Chapter 1
Science of the Nail Apparatus

David de Berker1, Beth S. Ruben2, and Robert Baran3

1 Bristol Dermatology Centre, Bristol Royal Infirmary, Bristol, UK

2 Dermatology/Dermatopathology, University of California, San Francisco

Dermatopathology, Palo Alto Medical Foundation, Palo Alto, CA, USA

3 Hon. Pr. of the University of Franche‐Comté; Nail Disease Centre, Cannes, France

CHAPTER MENU

• Gross anatomy and terminology
• Embryology
• Morphogenesis
• Tissue differentiation
• Factors in embryogenesis
• Histological preparation
• Regional anatomy
• Nail matrix and lunula
• Nail bed and hyponychium
• Nail folds
• Nail plate
• Vascular supply
• Arterial supply
• Venous drainage
• Effects of altered vascular supply
• Nail fold vessels
• Glomus bodies
• Nerve supply
• Comparative anatomy and function
• The nail and other appendages
• Phylogenetic comparisons
• Physiology
• Nail production
• Normal nail morphology
• Nail growth
• Nail plate biochemical analysis
• Physical properties of nails
• Strength
• Permeability
• Radiation penetration

Gross anatomy and terminology

Knowledge of nail unit anatomy and terms is important for clinical and scientific work [1, 2]. The nail is an opalescent window through to the vascular nail bed. It is held in place by the nail folds, origin at the matrix, and attachment to the nail bed. It ends at a free edge distally, overlying the hyponychium. These structures are illustrated in Figs 1.1–1.4. Definitions of the components of the nail unit are as follows.

• Nail plate (nail): durable keratinized structure which continues growing throughout life.
• Lateral nail folds: the cutaneous folded structures providing the lateral borders to the nail.
• Proximal nail fold (posterior nail fold): cutaneous folded structure providing the visible proximal border of the nail, continuous with the cuticle. On the undersurface this becomes the dorsal matrix.
• Cuticle (eponychium): the layer of epidermis extending from the proximal nail fold and adhering to the dorsal aspect of the nail plate.
• Nail matrix (nail root): traditionally, this can be split into three parts [3]. The dorsal matrix is synonymous with the ventral aspect of the proximal nail fold. The intermediate matrix (germinative matrix) is the epithelial structure starting at the point where the dorsal matrix folds back on itself to underlie the proximal nail. The ventral matrix is synonymous with the nail bed and starts at the border of the lunula, where the intermediate matrix stops. It is limited distally by the hyponychium.
• Lunula (half moon): the convex margin of the intermediate matrix seen through the nail. It is paler than the adjacent nail bed. It is most commonly visible on the thumbs and great toes. It may be concealed by the proximal nail fold.
• Nail bed (ventral matrix, sterile matrix): the vascular bed upon which the nail rests, extending from the lunula to the hyponychium. This is the major territory seen through the nail plate.
• Onychodermal band: the distal margin of the nail bed has a contrasting hue in comparison with the rest of the nail bed [4]. Normally, this is a transverse band of 1–1.5 mm of a deeper pink (white) or brown (Afro‐Caribbean). Its color, or presence, may vary with disease or compression, which influences the vascular supply (Fig. 1.5). Sonnex et al. [5] examined 1000 nails from thumbs and fingers in 100 subjects, alive and dead. In addition to clinical observation, they obtained histology from cadavers. Their findings are summarized in Table 1.1. The onychodermal band represents the first barrier to penetration of materials beyond the nail plate. Disruption of this barrier by disease or trauma precipitates a range of further events affecting the nail bed. The white appearance of the central band represents the transmission of light from the digit tip through the stratum corneum and up through the nail. If the digit is placed against a black surface, the band appears dark.
• Hyponychium: the cutaneous margin underlying free nail, bordered distally by the distal groove (Fig. 1.6).
• Distal groove (limiting furrow): a cutaneous ridge demarcating the border between subungual structures and the digit pulp.

Figure 1.1 (a,b) Longitudinal section of a digit showing the dorsal nail apparatus.

Figure 1.2 The tip of a digit showing the component parts of the nail apparatus.

Figure 1.3 Proximal nail unit in longitudinal excision. Longitudinal view. A, Cuticle. B, Proximal nail fold. C, Distal matrix including keratogenous zone. D, Proximal matrix. E, Dorsal matrix. F, Ventral aspect of proximal nail fold.

Figure 1.4 Distal nail matrix and nail bed in longitudinal excision. (a,b) Longitudinal view of the distal matrix and nail bed epithelium with overlying nail plate. (c,d) Transverse sections of the nail unit taken from an amputation for melanoma illustrating the nail bed longitudinal ridges in cross‐section (C1), lateral nail groove (C2), and underlying phalanx (C3).

Figure 1.5 (a) Onychodermal band. (b) Diagrammatic representation of the morphological features of the normal nail; detail of the distal physiological color bands are shown.

Courtesy of T.S. Sonnex and W.A.D. Griffiths.

Table 1.1 Clinical appearance of distal zones of the nail bed.

Figure 1.6 Distal nail unit in longitudinal excision. Onychodermal band (A) at the junction with the hyponychium (B) where a granular zone is again found along with acral compact stratum corneum.

Embryology

Morphogenesis

8–12 weeks

Individual digits are discernible from the 8th week of gestation [3]. The first embryonic element of the nail unit is the nail anlage, present from 9 weeks as the epidermis overlying the dorsal tip of the digit. At 10 weeks, a distinct region can be seen and is described as the primary nail field. This almost overlies the tip of the terminal phalanx, with clear proximal and lateral grooves in addition to a well‐defined distal groove. The prominence of this groove is partly due to the distal ridge, thrown up proximally, accentuating the contour. The primary nail field grows proximally by a wedge of germinative matrix cells extending back from the tip of the digit. These cells are proximal to both the distal groove and ridge. The spatial relationship of these two latter structures remains relatively constant as the former becomes the vestigial distal groove and the latter the hyponychium (Fig. 1.7).

Figure 1.7 Embryogenesis of the nail apparatus. 10 weeks, The primary nail field can be seen with proximal, lateral, and distal grooves. The latter is accentuated by a distal ridge. 13 weeks, A wedge of matrix primordium moves proximally, with the invagination of the proximal nail fold above. 14 weeks, The nail plate emerges. 17 weeks, The nail plate covers most of the nail bed and the distal ridge starts to flatten. 20 weeks, The nail plate extends to the distal ridge, now termed hyponychium. Finger and nail grow roughly in tandem from now on. Fetuses are one‐fifth of the actual size.

13–14 weeks

Differential growth of the slowly developing primary nail field and surrounding tissue results in the emergence of overhanging proximal and lateral nail folds. Depending on the point of reference, the nail folds may be interpreted as overhanging [6] or the matrix as invaginating. By 13 weeks the nail field is well defined in the finger, with the matrix primordium underlying a proximal nail fold. By 14 weeks the nail plate is seen emerging from beneath the proximal nail fold, with elements arising from the lunula as well as more proximal matrix.

17 weeks to birth

At...