Nail Isthmus: A Distinct Region of the Nail Apparatus

The nail unit is constructed by distinctly regulated components. The nail isthmus is a lately proposed region as a transitional zone between the most distal part of the nail bed and the hyponychium. It is difficult to recognize the nail isthmus in the normal nail, but it is easy to identify the region in nail disorders such as pterygium inversum unguis and ectopic nail. We describe structure and putative function of the nail isthmus via histopathologic features of pterygium inversum unguis and ectopic nail. 1. The Nail Isthmus The nail unit has distinct structure. The concept of the nail isthmus was recently proposed by Perrin in 2007 [1]. The region is present in the transitional zone between the most digital part of the nail bed and the hyponychium (Figure 1) [1–3]. Perrin described the four typical features of the nail isthmus: (i) the maintenance of the longitudinal ridge pattern of the nail bed, (ii) a discontinuous and thin granular layer, (iii) a peculiar and thin compartment of pale and nucleated corneocytes, and (iv) a profile of transitional keratin expression [1–3]. The nail isthmus is almost invisible in the normal nail, but it is able to be identified in nail disorders such as pterygium inversum unguis [4] and ectopic nail [5]. Figure 1: A diagram of the nail apparatus. The nail isthmus is present in the transitional zone between the most digital part of the nail bed and the hyponychium. 2. The Structure of the Nail Isthmus The nail isthmus is composed of two distinct parts. A histopathological study of the nail isthmus with a case of pterygium inversum unguis identified two substances: (i) a marked, highly eosinophilic, keratinized substance attaching the distal and visceral nail plate and (ii) a whorled, highly eosinophilic, keratinized substance into the horny layer of the finger tip (Figures 2 and 3) [4]. The former substance is constructed by an extraordinary mode of keratinization with a compartment of pale and nucleated corneocytes [1, 4]. Another histopathological study with a case of ectopic nail (Figures 4 and 5) showed two distinct parts: (i) a proximal and narrow part anchored to the nail plate and (ii) a distal and wide part constructed with highly eosinophilic structure [5]. Figure 2: A ventral appearance of pterygium inversum unguis in a 16-year-old Japanese man. Marked subungual keratotic thickening is present on the distal nail unit of the left fingers (arrows). Figure 3: A biopsy specimen from the left second finger showed (i) a marked, highly eosinophilic, keratinized substance attaching the distal and visceral nail plate