Patterns of expression of trichocytic and epithelial cytokeratins in mammalian tissues. II. Concomitant and mutually exclusive synthesis of trichocytic and epithelial cytokeratins in diverse human and bovine tissues (hair follicle, nail bed and matrix, lingual papilla, thymic reticulum)

Comparative Analysis of Type I Keratin Expression By Nail Consistency: An Immunohistochemistry Study

The nail plate is one of the essential structures of the nail apparatus and is highly keratinized, making it difficult to handle this tissue experimentally. Different types of nail consistency were identified by applying distal pressure to the nail plate. To analyze the relationship between the keratins expressed in the nail plate and nail consistency, we chose a sample of 32 adult individuals (age 49.81±3.21 y) with the same number of each sex, who had a similar percentage of nail consistency types (56.25% hard consistency nails and 43.75% soft consistency nails). Immunohistochemical analyses showed that hard consistency nails contain more keratin 17 than soft consistency nails (P=0.026). These novel results allow nail consistency to be defined by the differential expression of keratins in the nail plate, and have potential clinical implications for the diagnosis of possible nail disorders and/or systemic disease.

Vertebrate keratinization evolved into cornification mainly due to transglutaminase and sulfhydryl oxidase activities on epidermal proteins: An immunohistochemical survey

The epidermis of vertebrates forms an extended organ to protect and exchange gas, water, and organic molecules with aquatic and terrestrial environments. Herein, the processes of keratinization and cornification in aquatic and terrestrial vertebrates were compared using immunohistochemistry. Keratins with low cysteine and glycine contents form the main bulk of proteins in the anamniote epidermis, which undergoes keratinization. In contrast, specialized keratins rich in cysteine-glycine and keratin associated corneous proteins rich in cysteine, glycine, and tyrosine form the bulk of proteins of amniote soft cornification in the epidermis and hard cornification in scales, claws, beak, feathers, hairs, and horns. Transglutaminase (TGase) and sulfhydryl oxidase (SOXase) are the main enzymes involved in cornification. Their evolution was fundamental for the terrestrial adaptation of vertebrates. Immunohistochemistry results revealed that TGase and SOXase were low to absent in fish and amphibian epidermis, while they increased in the epidermis of amniotes with the evolution of the stratum corneum and skin appendages. TGase aids the formation of isopeptide bonds, while SOXase forms disulfide bonds that generate numerous cross-links between keratins and associated corneous proteins, likely increasing the mechanical resistance and durability of the amniote epidermis and its appendages. TGase is low to absent in the beta-corneous layers of sauropsids but is detected in the softer but pliable alpha-layers of sauropsids, mammalian epidermis, medulla, and inner root sheath of hairs. SOXase is present in hard and soft corneous appendages of reptiles, birds, and mammals, and determines cross-linking among corneous proteins of scales, claws, beaks, hairs, and feathers.

Keratin 36, a specific marker of tongue filiform papillae, is downregulated in squamous cell carcinoma of the mobile tongue

Human keratin 36 (K36) is a member of the hair keratin family and is a marker of hair cortex differentiation. The human KRT36 gene is located on the long arm of chromosome 17 and belongs to the cluster of structurally unrelated acidic hair keratins. Recently, it has been reported that KRT36 mRNA is specifically expressed in normal tongue epithelium and downregulated in squamous cell carcinomas of the mobile tongue. Furthermore, KRT36 levels have been reported to be downregulated in clinically normal mobile tongue tissue that is adjacent to tumours, suggesting it could be a marker of pre-neoplastic changes. However, the exact role and the potential role of K36 in tongue tumour formation remains unclear. The aim of the present study was to investigate expression of K36 in a series of squamous cell carcinomas of the mobile tongue, normal mobile tongue and a small panel of other human tissues (normal tissue from the appendix, cervix, hair, lip, mamilla, nail, oesophagus, skin, thymus and vagina) and selected cancer tissue (cervical cancer, melanoma and basal cell carcinoma). Affinity purified polyclonal antibodies against K36 were generated and used for immunohistochemical analysis. The results revealed that in the normal tongue, K36 was detected specifically in the filiform papillae of the dorsal surface of the tongue. Additionally, none of the tongue cancer tissue samples were K36-positive. Immunostaining also revealed that K36 was expressed in nail beds, Hassal's corpuscles in the thymus and the hair cortex. However, K36 was not expressed in the squamous epithelia of the skin, cervix and oesophagus, and the squamous cells of cervical carcinomas, basal cell carcinoma or melanoma. The present data indicated that K36 may be inactivated in tumours of the tongue. However, whether this is part of the tumoural process or if it is an effect of the tumour itself remains to be elucidated.

Cytokeratins in the Histological Diagnosis of Malignant Tumors

Cytokeratins, which comprise a multigene family of 20 related polypeptides (CKs 1–20), are constituents of the intermediate filaments of epithelial cells, in which they are expressed in various combinations depending on the epithelial type and the degree of differentiation. Of these, CK 19 (400 amino acids; 44.1 kilodaltons) is an example of a widely distributed CK, being expressed in various epithelia, including many simple epithelia. In contrast, the recently identified CK 20 (424 amino acids; 48.6 kilodaltons) is essentially confined to gastrointestinal epithelia, the urothelium and Merkel cells. The differential expression of individual CKs in various types of carcinomas makes them useful markers for histopathological carcinoma subtyping, providing relevant information concerning the differentiation and origin of carcinomas, especially when tumors first present as metastases. The CKs that are of particular value for differential diagnosis include CK 20, as it is mainly expressed in carcinomas derived from CK 20-positive epithelia; it is also found in bile-tract, pancreatic and mucinous ovarian adenocarcinomas, being absent in most other carcinomas. In certain carcinoma types, the changes in the expression of individual CKs that may occur during tumor progression could be of prognostic relevance. It remains to be established whether the serological detection of fragments of not only widely distributed but also more restrictedly expressed CKs may provide useful serological tumor markers in the future.

