Why do nails grow vertically?

Sir,

This issue of Indian Dermatology Online Journal carries an interesting case report by Prachi Barad, Joycelin Fernandes, Rakhi Ghodge, Pankaj Shukla describing a “vertically growing nail”.[1] This report is thought provoking, not just because of the case it presents, but also because of the age-old debate, why do the nails grow as they grow? The human nail is a set of complex structures, better described as the Nail Unit [Figure 1].[2] It consists of six components which form, ensheathe, support, anchor, and frame the nail plate. These components are the nail matrix, nail plate, cuticular system, nail bed mesenchyme and phalangeal bones, anchoring ligaments, and nail folds.

Figure 1

Sagittal anatomy of the nail unit revealing the location and constitution of the “cul-de-sac” from which the nail plate originates. (PNF refers to Proximal Nail Fold)

Why do the nails grow “outwards” and not “upwards”? This important role has been attributed to different parts of the nail unit over the years.

According to Kligman, the “cul-de-sac” could be ensuring this.[3] Based on his observations on experimental removal of the proximal nail fold (PNF) and transplantation of nail matrix tissue in human forearm, he concluded that in the absence of modulating effect of PNF, the nail tended to grow out vertically.[3] Today, we better understand this “cul-de-sac” as the area under the PNF comprising of the proximal and intermediate matrix, which surrounds the dorsal and the ventral part, respectively, of the newborn nail plate. As can be seen in Figures 1 and 2, the proximal matrix sits on the ventral surface of the PNF, and contributes to the superficial part of the nail plate, whereas the intermediate (or germinative) matrix lies below the nail plate and is responsible for producing its bulk.[2] Kikuchi et al. also reported a case of vertically growing ectopic nail of congenital origin.[4] The vertical growth in this case was attributed to a cul-de-sac, directed toward the skin surface.[4] However, evidently, the role played by the “cul-de-sac” is not that crucial because patients with chronic paronychia undergoing elliptical excision of PNF, are known to develop a normal nail plate thereafter.[56]

Figure 2

Mechanism of formation of the nail plate. The bold arrows highlight the contribution of the different components of the matrix to the direction of nail growth. (1) The proximal matrix contributes to the upper surface of the nail plate; (2) the intermediate matrix contributes to the bulk of the nail plate; (3) the nail bed (sterile matrix) contributes to the undersurface of the nail plate. Any imbalance between the relative contributions of these components can result in altered shape of the nail plate formed

The underlying phalanx could be determining the growth.[5] An absence of underlying phalanx could be the main reason behind Kligman's experimental nail matrix transplants producing a vertically oriented nail plate.

As proposed by Hashimoto[7] and supported by Baran,[5] The matrix orientation/structure could be mainly responsible as the centroaxial orientation of matrix cells is quite unique as compared with the surrounding tissue. This could ensure the unique orientation of the nail plate cells. A definite role of proximally directed rete comprising the matrix has been reinforced, with possibly a containing effect of lateral nail folds.

Subsequently, the effect of the nail bed was also proposed as the determining factor by Kato.[8] Although the nail bed (sterile matrix) makes a small contribution toward nail plate, this nevertheless, is an important one. Patients with nail bed scarring or loss of attachment between nail plate and bed could develop a vertically growing nail.

While we are debating the mechanisms of growth of a “vertical nail,” another important point which needs to be kept in mind is the possibility that a “vertical nail” could just be a more severe pathological analogue of onychogryphosis. Onychogryphosisis is a nail deformity characterized by an opaque, discolored nail plate, which is skewed, grossly thickened, and partly curved. In fact, the grossly abnormal architecture and surface of this nail qualifies the present case as onychogryphosis. Nath et al. had described a similar case (named a “ leaning tower nail” by the authors), which they qualified as an “extreme form of onychogryphosis.”[9]

The causes of onychogryphosis [Table 1] are many; but local trauma is one of the most important. Extrapolating to the index cases, we can see that the ill effects of misplaced (not indicated) nail avulsions could have lead to this deformity over a period of time. Zaias proposed that a differential growth rate in different sections of the matrix could result in a differential growth of parts of the nail plate, thus resulting in onychogryphosis. The faster growing side determines the direction with the nail plate tilting toward the slowly growing side.[10] Ohata et al. described an extreme case as “inychogryphosis,” that is, an onychogryphotic nail plate which grows backward and penetrates the PNF.[11] Essentially, the differential growth rate in different parts of the nail, could have led to a particularly deformed, vertically growing nail; the difference being just in the degree of angulation it produces, as highlighted in different cases in the literature [Figure 2].[19]

Table 1

Causes of onychogryphosis

A future thorough evaluation of such cases can throw more light on the underlying pathomechanisms, if a histopathological examination is done to visualize changes, if any, in the nail bed and nail matrix. Keratin typing or genetic analyses could also throw some light in case a larger number of nails are affected.