The ultrastructure and growth of human nails

Exposome Impact on Nail Health

Background

The nail unit is a complex system with various components, each serving distinct functions. The exposome, encompassing external and internal factors such as UV radiation, air pollution, dietary habits, and cosmetic product usage, substantially influences nail health and can lead to premature nail aging.

Summary

Internal and external exposomal factors can impact differently on nail health, inducing a variety of different clinical conditions. Effective therapeutic strategies exist, but a comprehensive understanding of how the exposome affects nails is lacking. This article aims to bridge this knowledge gap by exploring the relationship between the exposome and nail health, emphasizing it as a central focus of our analysis.

Key Messages

(1) The exposome, comprising various external and internal factors, may significantly influence nail health negatively, leading to premature nail aging. (2) Different nail conditions may arise due to the exposomal influence on nails. (3) Understanding the exposome's impact on nail health is crucial for developing solutions to mitigate negative effects and improve overall nail well-being.

Effects of two types of surface treatments on the structural elasticity of human fingernails

BACKGROUND

The human nail has a three-layered structure. Although it would be useful to quantitatively evaluate the changes in deformability of the nail due to various surface treatments, few studies have been conducted.

METHODS

The effects of two types of surface treatment-a chemically acting nail softener and a physically acting nail strengthener-on the deformability of human fingernails were investigated. The Young's modulus of each plate of the nail samples before and after softening treatment was determined by nanoindentation. The Young's modulus of the strengthener was determined by conducting a three-point bending test on a polyethylene sheet coated with the strengthener.

RESULTS

Young's modulus decreased in order from the top plate against the softening treatment time, and the structural elasticity for bending deformation (SEB) of the nail sample, which expresses the deformability against bending deformation independent of its external dimensions, decreased to 60% after 6 h of treatment. The Young's modulus of the nail strengthener was 244.5 MPa, which is less than 10% of the SEB of the nail. When the nail strengthener was applied to the nail surface, the SEB decreased to 73%, whereas the flexural rigidity increased to 117%.

CONCLUSION

Changes in nail deformability caused by various surface treatments for softening and hardening were quantitatively evaluated successfully.

Topical drug delivery strategies for enhancing drug effectiveness by skin barriers, drug delivery systems and individualized dosing

Topical drug delivery is widely used in various diseases because of the advantages of not passing through the gastrointestinal tract, avoiding gastrointestinal irritation and hepatic first-pass effect, and reaching the lesion directly to reduce unnecessary adverse reactions. The skin helps the organism to defend itself against a huge majority of external aggressions and is one of the most important lines of defense of the body. However, the skin's strong barrier ability is also a huge obstacle to the effectiveness of topical medications. Allowing the bioactive, composition in a drug to pass through the stratum corneum barrier as needed to reach the target site is the most essential need for the bioactive, composition to exert its therapeutic effect. The state of the skin barrier, the choice of delivery system for the bioactive, composition, and individualized disease detection and dosing planning influence the effectiveness of topical medications. Nowadays, enhancing transdermal absorption of topically applied drugs is the hottest research area. However, enhancing transdermal absorption of drugs is not the first choice to improve the effectiveness of all drugs. Excessive transdermal absorption enhances topical drug accumulation at non-target sites and the occurrence of adverse reactions. This paper introduces topical drug delivery strategies to improve drug effectiveness from three perspectives: skin barrier, drug delivery system and individualized drug delivery, describes the current status and shortcomings of topical drug research, and provides new directions and ideas for topical drug research.

Nanoindentation study of human fingernail for determining its structural elasticity

BACKGROUND

Human nails play an important role in transmitting force to the fingertips, and their mechanical properties are important indices. The nail has a three-layered structure consisting of top dorsal, middle intermediate, and under ventral plates, and its internal structure is believed to affect its mechanical properties. However, this has not been investigated in previous studies.

METHODS

The Young's moduli of the top, middle, and under plates were measured using nanoindentation, and a theoretical model was developed to estimate the structural elasticity for the bending deformation of human nails, which is an index describing the deformability of the nail without depending on its external dimensions. The structural elasticity of human nails was compared with that of human hair collected from the same person. The effect of the softening treatment on the nails was also evaluated.