In vitro Cytokeratin Expression Profiling of Human Oral Mucosa Substitutes Developed by Tissue Engineering

In this work we performed a study of cytokeratin (CK) expression profiling on human artificial oral mucosa developed in vitro by tissue engineering at different stages of maturation (from immature to well-developed stages) at the protein and mRNA levels. Human artificial oral mucosa was generated in the laboratory using fibrin-agarose biomaterials. As controls, we used human native normal oral mucosa and embryonic oral tissues. Our results demonstrated that human embryonic oral tissues tended to express CK8 and CK19. In contrast, monolayered bioengineered oral mucosa did not show any CK expression by immunohistochemistry whereas bilayered and multilayered artificial oral mucosa showed several markers of stratified epithelia, but did not express CK10. These results suggest that the CK expression pattern is strongly dependent on the maturation state of the artificial tissues and that the CK expression profile of our model of artificial oral mucosa was partially similar to that of the non-keratinized human adult oral mucosa. However, the expression of CK8 by the artificial oral mucosa suggests that these samples correspond to an early stage of development while kept in vitro.

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.

Glycation of nail proteins: from basic biochemical findings to a representative marker for diabetic glycation-associated target organ damage

Background

Although assessment of glycated nail proteins may be a useful marker for monitoring of diabetes, their nature and formation are still poorly understood. Besides a detailed anatomical analysis of keratin glycation, the usefulness of glycated nail protein assessment for monitoring diabetic complications was investigated.

Methods

216 patients (94 males, 122 females; mean age ± standard deviation: 75.0 ± 8.7 years) were enrolled. Glycation of nail and eye lens proteins was assessed using a photometric nitroblue tetrazolium-based assay. Following chromatographic separation of extracted nail proteins, binding and nonbinding fractions were analyzed using one-dimensional gel electrophoresis. Using a hand piece containing a latch-type-bur, a meticulous cutting of the nail plate into superficial and deep layers was performed, followed by a differential analysis of fructosamine.

Results

Using SDS PAGE, four and two bands were identified among the nonglycated and glycated nail fraction respectively. Significantly lower fructosamine concentrations were found in the superficial nail layer (mean: 2.16 ± 1.37 μmol/g nails) in comparison with the deep layer (mean: 4.36 ± 2.55 μmol/g nails) (P<0.05). A significant higher amount of glycated eye lens proteins was found in diabetes mellitus patients (mean: 3.80 ± 1.57 μmol/g eye lens) in comparison with nondiabetics (mean: 3.35 ± 1.34 μmol/g eye lens) (P<0.05). A marked correlation was found between glycated nail and glycated eye lens proteins [y (glycated nail proteins) = 0.39 + 0.99 x (eye lens glycated proteins); r² = 0.58, P<0.001]. The concentration of glycated eye lens proteins and the HbA1c level were found to be predictors of the concentration of glycated nail proteins.

Conclusions

Glycation of nail proteins takes place in the deep layer of finger nails, which is in close contact with blood vessels and interstitial fluid. Glycation of nail proteins can be regarded as a representative marker for diabetic glycation-associated target organ damage.

Effects of hyperbaric oxygen therapy on rat facial skin

INTRODUCTION

We used immunohistochemistry to investigate the histopathologic effects of hyperbaric oxygen (HBO) on the facial skin of rats.

MATERIAL AND METHODS

Rats in the HBO group (n = 6) were placed in a 20 L HBO chamber at 2.5 atmospheres absolute at 25-26°C with 100% oxygen for 90 min/day for 7 days. Following euthanasia, sections of facial skin were removed for examination.

RESULTS

Epidermal hyperplasia and degeneration, basal-cell hypertrophy, subepithelial fibrosis, and increased connective tissue were observed in the HBO group. E-cadherin expression was reduced in the epidermis, hair follicles, and sebaceous glands in HBO-treated rats relative to control animals. HBO treatment was associated with vimentin immunoreactivity in fibroblasts, endothelial cells, and the bulbus pilorum of a subset of hair follicles. It also resulted in increased type IV collagen expression within the connective tissue in the hair follicles and sebaceous glands.

CONCLUSION

The HBO group demonstrated epidermal hyperplasia and degeneration, basal-cell hypertrophy, and subepithelial fibrosis. In addition, HBO decreased E-cadherin expression, which suggests that HBO may impair intracellular adhesion. Expression of vimentin and type IV collagen was also observed in the dermis. Increased connective tissue, hemorrhage, and mononuclear cell infiltration were observed in the dermis of HBO-treated animals. Thus, HBO has effects on the structures of the epidermis and dermis.

