Ultrastructure of the human toenail

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.

Brittle nail syndrome: a pathogenesis-based approach with a proposed grading system

Brittle nail syndrome is a heterogeneous abnormality, characterized by increased fragility of the nail plate. Brittle nails affect about 20% of the population and women are affected twice as frequently as men. The vast majority of patients experience brittle nails as a significant cosmetic problem and a substantial number indicate that these nail abnormalities are painful, impair daily activities, and may have a negative impact on occupational abilities. Pathogenic factors leading to brittle nails are factors that impair intercellular adhesion of the corneocytes of the nail plate or factors that cause a pathologic nail formation by involving the matrix. Clinical features of brittle nail syndrome are onychoschizia and onychorrhexis: the impairment of intercellular adhesive factors of the nail plate is expressed as onychoschizia, whereas the involvement of the nail matrix is expressed as onychorrhexis. Although impairment of life quality has not been evaluated for patients with brittle nail syndrome, the reduction of life quality in other nail problems has been studied and is evident. A proposed scoring system of key features of brittle nails is presented, and therapeutic approaches focussed on the pathogenic factors are discussed.

Membrane-coating granules and the intercellular cementing substance (membrane-coating material) in the epidermis in different regions of the equine hoof

This study aimed to carry out a comprehensive morphological examination of the structure and ultrastructure of the membrane-coating granules (MCG) and membrane-coating material (MCM) common to all hoof segments as well as segment-specific features in the MCG and MCM regarding their synthesis, structure and biochemical composition. Tissue samples from the hooves of the left front and right hind limb of 20 adult riding horses were used. Samples were preserved and embedded for histological routine methods as well as for histochemical techniques and tracer techniques for light and electron microscopy. The MCM shows a segment-specific structure, biochemical composition and function, which were demonstrated for the different parts of the hoof. The MCM and its properties are related to the segment-specific horn quality. Besides intercellular structural factors the MCM is of major importance for the structural integrity and the biomechanical properties of the hoof horn. The physiological structure and composition of MCM provides explanation for the existence of clinically well-known sites of predilection for hoof disorders such as the White Line Disease. Alterations of the MCM play a role in dysfunction and diseases of the hoof epidermis; they are particularly involved in the pathogenesis of infectious diseases of the hoof.

Spiny keratoderma--a demonstration of hair keratin and hair type keratinization

Six cases of spiny keratoderma were analyzed with hair specific antikeratin antibodies (AE13, AE14) and by electron microscopy. The keratotic column exhibited a different keratin birefringence and the underlying viable epidermis was less eosinophilic than the surrounding epidermis. AE13, which is specific for hair cortex, was positive in the lower column and variably positive in the viable epidermis, often beyond the columnar lesion. AE14 was negative in the lesion. Electron microscopy demonstrated features of keratinization of normal hair cortex, i.e. by the accretion of keratin filaments without production of keratohyalin or trichohyalin granules. Cementsomes (lamellar granules) and marginal bands were not produced as they are not formed in normal cortical keratinization. It was suggested that spiny keratoderma represents an ectopic hair formation of palms and soles.

CHILD syndrome: analysis of abnormal keratinization and ultrastructure

A new patient with CHILD syndrome (congenital hemidysplasia, ichthyosiform erythroderma, and limb defects), the thirtieth in the literature, was observed for over three years. Initially, the right-sided lesion spared the breast area. At 10 months of age the trunk lesion extended to cover the entire area of the right chest. At age 20 months the patient developed linear, bandlike, keratotic, brown-black lesions on her left thigh that subsided within six weeks, leaving a slight hyperpigmentation. This patient was studied by routine histologic methods as well as with markers of keratinization and electron microscopy. In hematoxylin and eosinstained sections, parakeratosis and orthokeratosis alternated. In some parakeratotic areas, large granular cells, and in others, ghost granular cells, were present. The latter showed basophilic cytoplasm, and palestaining or vacuolated nucleus and were seen either above the normal granular layer or without it. Although regional variations existed, basal cell-type keratins as recognized by AE1 continued to be expressed in suprabasal layers. Filaggrin- and involucrin-positive layers were expanded, particularly the latter, down to the lower prickle cell layer. Ultrastructurally, numerous lamellar or membranous structures were found in upper layers of the epidermis, both intracellulary and intercellularly. Normal cementsomes coexisted with these abnormal lamellar structures, and it was thought that the latter represent modified cementsomes because the discharge of those from the cell periphery was often detected.