RESULTS

The Young's moduli of the top, middle, and under plates measured using nanoindentation were 2.9, 3.1, and 2.8 GPa, respectively. The structural elasticity of the nail was estimated to be 2.9 GPa, approximately 75% that of hair. Moreover, softening treatment with a urea cream reduced the structural elasticity of the nail to 70%.

CONCLUSION

This paper proposed a method for estimating the structural elasticity of a human nail with a three-layered structure. This index is a mechanical property with "Pa" as a unit, and is useful for comparing deformability with the Young's modulus of other homogeneous materials or for investigating the effect of various treatments quantitatively.

Onychomycosis: Current Understanding and Strategies for Enhancing Drug Delivery into Human Nail Tissue

BACKGROUND

Onychomycosis is by far the most common finger or toe nail fungal infectious disease caused by dermatophytes, non-dermatophytic molds or yeast. It accounts for 50% of the total nail disorders, and affects patients physically, socially, and psychologically and can seriously influence their quality of life.

OBJECTIVES

Oral antifungals are routinely used to treat the nail fungal disease; however oral therapy is associated with severe side effects and longer treatment times. In recent years, drug delivery directly into the nail or nail bed has gained attention and various topical products have been tested that can cure the disease when applied topically or transungually. Nevertheless, drug penetration into and through the nail is not straightforward and requires chemicals to improve its permeability or by applying physical stress to promote drug penetration into and through the nail. This lucid review presents an overview of various causes of onychomycosis, current therapeutic approaches, and efforts aimed at increasing the permeability of nails through various strategies such as chemical, physical and mechanical methods for permeation enhancement.

CONCLUSION

Various strategies have been proposed for the treatment of onychomycosis, however, much research into a more precise and effective therapy is still required.

Treatment and management strategies of onychomycosis

Onychomycosis is one of the most prevalent and severe nail fungal infections, which is affecting a wide population across the globe. It leads to variations like nail thickening, disintegration and hardening. Oral and topical drug delivery systems are the most desirable in treating onychomycosis, but the efficacy of the results is low, resulting in a relapse rate of 25-30%. Due to systemic toxicity and various other disadvantages associated with oral therapy like gastrointestinal, hepatotoxicity, topical therapy is commonly used. Topical therapy improves patient compliance and reduces the cost of treatment. However, due to poor penetration of topical therapy across the nail plate, research is focused on different chemical, mechanical and physical methods to improve drug delivery. Penetration enhancers like Thioglycolic acid, Hydroxypropyl-β-cyclodextrin (HP-β-CD), Sodium lauryl sulfate (SLS), carbocysteine, N-acetylcysteine etc. have shown results enhancing the drug penetration across the nail plate. Results with physical techniques such as iontophoresis, laser and Photodynamic therapy are quite promising, but the long-term suitability of these devices is in need to be determined. In this article, a brief analysis of the treatment procedures, factors affecting drug permeation across nail plate, chemical, mechanical and physical devices used to increase the drug delivery through nails for the onychomycosis management has been achieved.

Transungual Delivery of Ciclopirox Is Increased 3⁻4-Fold by Mechanical Fenestration of Human Nail Plate in an In Vitro Model

Onychomycosis is a fungal infection of nails that is widespread and difficult to treat because of the impermeable nature of human nails. Topically applied anti-fungal agents cannot penetrate this structure, and treatment regimens often resort to systemic antifungals with concomitant side effects. One recent clinical study suggested that mechanical fenestration of the nail using an intelligent nail drill might be a possible solution to this problem. In this work, an in vitro model of the transungual delivery of antifungal agents is presented, which utilizes real nail tissue and an inline flow system. This system was deployed to measure transungual delivery of ciclopirox and determined that nail fenestration improved drug delivery by 3–4-fold after 42 days. This study bolsters the argument that nail fenestration should be accepted as a pretreatment for onychomycosis and offers a way of evaluating new drugs or formulations designed to combat this condition.

Understanding the formidable nail barrier: A review of the nail microstructure, composition and diseases

The topical treatment of nail fungal infections has been a focal point of nail research in the past few decades as it offers a much safer and focused alternative to conventional oral therapy. Although the current focus remains on exploring the ways of enhancing permeation through the formidable nail barrier, the understanding of the nail microstructure and composition is far from complete. This article reviews our current understanding of the nail microstructure, composition and diseases. A few of the parameters affecting the nail permeability and potential causes of the recurrence of fungal nail infection are also discussed.