Bison and bovine rectoanal junctions exhibit similar cellular architecture and Escherichia coli O157 adherence patterns

BACKGROUND

Escherichia coli O157 (E. coli O157) has been isolated from bison retail meat, a fact that is important given that bison meat has been implicated in an E. coli O157-multistate outbreak. In addition, E. coli O157 has also been isolated from bison feces at slaughter and on farms. Cattle are well documented as E. coli O157 reservoirs, and the primary site of E. coli O157 persistence in such reservoirs is the rectoanal junction (RAJ), located at the distal end of the bovine gastrointestinal tract. Since bison and cattle share many genetic similarities manifested as common lineage, susceptibility to infection and the nature of immune responses to infectious agents, we decided to evaluate whether the RAJ of these animals were comparable both in terms of cellular architecture and as sites for adherence of E. coli O157. Specifically, we compared the histo-morphologies of the RAJ and evaluated the E. coli O157 adherence characteristics to the RAJ squamous epithelial (RSE) cells, from these two species.

RESULTS

We found that the RAJ of both bison and cattle demonstrated similar distribution of epithelial cell markers villin, vimentin, cytokeratin, E-cadherin and N-cadherin. Interestingly, N-cadherin predominated in the stratified squamous epithelium reflecting its proliferative nature. E. coli O157 strains 86-24 SmR and EDL 933 adhered to RSE cells from both animals with similar diffuse and aggregative patterns, respectively.

CONCLUSION

Our observations further support the fact that bison are likely 'wildlife' reservoirs for E. coli O157, harboring these bacteria in their gastrointestinal tract. Our results also extend the utility of the RSE-cell assay, previously developed to elucidate E. coli O157-cattle RAJ interactions, to studies in bison, which are warranted to determine whether these observations in vitro correlate with those occurring in vivo at the RAJ within the bison gastrointestinal tract.

Nail stem cells

Our knowledge on stem cells of the hair follicle has increased exponentially after the bulge was characterized as the stem cell niche two decades ago. In contrast, little is known about stem cells in the nail unit. Whereas hair follicles are plentiful and easy to access, the human body has only twenty nails and they are rarely biopsied. Therefore, examining fetal material offers unique advantages. In the following mini-review, our current knowledge on nail stem cells is summarized and analogies to the hair follicle stem cells are drawn.

Expression of caspase-14 and keratin-19 in the human epidermis and appendages during fetal skin development

Caspase-14 is a seemingly non-apoptotic caspase involved in keratinocyte differentiation and cornification of the skin. Keratin-19 is an epithelial marker and a potential marker of epidermal stem cells that is expressed during human fetal skin development. We examined the immunohistochemical expression of caspase-14 in relation to CK-19 in the human fetal skin during development and perinatally, to assess their role in human skin maturation. Skin samples were received at autopsy. In the fetal epidermis, caspase-14 was predominantly expressed in the more differentiated layers, gradually disappearing from the basal layer toward term. By contrast, keratin-19 expression gradually decreased with epidermal maturation through gestation (rho = -0.949; p = 0.0001) and was a marker of the germinative layers. Keratin-19 was preserved in scarce basal cell nests at term and postnatally. Caspase-14 and keratin-19 were inversely expressed in the differentiating epidermal layers through gestation (p < 0.0001). Concerning the appendages, in hair follicles and sebaceous glands, caspase-14 located preferentially in the more differentiated layers of the inner root sheath, whereas keratin-19 was expressed in the outer sheath. Eccrine sweat glands showed a variable pattern of caspase-14 and keratin-19 expression. In conclusion, caspase-14 emerged as a marker of human skin differentiation during development, while keratin-19 marked the germinative epithelial layers in the fetal epidermis and appendages and possibly the nests of epidermal stem cells.

The ventral proximal nail fold: stem cell niche of the nail and equivalent to the follicular bulge--a study on developing human skin

BACKGROUND

Although the bulge is well characterized as a stem cell niche of the hair follicle, comparatively little is known about the location of stem cells in the nail. Herein, we describe the spatiotemporal expression pattern of six stem cell markers in the developing human nail and compared it with the embryonic and fetal human hair follicle. The areas of proliferative activity were additionally examined using labeling with Ki-67.

METHODS

We examined immunohistochemically samples from embryonic and fetal human nail, hair and skin for the expression of cytokeratin 15 (CK15, two clones), cytokeratin 19 (CK19), PHLDA1, CD200, nestin and Ki-67 using standard techniques.

RESULTS

CK15 (clone LHK15), CK19 and PHLDA1 are negative in the nail and hair matrix but positive in the ventral proximal nail fold and in the follicular bulge. Over the course of embryogenesis they display a highly specific spatiotemporal expression pattern both in the nail and in the hair follicle.

CONCLUSIONS

We propose that at least during embryogenesis the proximal ventral nail fold represents the niche for the nail stem cells. In contrast to animal experiments, autoradiographic pulse-chasing studies cannot be performed in human, and immunohistochemical studies are a valid alternative although they have their limitations. Further studies on adult human nail units are suggested.

Msx2 and Foxn1 regulate nail homeostasis

Epithelial-mesenchymal interactions underlie the foundation for ectodermal appendage formation. Signal molecules such as BMPs and WNTs mediate crosstalk between the two tissue layers and coordinate both the induction and morphogenesis of ectodermal appendages. Here, we analyzed the function of two BMP downstream transcription factors, Msx2 and Foxn1, in nail differentiation. First, we show that Msx2 function is required during onychocyte (nail cell) terminal differentiation. Second, the Msx2/Foxn1/hair keratin pathway controlling hair differentiation is also conserved during onychocyte differentiation. Finally, the Msx2-/-; Foxn1-/- double-mutant nails exhibit a more severe phenotype than either single mutant including nail bed hyperplasia. Together, our data implicate important functions for Msx2 and Foxn1 in regulating differentiation of the keratogenous zone, proliferation of distal nail matrix cells, and organization of the nail bed.