Fine structure of the dorsal lingual epithelium of the crab-eating monkey, Macaca irus

Light and electron microscopic observations of the dorsal lingual epithelium of the crab-eating monkey, Macaca irus, revealed three different regions: the epithelium on the anterior side of the filiform papillae, the epithelium on the posterior side of the filiform papillae, and the interpapillar epithelium. Whereas the basal and suprabasal cells are similar throughout, differences characterize the intermediate and surface layers. Keratohyalin granules appear predominantly in the intermediate layer of the epithelium on the anterior side of filiform papillae. In the epithelium on the posterior side of the filiform papillae, no keratohyalin granules are seen and, instead, tonofibrils are prominent. The cells begin to be significantly flattened. In the interpapillar epithelium, no keratohyalin granules and tonofibrils are seen, and the tonofilaments occupy almost the entire cytoplasm of the cells of the intermediate and surface layers, with the cells having larger volumes in these layers.

Structure and biochemistry of mammalian hard keratin

In this review, the structure and biological formation of hard alpha-keratin are drawn together. The hard keratins comprising wool, hairs, quills, hooves, horns, nails and baleen contain partly alpha-helical polypeptides which show homology with epidermal polypeptides only in the helical regions. These polypeptides (about 32 chains) are organized into intermediate filaments (IFs) of 7.5 nm diameter which are embedded in variable amounts of a matrix of non-helical cystine-rich proteins and glycine-tyrosine-rich proteins. The total number of proteins may exceed 100. In addition keratins contain a variety of lipid components. Wool and hair are produced in follicles in a multistep procedure. In the lower levels of the follicle, IFs without associated matrix are found. Subsequently matrix proteins are laid down between the IFs and further synthesis takes place concurrently. Finally the proteins are insolubilized by the oxidative formation of disulphide bonds. Keratinized fibres shows considerable complexity and diversity in the structural arrangement of IFs and matrix within cortical cells. Typically the IFs show hexagonal packing or give a whorl-like appearance in cross-section.

Pathogenesis of onychoschizia (lamellar dystrophy)

Onychoschizia or lamellar dystrophy of the nails is common, especially in adult women, but little information is available about its cause. Most theories involve environmental factors, but supportive experimental data are scarce. Therefore we studied the in vitro nail changes produced by several organic solvents, detergents, water, other polar materials, and both acidic and basic solutions. Challenged and control fingernail clippings were examined grossly, microscopically, and by scanning electron microscopy at regular intervals. There was a progressive increase in severity with prolonged wetting and drying. By 3 weeks, scanning electron microscopy demonstrated unattached individual cells in empty spaces in which separation was more prominent. Basic solutions caused some softening, but layering (peeling) was seen only after repeated hydration and dehydration. Although other factors may influence onychoschizia, the typical changes can be produced in normal nails after a 21-day challenge of repeated exposure to water followed by dehydration. These findings suggest a probable cause for the condition and a logical approach to management.

Effect of enzyme digestion on the permeability barrier in keratinizing and non-keratinizing epithelia

In an attempt to characterize the permeability barrier of the oral mucosa and skin, small pieces of keratinized and non-keratinized oral epithelia and epidermis were digested with specific enzymes. These enzymes were selected for their effect on carbohydrate-protein, or carbohydrate-lipid compounds and phospholipids. The effect of the enzyme treatment was monitored by exposing the digested tissue to horseradish peroxidase. Electron microscopic examination of tissue treated with phospholipases revealed considerable damage to membrane structures but not to the integrity of the permeability barrier. Hyaluronidase and neuraminidase caused less structural damage but did not impair barrier function; this was only seen after treatment with chondroitinase ABC. This enzyme may degrade certain of the polar molecules thought to be necessary to stabilize the neutral lipid bilayers of the intercellular barrier and thus disrupt its barrier properties.

