Clinical Experimental Dermatology 1988; L3: 160-163. Congenital and hereditary nail dystrophies^an embryological approach to classification N.R.TELFER, J.H.BARTH AND R.P.R. DAWBER Department of Dermatology, The S lade Hospital, Oxford OX3 7JH, UK Accepted for publication 4 November 1987 Summary In many congenital and hereditary ectodermal syn- dromes, nail changes can often provide the earliest definitive and consistent signs by which the condition may be identified. In the dermatological, paediatric and genetic hterature the nail signs are usually described on purely morphologi- cal grounds with little attention being paid to the embryological defect or defects underlying such changes. Consequently, any secondary changes produced by trauma to the nails may be confused with, and included in, the primary 'diagnostic' features. We propose a pathophy.siological approach to the nail changes in congenital and inherited conditions: these will be classified as defects in the nail matrix, defects in the nail field or bed, and combined disorders originating from ectodermal and mesodermal tissues (Table 1). This is a simplistic classification and many disorders will result from a combination of these defects. Each of the defects will be illustrated by genodermatoses in which the knowledge of normal nail embryology and intra-uterine development aids description and diagnosis. Embryology of the normal nail'"' Human nail development begins at 9 weeks (Fig. 1). The first visible sign is the formation of a rectangular area on the dorsum of the terminal digit which is delineated by a continuous shallow groove. This is the nail field and is the site of the future nail. The nail matrix forms at the proximal border of the nail field with the development of the nail matrix primor- dium. A solid wedge of differentiating epithelial cells extends downwards and proximally into the dermis. This divides into superficial and deep components. The superficial part becomes the epidermis of the underside of the proximal nail fold and the deeper part becomes the nail matrix. Correspondence: N. R. Teller, Department of Dermatology, Tbe Slade Hospital, Oxford OX3 7JH, UK. Table \. Classification of congenital and hereditary nail dystrophies size position quality Matrix defects Field defects Nail bed defects Combined ectodermal-mesodermal defects At 11 weeks, the fingers are well formed with discern- ible interphalangeal joints and the proximal and lateral nail grooves are well demarcated. At the distal border of the nail field, two structures have appeared: the distal groove and the distal ridge (DR). The distal groove is a shallow depression lying at the distal margin of the nail field; it disappears in man but is large and well defined in other primates. The DR lies parallel and immediately proximal to the distal groove and later becomes the hyponychium. The area of nail field between the DR, the two lateral grooves and the lunula of the developing matrix is termed the nail bed. Keratinization of the nail bed begins at the DR at 11 9weeks 11 weeks NP 13 weeks 22 weeks 32 weeks Figure 1. The embryological development of the nail^—see text for details, (nail field (NF), dorsal ridge (DR) and nail plate (NP)). 160