Characterization of the expression of cytokeratins 5, 8, and 14 in mouse thymic epithelial cells during thymus regeneration following acute thymic involution

The thymus is a central lymphoid organ for T cell development. Thymic epithelial cells (TECs) constitute a major component of the thymic stroma, which provides a specialized microenvironment for survival, proliferation, and differentiation of immature T cells. In this study, subsets of TECs were examined immunohistochemically to investigate their cytokeratin (CK) expression patterns during thymus regeneration following thymic involution induced by cyclophosphamide treatment. The results demonstrated that both normal and regenerating mouse thymuses showed a similar CK expression pattern. The major medullary TECs (mTEC) subset, which is stellate in appearance, exhibited CK5 and CK14 staining, and the minor mTEC subset, which is globular in appearance, exhibited CK8 staining, whereas the vast majority of cortical TECs (cTECs) expressed CK8 during thymus regeneration. Remarkably, the levels of CK5 and CK14 expression were enhanced in mTECs, and CK8 expression was upregulated in cTECs during mouse thymus regeneration after cyclophosphamide-induced acute thymic involution. Of special interest, a relatively high number of CK5⁺CK8⁺ TEC progenitors occurred in the thymic cortex during thymus regeneration. Taken together, these findings shed more light on the role of CK5, CK8, and CK14 in the physiology of TECs during mouse thymus regeneration, and on the characterization of TEC progenitors for restoration of the epithelial network and for concomitant regeneration of the adult thymus.

Proteomic analysis of human nail plate

Shotgun proteomic analysis of the human nail plate identified 144 proteins in samples from Causcasian volunteers. The 30 identified proteins solubilized by detergent and reducing agent, 90% of the total nail plate mass, were primarily keratins and keratin associated proteins. Keratins comprised a majority of the detergent-insoluble fraction as well, but numerous cytoplasmic, membrane, and junctional proteins and histones were also identified, indicating broad use by transglutaminases of available proteins as substrates for cross-linking. Two novel membrane proteins were identified, also found in the hair shaft, for which mRNAs were detected only at very low levels by real-time polymerase chain reaction in other tissues. Parallel analyses of nail samples from volunteers from Inner Mongolia, China gave essentially the same protein profiles. Comparison of the profiles of nail plate and hair shaft from the latter volunteers revealed extensive overlap of protein constituents. Analyses of samples from an arsenic-exposed population revealed few proteins whose levels were altered substantially but raised the possibility of detecting sensitive individuals in this way.

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.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Substrate adaptation of Trichophyton rubrum secreted endoproteases

Trichophyton rubrum is the most common pathogen caused the dermatophytosis of nail and skin in human. The secreted proteases were considered to be the most important virulence factors. However, the substrates adaptation of T. rubrum secreted proteases is largely unknown. For the first time, we use the keratins from human nail and skin stratum corneum as the growth medium to investigate the different expression patterns of T. rubrum secreted endoproteases genes. During grow in both keratin-containing media SUB7 and MEP2 were the highest expressed gene in each family. These results indicated that SUB7 and MEP2 may be the dominant endoproteases secreted by T. rubrum during host infection and the other proteases may play a supplementary role. The direct comparison of T. rubrum grown on skin and nail medium showed different substrate favorite of secreted endoproteases. The genes MEP2, SUB5, SUB2 and SUB3 were more active during growth in skin medium, possibly these proteases have a higher affinity for skin original keratins. While the structures of SUB1, SUB4, and MEP4 maybe more suitable for the degradation of nail original keratins. This work presents useful molecular details for further understanding the pathogenesis of secreted proteases and the wide adaptation of T. rubrum.

The human keratins: biology and pathology

The keratins are the typical intermediate filament proteins of epithelia, showing an outstanding degree of molecular diversity. Heteropolymeric filaments are formed by pairing of type I and type II molecules. In humans 54 functional keratin genes exist. They are expressed in highly specific patterns related to the epithelial type and stage of cellular differentiation. About half of all keratins—including numerous keratins characterized only recently—are restricted to the various compartments of hair follicles. As part of the epithelial cytoskeleton, keratins are important for the mechanical stability and integrity of epithelial cells and tissues. Moreover, some keratins also have regulatory functions and are involved in intracellular signaling pathways, e.g. protection from stress, wound healing, and apoptosis. Applying the new consensus nomenclature, this article summarizes, for all human keratins, their cell type and tissue distribution and their functional significance in relation to transgenic mouse models and human hereditary keratin diseases. Furthermore, since keratins also exhibit characteristic expression patterns in human tumors, several of them (notably K5, K7, K8/K18, K19, and K20) have great importance in immunohistochemical tumor diagnosis of carcinomas, in particular of unclear metastases and in precise classification and subtyping. Future research might open further fields of clinical application for this remarkable protein family.