A qualitative ultrastructural study of the intercellular spaces between epithelial cells treated in vivo with DMBA

One of the features of epithelial dysplasia at the histological level is known as "loss of cellular adherence" in which adjacent epithelial cells appear more widely separated from each other than in normal tissues. In this study we examine the effects of the carcinogen DMBA on the epithelium of the hamster cheek-pouch with particular emphasis on the dimensions of the intercellular spaces. DMBA-induced lesions were processed for electron microscopy and assigned to hyperplasia, dysplasia and carcinoma groups, using defined criteria on toluidine blue-stained 1 micron Araldite sections. Untreated pouches were used as a control. At the light-microscopical level, intercellular spaces in hyperplastic epithelium appeared similar to those present in untreated tissue but increased progressively in dysplastic and carcinomatous lesions. Spaces were generally wider between basal and spinous cells than between granular cells, although wide variations were observed between tissue blocks demonstrating similar histological features and also within adjacent areas of the same block. At the ultrastructural level, untreated and hyperplastic tissue showed only occasional focal separations of adjacent plasma membranes; these spaces were more frequent between cells of lower strata. In sections from dysplasias and carcinomas, spaces were always extensive and were occupied by numerous villous or foliate membrane-bound cytoplasmic extensions. These were often attached to each other by desmosomes of apparently normal morphology but of a lower frequency than in untreated epithelium. The increased epithelial separation as indicated by the increased intercellular spaces during chemical carcinogenesis may be a result of any or all of the following factors: desmosomal disruption or their failure to develop; the production of cell-surface molecules which are less adhesive; inflammatory oedema and direct alterations on intercellular junctions and cell-surface components by infiltrating inflammatory cells.

Alterations in the volume of the intercellular space between epithelial cells of the hamster cheek-pouch: quantitative studies of normal and carcinogen-treated tissues

The present report investigated the extent of the epithelial dysplastic feature known as "loss of cellular adherence" at the ultrastructural level by quantifying the volume of the intercellular space during hamster cheek-pouch carcinogenesis. Following topical application of DMBA to cheek-pouches, lesions were classified as hyperplasia, dysplasia and carcinoma, with untreated pouches serving as a control. Stereological point counting procedures were used to determine the volume density of intercellular space in defined basal, spinous and granular layers for each group. In general, progressive increases in volume density were detected within each stratum during carcinogenesis. These results indicate that increasing separation of epithelial cells occurs during carcinogenesis, although it is not yet known whether this results from loss of cohesion between specialised (i.e., desmosomal) or nonspecialised membrane areas. In addition, a simple indicator of pathological alteration, the Pathological Alteration Ratio (PAR), is described and was used to evaluate existing published data for intercellular spaces in various oral mucosal conditions. Values of the PAR were found to be substantially higher in carcinogen-treated epithelia than in reports describing changes in wound healing, lichen planus and leukoplakia simplex. These objective techniques are of value for investigating the pathogenesis of diseased epithelium and may find applications in the diagnosis of oral premalignant lesions.