Nail biology and nail science

The nail plate is the permanent product of the nail matrix. Its normal appearance and growth depend on the integrity of several components: the surrounding tissues or perionychium and the bony phalanx that are contributing to the nail apparatus or nail unit. The nail is inserted proximally in an invagination practically parallel to the upper surface of the skin and laterally in the lateral nail grooves. This pocket-like invagination has a roof, the proximal nail fold and a floor, the matrix from which the nail is derived. The germinal matrix forms the bulk of the nail plate. The proximal element forms the superficial third of the nail whereas the distal element provides its inferior two-thirds. The ventral surface of the proximal nail fold adheres closely to the nail for a short distance and forms a gradually desquamating tissue, the cuticle, made of the stratum corneum of both the dorsal and the ventral side of the proximal nail fold. The cuticle seals and therefore protects the ungual cul-de-sac. The nail plate is bordered by the proximal nail fold which is continuous with the similarly structured lateral nail fold on each side. The nail bed extends from the lunula to the hyponychium. It presents with parallel longitudinal rete ridges. This area, by contrast to the matrix has a firm attachment to the nail plate and nail avulsion produces a denudation of the nail bed. Colourless, but translucent, the highly vascular connective tissue containing glomus organs transmits a pink colour through the nail. Among its multiple functions, the nail provides counterpressure to the pulp that is essential to the tactile sensation involving the fingers and to the prevention of the hypertrophy of the distal wall tissue, produced after nail loss of the great toe nail.

Expression of Cadherin/Catenin Cell—Cell Adhesion Molecules in a Onychomatricoma

Onychomatricoma is a rare nail tumor with a distinctive architecture. Proximally, there are serum-filled invaginations of nail matrix epithelium into the stroma, and distally, dermal protrusions perforate the nail plate. Because other matrical tumors of follicular and odontogenic origin express nuclear β-catenin, we examined the expression of cadherin/catenin proteins in this onychomatricoma case. The patient presented with a toenail yellow streak, and the biopsy revealed an onychomatricoma. E-cadherin and β-catenin were at the cell membrane in the epithelial invaginations. P-cadherin was restricted to basal cells. In contrast to other matrical tumors, nuclear β-catenin was not present. These results suggest that onychomatricoma may lack the transcriptional activating role of β-catenin that characterizes follicular and odontogenic matrical tumors. This is the first report on the expression of cadherin/ catenin cell—cell adhesion proteins in this rare nail tumor.

Nail swelling as a pre-formulation screen for the selection and optimisation of ungual penetration enhancers

INTRODUCTION

Targeting drug treatment to fungal infections that reside within or below the nail plate is problematic due to the highly restrictive barrier of the human nail. To optimise topical formulations for ungual drug delivery, inclusion of an effective penetration enhancer (PE) is imperative. At present, in vitro nail permeation studies can take weeks or months in order to obtain any meaningful data because the lack of a simple in vitro model to identify and develop nail PEs makes the selection and optimisation of novel PEs an empirical and inefficient process. The aim of this study was to compare three methods for pre-formulation screening of putative ungual PEs and then to select the most suitable technique for screening candidates that may enhance the permeation of therapeutic agents through the human nail.

METHODS

Three screening techniques were evaluated; nail swelling (weight increase of human nail clippings), horse hoof swelling (weight increase of horse hoof clippings) and nail penetration of a radiolabelled permeability probe. Four test PEs were evaluated using each screening method and nail swelling was identified as a simple, rapid, economic, relevant and reliable technique. This screen was then used to evaluate 20 potential PEs. Thioglycolic acid (TA), hydrogen peroxide (H(2)O(2)) and urea H(2)O(2) produced the greatest nail weight increases; 71.0+/-4.6%, 69.2+/-6.6%, and 69.0+/-9.9 respectively. To confirm the relationship between human nail swelling and altered ungual barrier function, a permeation study was performed in human nails using caffeine as a model penetrant.

RESULTS AND DISCUSSION

Human nails pre-treated with TA in vitro had a 3.8-fold increase in caffeine flux compared to the control (TA-free solution). This study illustrated the potential to use human nail clipping swelling as a surrogate marker of PE activity for topical ungual drug delivery.