Vibrissa hair bulge houses two populations of skin epithelial stem cells distinct by their keratin profile

Defining the properties of postnatal stem cells is of interest given their relevance for tissue homeostasis and therapeutic applications, such as skin tissue engineering for burn patients. In hair follicles, the bulge region of the outer root sheath houses stem cells. We show that explants from the prominent bulge area, but not the bulb, in rodent vibrissa follicles can produce epidermis in a skin model of tissue engineering. Using morphological criteria and keratin expression, we typified epithelial stem cells of vibrissa bulge. Two types of slow-cycling cells (Bb, Bs1) featuring a high colony-forming capacity occur in the bulge. Bb cells are located in the outermost basal layer, express K5, K15, K17, and K19, and feature a loosely organized keratin network. Bs1 cells localize to the suprabasal layers proximal to Bb cells and express K5/K17, correlating with a network of densely bundled filaments. These prominent bundles are missing in K17-null mice, which lack vibrissa. Atypically, both the Bb and Bs1 keratinocytes lack K14 expression. These findings show heterogeneity within the hair follicle stem cell repository, establish that a subset of slow-cycling cells are suprabasal in location, and point to a special role for K5/K17 filaments in a newly defined subset of stem cells. Our results are discussed in the context of long-term survival of engineered tissues after grafting that requires the presence of stem cells.

Hair follicle-specific keratins and their diseases

The human keratin family comprises 54 members, 28 type I and 26 type II. Out of the 28 type I keratins, 17 are epithelial and 11 are hair keratins. Similarly, the 26 type II members comprise 20 epithelial and 6 hair keratins. As, however, 9 out of the 37 epithelial keratins are specifically expressed in the hair follicle, the total number of hair follicle-specific keratins (26) almost equals that of those expressed in the various forms of epithelia (28). Up to now, more than half of the latter have been found to be involved in inherited diseases, with mutated type I and type II members being roughly equally causal. In contrast, out of the 26 hair follicle-specific keratins only 5 have, at present, been associated with inherited hair disorders, while one keratin merely acts as a risk factor. In addition, all hair follicle-specific keratins involved in pathologies are type II keratins. Here we provide a detailed description of the respective hair diseases which are either due to mutations in hair keratins (monilethrix, ectodermal dysplasia of hair and nail type) or hair follicle-specific epithelial keratins (two mouse models, RCO3 and Ca(Rin) as well as pseudofolliculitis barbae).

Identification of a Keratin 4 Mutation in a Chemically Induced Mouse Mutant that Models White Sponge Nevus

With the goal of increasing the number of genetic entry points for studying physiologic processes and human disease, large-scale, systematic, chemical mutagenesis projects in mice have been initiated in several different centers. We have been studying mouse mutants that exhibit dominantly inherited defects in either skin and/or hair color. Here, we describe a bright coat color mutant, Bright coat color 1 (Bcc1), which develops light-colored hair at 4 weeks of age, and when homozygous exhibits oral leukoplakia and blistering, and growth retardation. We identified a missense mutation in mutant animals that predicts an N154S amino-acid substitution in the 1A domain of Keratin 4 (encoded by the Krt2-4 gene), a region known to be mutated in human patients with white sponge nevus (WSN). Bcc1 recapitulates the gross pathologic, histologic, and genetic aspects of the human disorder, WSN.

Isolation and characterization of proinsulin-producing medullary thymic epithelial cell clones

Proinsulin, like many tissue-specific antigens, is expressed by rare (1-3%) cells of the thymus medullary stroma, presumably for the purpose of self-tolerance. Levels of this expression are associated with type 1 diabetes susceptibility in humans and in the NOD mouse. To further understand the mechanism of central tolerance induction by these rare cells, we have isolated and cultured two proinsulin-producing epithelial cell clones from murine thymus. These cells have a typical epithelial morphology and, by flow cytometry, a surface phenotype representative of mature thymic medullary epithelial cells (G8.8(+)/UEA-1(+)/DEC205(-)/CD45(-)/MHC II(+)). By RT-PCR, they express predominantly Ins2 as opposed to Ins1, as does whole thymus. Expression of the transcription factor Aire, implicated in enhancing promiscuous thymic expression of tissue-specific antigens, fell to very low levels after a few passages but increased 20-fold upon exposure to an agonistic anti-lymphotoxin B antibody, concurrent with 2.5-fold enhanced insulin expression. RNA of Pdx-1, Glut-2, and Gck was detectable by RT-PCR in whole thymus but not in the clones, suggesting thymic proinsulin expression is Pdx-1 independent and that Pdx-1, Glut-2, and Gck are likely expressed in the thymus as antigens, not as regulatory molecules.

K25 (K25irs1), K26 (K25irs2), K27 (K25irs3), and K28 (K25irs4) represent the type I inner root sheath keratins of the human hair follicle

The recent elucidation of the human type I keratin gene domain allowed the completion of the so far only partially characterized subcluster of type I keratin genes, KRT25-KRT28 (formerly KRT25A-KRT25D), representing the counterparts of the type II inner root sheath (IRS) keratin genes, KRT71-KRT74 (encoding proteins K71-K74, formerly K6irs1-K6irs4). Here, we describe the expression patterns of the type I IRS keratin proteins K25-K28 (formerly K25irs1-K25irs4) and their mRNAs. We found that K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) occur in the Henle layer, the Huxley layer, and in the IRS cuticle. Their expression extends from the bulb region up to the points of terminal differentiation of the three layers. In contrast, K26 (K25irs2) is restricted to the upper IRS cuticle. Apart from the three IRS layers, K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) are also present in the hair medulla. Based on previous, although controversial claims of the occurrence in the IRS of various "classical" epithelial keratins, we undertook a systematic study using antibodies against the presently described human epithelial and hair keratins and show that the type I keratins K25-K28 (K25irs1-K25irs4) and the type II keratins K71-K74 (K6irs1-K6irs4) represent the IRS keratins of the human hair follicle.