Recent advances in oral mucosal research

Major progress in investigation of the normal structure and function of the oral mucosa has been made within the last ten years and has come principally from the application of various techniques developed in basic science disciplines to specific mucosal problems. However, it is apparent that many gaps still exist in our knowledge of the oral mucosa and, although it is to be expected that different workers will have distinct views on which of these are the most significant, some basic areas for further investigation can clearly be identified. For example, little is known about epithelial control systems and their disturbance by epithelial disease processes, about the nature of the interactions occurring during development and maintenance of the oral mucosal epithelia, or about the epithelial cell surface and its role in normal function. The specific properties and behavior of the cell populations of the subepithelial connective tissues appears to be poorly understood and the existence and significance of functional changes in mucosa with age and malnutrition are uncertain. It is increasingly apparent that successful progress in such investigations involves approaches using diverse methodologies. For example, epithelial-mesenchymal interactions are likely to involve multiple mesenchymal factors acting in concert to establish and maintain epithelial form and, because of this complexity, the nature of the inductive influences is not likely to be elucidated in model systems unless individual variables can be rigidly controlled. Defining the cellular and acellular elements in mesenchyme and reconstructing a functional mesenchyme from purified components may not be a simple task, but with current methods for cultivating mucosal keratinocytes and fibroblasts, as well as for purifying various components of the ground substance, it should be possible to initiate such a program of study. Some of the most dramatic advances made over the past 5-6 years in epidermal research have come about through the utilization of newly developed biochemical investigative techniques, examples of which include the use of gene cloning to study the organization of the keratin gene family, and the use of immuno-fluorescence with monoclonal antibodies to discern when various keratin proteins appear during differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Physicochemical characterization of the human nail: permeation pattern for water and the homologous alcohols and differences with respect to the stratum corneum

In order to develop a basic concept of the permeability of the human nail plate and thus create a better understanding of the toxic potentials and therapeutic possibilities of substances applied to the nail, avulsed cadaver nails have been placed in specially constructed diffusion chambers and their permeation by water and the n-alkanols through dodecanol, all in high aqueous dilution, has been investigated. The permeability coefficient of water is 16.5 X 10(-3) cm h-1 and that for methanol is 5.6 X 10(-3) cm h-1. Ethanol's permeability coefficient measured 5.8 X 10(-3) cm h-1. Permeability coefficients decreased systematically thereafter to a low value of 0.27 X 10(-3) cm h-1 at n-octanol. The middle chain length alkanols, n-pentanol through n-octanol, have similar permeability coefficients but n-decanol and n-dodecanol show higher rates of permeation. The data suggest that, as a membrane, the hydrated human nail plate behaves like a hydrogel of high ionic strength to the polar and semipolar alcohols. Declining permeability rates appear linked to decreased partitioning into the complex matrix of the plate as the compounds become hydrophobic. The results for n-decanol and n-dodecanol introduce the possibility that a parallel lipid pathway exists which favours the permeation of these exceedingly hydrophobic species.

On the so-called membrane coating granules in keratinized lichen planus lesions of the buccal mucosa

Serial sections of the so-called membrane-coating granules have been examined in keratinized oral epithelium of lichen planus lesions. As with 'granules' apparent in non-keratinized epithelium, it is found they do not represent specialized intra-cytoplasmic organelles, but are the result of sectioning at different areas, levels and planes through the plasma membrane of interdigitating cell processes. Such 'granules' appear mostly in the superficial, but not deep, part of the cytoplasm of the upper prickle cells. This is considered to be due to topographic differences between the upper and under surfaces of these cells and the presence of narrower intercellular spaces than those between deeper epithelial cells. Such arrangement often results in cell processes in sections appearing free in the superficial part of the cell below. The appearance of 'granules' arises when the plane of section is not at right angles to the two plasma membranes surrounding these processes.

Ultrastructural study of hydrocortisone action in cultured human epidermis

Adult human skin was maintained in organ culture in a chemically defined medium without serum for up to 10 days. Under these conditions cultured epidermis revealed ultrastructural changes suggesting alterations in keratinization. Half of the newly formed stratum corneum became parakeratotic. The number and size of keratohyalin granules were reduced, and only few keratinosomes were present after the 5th day of culture. The effect of hydrocortisone at final concentrations of 10(-5)-10(-9) mol/l was studied by applying stereological techniques for electron microscopic morphometry on systematically sampled tissues. The volume densities of tissue components and cell organelles were assessed at three different strata of epidermis on the 5th day of culture. Changes of keratinization due to the culture conditions were partly reversed by the hormone, i.e. hydrocortisone increased the volume densities of keratohyalin granules, keratinosomes and tonofilaments. Minute effects due to hormonal action were seen on tissue components or the volume densities of other cellular organelles.

