the nail apparatus, but some adipocyte clusters may be observed between the nail bed dermis and the periosteum.

The nail plate is made up of parallel layers of keratinized, flat, and completely differentiated cells called onychocytes. The latter are firmly adherent and do not desquamate, in contrast to corneocytes. Remnants of nuclei are frequently observed in the proximal, ventral part of the nail plate.

Section snippets

Nail pathology

This paper will only describe pathologic aspects, as clinical aspects can be found in other relevant articles of this issue. Melanocytic lesions, nonmelanocytic nail tumors, inflammatory nail conditions, and infections will be successively considered.

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