Organ specific autoantigens and the autoreactiveT cell repertoire: the case of myelin oligodendrocyte glycoprotein

There is strong evidence that immunological self tolerance critically relies on the elimination of potentially autoaggressive T lymphocyte clones from the emerging immune repertoire during intrathymic T cell differentiation. These 'forbidden' T cells are deleted as a result of a confrontation with their specific self antigen as presented on medullary stroma cells. But this purging mechanism is remarkably leaky, allowing numerous autoreactive T cells to join the healthy immune repertoire. A paper in this issue of the European Journal of Immunology studies the effect of organ-specific autoantigen expression on the cognate T cell repertoire. Myelin oligodendrocyte glycoprotein (MOG), a putative autoantigen in human multiple sclerosis, is used as a model self antigen. T cell receptor profiles in wild-type mice were compared with those in MOG-knock-out mice. Surprisingly, significant differences were not found suggesting that, in this particular case, autoantigen expression does not affect the autoreactive T cell repertoire.

EEDA: a protein associated with an early stage of stratified epithelial differentiation

Using suppressive subtractive hybridization, we have identified a novel gene, which we named early epithelial differentiation associated (EEDA), which is uniquely associated with an early stage of stratified epithelial differentiation. In epidermis, esophageal epithelium, and tongue epithelium, EEDA mRNA, and antigen was abundant in suprabasal cells, but was barely detectable in more differentiated cells. Consistent with the limbal location of corneal epithelial stem cells, EEDA was expressed in basal corneal epithelial cells that are out of the stem cell compartment, as well as the suprabasal corneal epithelial cells. The strongest EEDA expression occurred in suprabasal precortical cells of mouse, bovine, and human anagen follicles. Developmental studies showed that the appearance of EEDA in embryonic mouse epidermis (E 15.5) coincided with morphological keratinization. Interestingly, EEDA expression is turned off when epithelia were perturbed by wounding and by cultivation under both low and high Ca2+ conditions. Our results indicate that EEDA is involved in the early stages of normal epithelial differentiation, and that EEDA is important for the "normal" differentiation pathway in a wide range of stratified epithelia.

Novel Application of Layered Expression Scanning for Proteomic Profiling of Plucked Hair Follicles

BACKGROUND

There is a need for a technology that can quantitatively assay multiple proteins from a single hair follicle while preserving the morphology of the follicle. For proteomic profiling, the technology should be less labor intensive, with a higher throughput, more quantitative and more reproducible than immunohistochemistry.

OBJECTIVE

To test the ability of a novel method, layered expression scanning of hair (LES-hair) to detect the levels and localization of proteins in plucked hair follicles.

METHODS

LES-hair was used to assay proteins in the plucked hair follicle.

RESULTS

LES-hair detected differential expression of proteins within discrete regions of the plucked hair follicle. These proteins included cleaved caspase 3, Ki-67 and the phosphorylated forms of c-Kit, epidermal growth factor receptor and vascular endothelial growth factor receptor.

CONCLUSION

LES-hair provides a research tool for studying the basic biology of plucked hair follicles and has potential clinical applications such as monitoring treatment of alopecia or using plucked hair follicles as a surrogate tissue to monitor pharmacodynamic effects of targeted cancer therapies.

The human type I keratin gene family: characterization of new hair follicle specific members and evaluation of the chromosome 17q21.2 gene domain

In general concurrence with recent studies, bioinformatic analysis of the chromosome 17q21.2 DNA sequence found in the EBI/Genebank database shows the presence of 27 type I keratin genes and five keratin pseudogenes present on 8 contiguous Bacterial Artificial Chromosome (BAC) sequences. This constitutes the 970 kb type I keratin gene domain. Inserted into this domain is a 350 kb region harboring 32 previously characterized keratin-associated protein genes. Of the 27 keratin genes found in this region, six have not been characterized in detail. This study reports the isolation of cDNA sequences for these keratin genes, termed K25irs1-K25irs4, Ka35, and Ka36, as well as cDNA sequences for the previously reported hair keratins hHa3-I, hHa7, and hHa8. RT-PCR analysis of 14 epithelial tissues using primers for the six novel keratins, as well as for keratins 23 and 24, shows that the six novel keratins appear to be hair follicle associated. Previous expression data, coupled with evolutionary analysis studies point to K25irs1-K25irs4 probably being inner root sheath specific keratins. Ka35 and Ka36 are, based on their exon-intron structure and expression characteristics, hair keratins. In contrast, K23 and K24 appear to be epithelial keratins associated with simple/glandular or stratified, non-cornified epithelia, respectively. A literature analysis coupled with the data presented here confirms that all of the 27 keratin genes found on this domain have been characterized at the transcriptional level. Together with K18, a type I keratin gene found on the type II keratin domain, this seems to be the entire complement of functional type I keratins in humans.

Keratins of the Human Hair Follicle

Substantial progress has been made regarding the elucidation of differentiation processes of the human hair follicle. This review first describes the genomic organization of the human hair keratin gene family and the complex expression characteristics of hair keratins in the hair-forming compartment. Sections describe the role and fate of hair keratins in the diseased hair follicle, particularly hereditary disorders and hair follicle-derived tumors. Also included is a report on the actual state of knowledge concerning the regulation of hair keratin expression. In the second part of this review, essentially the same principles are applied to outline more recent and, thus, occasionally fewer data on specialized epithelial keratins expressed in various tissue constituents of the external sheaths and the companion layer of the follicle. A closing outlook highlights issues that need to be explored further to deepen our insight into the biology and genetics of the hair follicle.