Terminal differentiation of nail matrix keratinocytes involves up-regulation of DNase1L2 but is independent of caspase-14 expression

Terminal differentiation of keratinocytes in the epidermis and in epidermal appendages results in specialized forms of cell death. Keratinocytes of the nail matrix differentiate into nail corneocytes, the building blocks of the nail plate. Here, we show that, in contrast to the abrupt breakdown of the nucleus during corneocyte formation of epidermal keratinocytes, chromatin undergoes progressive condensation over several nail matrix cell layers below the transition zone to the nail plate, where nuclear DNA disappears. Virtually all keratinocytes in the cell layer immediately beneath the nail plate contained terminal deoxynucleotidyl transferase-mediated fluorescein-dUTP nick end labeling-positive DNA fragments. Nail matrix keratinocytes lacked processed caspase-3, a marker of apoptosis, and did not express caspase-14, a protease up-regulated during terminal differentiation of epidermal keratinocytes. By contrast, DNase1L2, which is also up-regulated during the differentiation of epidermal keratinocytes and plays an essential role in differentiation-associated degradation of nuclear DNA in epidermal keratinocytes, was strongly expressed in the nail matrix-nail plate transition layer. Our results show that caspase-14 is not strictly, if at all, required for differentiation-associated keratinocyte cell death and implicates DNase1L2 in terminal differentiation of nail matrix keratinocytes.

Modulation of linear nail growth to treat diseases of the nail

Diseases affecting the nail can cause significant distress and interfere with an individual's self-esteem, personal relationships, and professional life. Often, hand and foot function is adversely affected. Certain diseases are characterized by accelerated nail growth while others show a decrease. In this review, drugs known to influence the growth rate of nails are examined, highlighting their potential use as adjunctive therapy in the treatment of nail disease. This approach, described recently in the context of the yellow nail syndrome,(1) may be extended to other common disorders such as nail psoriasis, brittle nails, and onychomycosis.

Scopulariopsis brevicaulis: a keratinophilic or a keratinolytic fungus?

The morphologic expression of human hair and nail invasion in vitro by Scopulariopsis brevicaulis isolates was studied by light microscopy on whole material and on semi-thin sections, and also by scanning and transmission electron microscopy. Only three isolates of the nine that were examined were keratinolytic, capable of both attacking keratinic substrates and demolishing their keratin. The process showed all the characteristic of enzymatic digestion and was in agreement with the biochemical and morphogenetic scheme proposed for other keratinolytic fungi during their invasion of human hair in vitro. All the active isolates were capable of developing structures related to surface erosion and radial penetration contemporaneously. However the extent and rate of keratinolysis were rather low when compared with the efficiency of other keratinolytic fungi. This finding suggests that S. brevicaulis is of secondary importance in the mineralization of keratinic substrates in natural environments. From the medical standpoint the mere demonstration of keratinolytic activity means that it may be regarded as a real cause of primary infection.

Drug permeation through the three layers of the human nail plate

The in-vitro permeation characteristics of a water soluble model drug, 5-fluorouracil, and a poorly water soluble model drug, flurbiprofen, were investigated through three layers of the human nail plate (namely, the dorsal, intermediate and ventral nail plates), using a modified side-by-side diffusion cell. The dorsal-filed nail plate, the ventral-filed nail plate and the dorsal-and-ventral-filed nail plate were prepared to known thicknesses and then used with the full-thickness nail plate to investigate the permeation characteristics of each single layer. Most of the lipids in the human nail plate were found in the dorsal and ventral layers. The rank orders of the permeation fluxes for 5-fluorouracil and flurbiprofen were both: dorsal-and-ventral-filed nail plate > dorsal-filed nail plate > ventral-filed nail plate > full-thickness nail plate. With respect to 5-fluorouracil permeation through each single layer, the permeability coefficient of the intermediate layer was higher than those of other single layers. However in the case of flurbiprofen, the permeability coefficient of the ventral layer was higher than other single layers. The diffusion coefficients of 5-fluorouracil and flurbiprofen in the dorsal layer were the lowest of any single layer. The drug concentration in each layer was estimated using each respective permeation parameter. The drug concentration in the nail plate was observed to be dependent on the solubility and the flux of the drug. From these findings, we suggest that the human nail plate behaves like a hydrophilic gel membrane rather than a lipophilic partition membrane and that the upper layer functions as the main nail barrier to drug permeation through its low diffusivity against the drugs.