A freeze-fracture study of the human nail plate

This study of freeze-fracture replicas of human nail plate fragments demonstrates that keratin filaments have a width of about 100 A. The 'marginal band' is probably formed by a row of particles, 50 A in diameter, arranged along the inner surface of the plasma membrane. These particles are often masked by keratin filaments. In nonjunctional areas, the plasma membranes are devoid of membrane-associated particles. The only type of junction observed is the desmosome. Even though the modes of keratinization are different, the structures of the keratin pattern, of cell membranes, and of the desmosomes are identical in replicas of wholly keratinized nail and human stratum corneum cells. The ultrastructural differences observed appear to be related to the barrier function carried out by the epidermis and by its desquamation process.

Morphology of corneocytes from human nail plates

A technique is described which permits the isolation of individual corneocytes from the superficial layers of the human nail plates. Tesa-film D is used to strip off the cells. The tape is mounted on a glass slide, stained with a mixture of methylene blue and rhodamine B. The parameters were size (surface mu2), shape (regular, irregular), nuclear inclusions and trabeculae. Specimens were obtained from 3 groups of patients (finger- and toe nails): (1) 60 healthy subjects with normal nails, males and females, in 3 age-groups (babies, adults, aged); (2) 10 patients with fast growing nails with psoriasis vulgaris and psoriatic nail involvement; (3) 9 patients with slow growing nails with lichen planus with nail involvement including one patient with Zinser-Engman-Cole-syndrome (dyskeratosis congenita). The nail growth rate was determined with a dissecting microscope-technique. Corneocytes of the dorsal nail plates of normal nails are of irregular polyedrical shape, not nucleated and show distinct but irregular trabecular network. Within each age-group, corneocytes are of rather uniform size but increase significantly (p less than or equal to .001) with age (e.g., thumbin males: 597 vs. 920 vs. 1008 mu2). Accelerated nail plate growth results in smaller corneocytes, and slowed down nail plate growth in larger corneocytes. It is concluded that cell proliferation (and abnormal keratinization) has a measurable effect on the size of corneocytes from the nail plates.

A quantitative study of membrane coating granules in follicles undergoing experimental comedo formation

Membrane coating granules (MCGs) may be required for normal keratinization. Comedone formation is associated with abnormal keratinization, and so the MCGs in sebacaous follicles were counted during experimental comedogenesis in the rabbit. During induced comedo formation with 50% oleic acid in paraffin oil, the numbers of MCGs decreased significantly; first, at the epidermis and mouth of the follicle and then later, at the follicular canal and common sebaceous duct. The maximum decrease of about 40% was reached by the 6th day in these regions. Throughout the treatment, the MCG count in the individual sebaceous ducts remained fairly constant.

Keratinization

Early studies have already shown that the tonofibrils of malpighian cells consist of a --SH containing fibrous alpha-protein. It was assumed that the highly resistant protective substance, keratin, was formed by the conversion of --SH groups into --S--S--bonds in this protein. This chemical reaction was regarded as the most significant event of the keratinization process. Recent studies show that keratinization proceeds by a synthetic and a degradative stage and that ultimately a complex protective substance is formed. Horny cells become filled with --SH-containing filaments embedded in a --S--S---rich amorphous matrix. This complex is encased by a thickened membrane rendered insoluble by --S--S bonds and an unknown, highly resistant bond. In the stratum corneum, the intercellular space is occupied by bipolar lipids originating from the discharged lamellae of membrane-coating granules.

On the structure of the normal nail. A scanning electron microscope study

A scanning electron microscopic (SEM) study of cut surfaces in normal human nails have confirmed the previous description of nail structure, i. e. the hard dorsal nail plate supported by the plastic intermediate nail plate.