Expression of hair keratins in the adult nail unit: an immunohistochemical analysis of the onychogenesis in the proximal nail fold, matrix and nail bed

BACKGROUND

Recently, the expression profiles of the members of the complex hair keratin family have been determined in the human anagen hair follicle. In contrast, the details of hair keratin expression in the human nail unit are poorly known.

OBJECTIVES

In order to fill this gap, we have performed an immunohistochemical study of the adult human nail unit by means of specific antibodies against nine hair keratins of both types (hHa2, hHb2, hHa5, hHb5, hHa1, hHb1, hHb6, hHa4 and hHa8) as well as three epithelial keratins (K5, K17 and K10).

METHODS

Formalin-fixed paraffin sections of adult nails were examined using monoclonal and polyclonal keratin antibodies, respectively. Longitudinal as well as transverse sections were investigated.

RESULTS

Our study revealed two types of epithelial tissue compartments in the nail unit. The first comprised the eponychium and hyponychium and the nail bed, which expressed only epithelial keratins. While keratins K5, K17 (basal) and K10 (suprabasal) were found in the orthokeratinizing eponychium and hyponychium, throughout, the nail bed epithelium expressed only K5 and K17. The second type comprised the apical and ventral matrix which exhibited a mixed pattern of epithelial and hair keratin expression. Thus, K5 and K17 were expressed in the entire multilayered basal cell compartment of the apical and ventral matrix; however, in the latter, K5 and K17 also occurred in the lowermost layers of the overlying keratogenous zone. The hair matrix keratin hHb5, but not its type II partner hHa5, was seen in the entire keratogenous zone of the apical and ventral matrix, but was also located in the uppermost cell layers of the basal compartment of the ventral matrix, where it overlapped with K5 and K17. Similar to their sequential expression in the hair follicle cortex, hair keratins hHa1, hHb1, hHb6 and hHa4 were consecutively expressed in the keratogenous zone of both the ventral and, albeit less distinctly, apical matrix, with hHa1 initiating in the lowermost cell layers. The expression of hHa8 in only single cortex cells of the hair follicle was also preserved in cells of the keratogenous zone. In the region of the so-called dorsal matrix, we observed two histologically and histochemically distinct types of epithelia: (i) a dominant type, histologically similar to the eponychium and an associated K5, K17 and K10 keratin pattern which clearly extended into the apical matrix, and (ii) a minor type, histologically resembling the postulated dorsal matrix without a granular layer and a cuticle, and exhibiting extended K5 expression as well as hair keratin expression in superficial cells.

CONCLUSIONS

The coexpression of hHb5 with K5 and K17 in the uppermost cell layers of the basal compartment and the lowermost layers of the keratogenous zone of the ventral matrix prompts us to designate this region the prekeratogenous zone of the ventral matrix. The two alternating types of histology and keratin expression in the dorsal matrix identify this region as a transitional zone between the eponychium and the apical matrix. Finally, our data clearly show that the ventral matrix is the main source of the nail plate. In addition, the mixed scenario of hair and epithelial keratins, including demonstrable amounts of K10, in superficial cells of the apical matrix, lends support to the notion that the dorsal portion of the nail is generated by the apical matrix.

Self-representation in the thymus: an extended view

The thymus has been viewed as the main site of tolerance induction to self-antigens that are specifically expressed by thymic cells and abundant blood-borne self-antigens, whereas tolerance to tissue-restricted self-antigens has been ascribed to extrathymic (peripheral) tolerance mechanisms. However, the phenomenon of promiscuous expression of tissue-restricted self-antigens by medullary thymic epithelial cells has led to a reassessment of the role of central T-cell tolerance in preventing organ-specific autoimmunity. Recent evidence indicates that both genetic and epigenetic mechanisms account for this unorthodox mode of gene expression. As we discuss here, these new insights have implications for our understanding of self-tolerance in humans, its breakdown in autoimmune diseases and the significance of this tolerance mode in vertebrate evolution.

Medullary epithelial cells of the human thymus express a highly diverse selection of tissue-specific genes colocalized in chromosomal clusters

Promiscuous expression of tissue-specific self-antigens in the thymus imposes T cell tolerance and protects from autoimmune diseases, as shown in animal studies. Analysis of promiscuous gene expression in purified stromal cells of the human thymus at the single and global gene level documents the species conservation of this phenomenon. Medullary thymic epithelial cells overexpress a highly diverse set of genes (>400) including many tissue-specific antigens, disease-associated autoantigens, and cancer-germline genes. Although there are no apparent structural or functional commonalities among these genes and their products, they cluster along chromosomes. These findings have implications for human autoimmune diseases, immuno-therapy of tumors, and the understanding of the nature of this unorthodox regulation of gene expression.

Hereditary 'white nails': a genetic and structural study

BACKGROUND

Hereditary subtotal leuconychia is a rare nail disease. The gene(s) underlying this phenotype is (are) not known. Immunohistochemical and ultrastructural studies of nails are performed infrequently.

OBJECTIVES

To perform genetic linkage analysis and to assess ultrastructure and soft/hard keratin expression in hereditary white nails.