The onychomatricoma: an electron microscopic study

The electron microscopic findings in the onychomatricoma are described. In the proximal zone of the onychomatricoma, basal cells have various aspects, some being lacunar while others have only a limited cytoplasmic rim containing mitochondria and tonofilaments. In the parakeratotic cell columns the cells elongate and homogenized tonofilaments appear. Around the lacunae the cells are poorly differentiated and their cytoplasm is granular. It can be concluded that in an onychomatricoma the basal cells have a decreased amount of tonofilaments and desmosomes and that their evolution is not uniform. The tumour can be considered as being the result of a disturbed differentiation of nail matrix cells.

Current management of onychomycosis. An overview

Until recently pedal onychomycosis, particularly when it affected several nails or involved a large nail plate area, was often regarded as untreatable. The advent of new therapies such as itraconazole, terbinafine, and fluconazole has been a significant and welcome addition to the armamentarium of therapies at the disposal of the physician. These drugs appear in the nail plate within days of starting oral therapy, being taken up by both the nail matrix and the nail bed. The duration required for effective therapy has been reduced, while the efficacy rates and cost-effectiveness have increased compared with the older treatments, such as griseofulvin. Some of the newer agents appear to have a wider spectrum of activity. Thus far, the newer agent have exhibited a low risk to benefit ratio. I may be possible to combine oral therapies with topical and surgical treatments, thereby further increasing efficacy rates and the cost-effectiveness while decreasing adverse effects and duration of oral therapy.

Acquired nail beading in patients receiving itraconazole--an indicator of faster nail growth? A study using optical profilometry

Nail surfaces of toe-nail fragments were analysed by optical profilometry in patients treated with two different dosages of itraconazole: 200 mg continuous dosing for 12 weeks, or a pulse-dose regimen of 1 week per month of 400 mg itraconazole daily for 3-4 months. The use of the latter regimen seemed clinically to increase nail growth, which was accompanied in several patients by the occurrence of nail surface irregularities. This aspect was studied by computerized optical profilometry. Nail beading was characterized by a higher number of peaks and a larger mean roughness value (Ra). Such findings are reminiscent of other nail alterations resulting from a faster matrix turnover. A substantially greater number of peaks and larger Ra were found in the patients receiving the pulse treatment than in those receiving the continuous-dose regimen. Pulse therapy with itraconazole therefore appears to modify the structure of the nail plate, probably as a result of, or in association with, an increased rate of growth.

A case-control study of oral cancer and pre-diagnostic concentrations of selenium and zinc in nail tissue

A case-control study of oral cancer was conducted in western Washington state between 1983 and 1987. Cases (n = 379) were identified through a population-based registry, and controls (n = 514) were selected by telephone using random digit dialing. Subjects participated in a personal interview, completed a food-frequency questionnaire, and submitted clippings from the nails of each great toe for the determination of selenium and zinc concentrations. The odds ratio (OR) for low selenium levels in nail tissue (lowest 25% of the distribution compared to the upper 75%) was 1.4 (95% confidence interval (CI) 1.0-2.2). Likewise, the odds ratio for low zinc levels in nails was 1.6 (95% CI 1.0-2.3), but for low dietary zinc was 1.0 (95% CI 0.7-1.7). Men with oral cancer had lower nail selenium levels than did the controls (OR = 1.9), but women with oral cancer did not (OR = 0.6). Individuals 20 to 39 years of age with oral cancer, in particular, were more likely to have lower selenium levels in nail tissue than controls (OR = 16.4). There was a significant interaction between selenium and ascorbic acid levels which could not be explained by cigarette use. Subjects at greatest risk had low levels of both nutrients (OR = 3.8 for smokers and OR = 5.7 for non-smokers). However, since the elements were deposited in the nail matrix close to the date of diagnosis, the differences in the element concentrations between cases and controls may have been a result of the disease. Further etiologic studies of selenium, vitamin intake and oral carcinoma are warranted.

Onychoschizia: scanning electron microscopy

The distal ends of the fingernails of four women with onychoschizia were studied with scanning electron microscopy. The dorsal surface and tip of each nail showed horizontal lamellar separations representing single cell layers. Some cleavage lines extended proximally into the nail plate, revealing remarkable sculptured cell surfaces deep within the plate. These observations indicate that the lamellar splitting of onychoschizia occurs between cell layers. This presumably results from repeated trauma to a nail plate with diminished adherence between cell layers, secondary to the dissolution of intercellular cement by detergents and nail polish solvents.