The 2 clinical subbands of the distal nail unit and the nail isthmus. Anatomical explanation and new physiological observations in relation to the nail growth.Am J Dermatopathol 2008; 30(3):216-21AJ
The distal region of the nail unit, which marks the transition of the nail bed to the hyponychium, is known clinically as the onychodermal band (ODB). For several authors, the ODB is described as 2 subbands of tonal values: slightly milky and pink from proximal to distal. However, its 2 subbands show individual variation and are often scarcely visible. It has been stated that the proximal subband, that is, the white band of Pinkus, corresponds histologically to the attachment of the compact orthokeratotic layer of the hyponychium to the nail plate. In 2 studies, I have individualized a transitional zone between the nail bed and the hyponychium: the nail isthmus. In these previous studies, the ODB was not visible. The purpose of this article is to analyze the anatomo-clinical correlations between the nail isthmus and the ODB and to highlight some new physiological information concerning the nail growth. A case showing clearly the 2 clinical subbands of ODB was investigated. In addition, all the longitudinal sections of the 5 normal nail unit specimens, used in the 2 previous studies, were reviewed to analyze the ventral surface of the nail plate in longitudinal planes. The nail isthmus can be described synthetically by 2 features: (1) a stair-like appearance of the epithelium of the distal nail bed with a marked depression of the epithelium below the inferior surface of the nail plate (2) a specialized mode of attachment of its horny layer to nail plate via a horizontal mode of differentiation. The white band of Pinkus corresponded histologically to the nail isthmus. Its white color was caused by alteration in light diffraction in the thin compartment of pale parakeratotic corneocytes squeezed between the nail plate and the epithelium of the nail isthmus. The pink middle band corresponded to the 2 horny layers surmounting the epidermis of the hyponychium: the compact horny layer of the hyponychium and the thin pale "tongue-like" parakeratotic extension of the nail isthmus. The variable thinness of both the suprapapillary plates of hyponychium and the 2 horny layers surmounting this area explained the variable visibility of the well-developed vascular network of the dermal papillae of hyponychium, that is, the variegation of the middle subband. The review of all step sections of the normal distal nail unit cut longitudinally showed that the ventral surface of the nail plate was slightly serrated in longitudinal planes. The nail isthmus belongs functionally to the nail bed. The strong adherence between the nail plate and the nail bed is replaced by an effective sealing between the thin pale tongue-like parakeratotic extension of the nail isthmus and the nail plate. In contrast, the hyponychium belongs functionally to fingertip and exhibits an epidermal-type horny layer, that is, the horny layer of the distal hyponychium is a supportive structure. As the ventral surface of the nail plate is undulated in its transversal and to a lesser extend in its longitudinal axis, the nail bed and the nail plate grow forward together. The nail plate is "pushed" out by 2 factors: (1) the matrix which makes a new plate, (2) the nail bed which moves slowly, parallel to the direction of the nail growth, toward the tiny grooves and ridges of the inferior border of the nail plate. According to the anatomo-clinical description of the distal nail unit, the nomenclature of this region could be simplified. The term nail isthmus should be proposed as equivalent of the white band of Pinkus. The hyponychium could be compared with the follicular infundibulum as a zone bordering the surrounding epidermis, and the term nail infundibulum could be considered to be synonymous for hyponychium and ODB.