METHODS

We have analysed microscopically and ultrastructurally the white nails of a patient from a family in which the trait is inherited in an autosomal dominant manner as an isolated symptom. No skin lesions or hair abnormalities could be detected. Genetic linkage studies were performed on DNA samples obtained from several members of the affected family. A longitudinal surgical biopsy of the nail from a great toe was split in two parts. One part was fixed in formalin and processed for histopathology. Another part was further subdivided and embedded either in Epon, following fixation in 2% glutaraldehyde, or in Lowicryl K4M, after fixation in 3% paraformaldehyde. Dewaxed nail sections and Lowicryl ultrathin sections were also stained with various antikeratin antibodies.

RESULTS

Genetic linkage studies of the family pointed to the disease gene mapping to the chromosomal 12q13 region. Genes mapping within this chromosomal region include the genes coding for type II (basic) cytokeratins and hard keratins. The nail matrix presented an abnormal hypergranulosis. The upper part of the nail plate, originating from the proximal nail matrix, had a nonhomogeneous lamellar appearance, with numerous intracellular 'lipidic' vacuoles and 'empty' spaces separating keratin filament bundles. These cells were progressively shed at the nail surface. The cell loss was compensated by hyperproliferation of the distal matrix and of the nail bed keratinocytes, with persistent marked parakeratosis and loose arrangement of keratin bundles. The distal matrix and the nail bed contributed equally to formation of the lower plate. This presented the characteristics of a tissue composed of soft keratins. Accordingly, there was virtually no labelling with the Hb1 antibody to a basic hard keratin in the white nail, whereas the labelling with AE3 antibody to all type II keratins and with KL1 recognizing suprabasal soft keratins was normal or even enhanced.

CONCLUSIONS

Genetic linkage indicates that the gene defect underlying the leuconychia in the family studied resides on chromosome 12q13. As the type II keratins map within this chromosomal interval, it is possible that a mutation in one of these keratin genes may be a cause of the hereditary leuconychia. The white appearance of nails in this disease seems to be due to an abnormal keratinization of cells originating from the proximal nail matrix, leading to the presence of abundant intracellular vacuoles and to a lesser compactness of keratins.

A novel promoter polymorphism (-71C>T) in KRTHB6 gene in Indian population

We have screened the basal promoter region, of KRTHB6 gene involving CAAT and TATA boxes in randomly selected 125 individuals of Indian origin by PCR-SSCP and DNA sequencing. We observed a novel promoter polymorphism (-71C>T) which could be differentiated by using LweI restriction enzyme. The frequency of -71 C allele, allele A (Accession no AY203963), was observed to be higher ( 0.712) in comparison to -71 T allele, allele B (0.288) (Accession no. AY037552).

A 4.2 kb Upstream Region of the Human Corneodesmosin Gene Directs Site-Specific Expression in Hair Follicles and Hyperkeratotic Epidermis of Transgenic Mice

Corneodesmosin (CDSN) is a desmosomal protein expressed in the epidermis during the late stages of differentiation and in the inner root sheath of hair follicles. The homophilic adhesive properties of the protein suggest that it reinforces keratinocyte cohesion in the upper layers of the epidermis (stratum granulosum and stratum corneum). In this study, we analyzed the expression of the CDSN gene in 16 human tissues. We confirmed the closely restricted expression pattern of CSDN. Indeed, apart from the skin, the mRNA was significantly detected only in the placenta and the thymus. As a step in elucidating the mechanisms of tissue-specific expression, transgenic mice bearing a 4.2 kb fragment of the human CSDN gene promoter linked to the LacZ gene were generated. The reporter-gene expression was detected in special areas of the inner root sheath of the hair follicles and the hair medulla but not in the epidermis. Induction of epidermis hyperproliferation however either by pharmacological agents or by wounding led to strong expression of the reporter gene in the keratinocytes of the stratum granulosum and the parakeratotic corneocytes of the stratum corneum. The data suggest that the genomic sequences and/or regulating factors responsible for the cell-specific expression of the human CDSN gene in the normal hair follicle as well as in the hyperproliferative epidermis are different from those necessary for expression in the normal epidermis.

Long-range, nonautonomous effects of activated Notch1 on tissue homeostasis in the nail

Using an antibody directed against gamma-secretase-generated antigen unique to activated Notch1, we mapped Notch1 activation strictly to suprabasal cells in epidermis, nail matrix, and other skin appendages during normal development. The consequences of Notch1 activation in keratinizing nail cells were investigated in a transgenic mouse model. Ectopic activation of Notch1 in postmitotic cells within the nail keratogenous zone resulted in longer nails. BrdU labeling revealed an increased number of mitotic cells in transgenic nails. The matrix and keratogenous zone expanded distally due to the increase in cell numbers. The mitosis-promoting effects by a gene product expressed exclusively in postmitotic cells indicates a long-range effect of transgenic Notch1 on regulation of nail homeostasis. We demonstrate that activation of Notch1 in the keratogenous zone resulted in ectopic activation of Wnt signaling, the first such evidence in vertebrates. However, we detected little or no beta-catenin activation in proliferating matrix cells, indicating that Wnt is at most an indirect mediator of Notch-induced proliferation. These data support the existence of a novel, cell-nonautonomous role for Notch in maintaining homeostasis of stratified squamous epithelia by indirectly promoting mitosis in basally located cells.

Mice with defects in HB-EGF ectodomain shedding show severe developmental abnormalities

Heparin-binding EGF-like growth factor (HB-EGF) is first synthesized as a membrane-anchored form (proHB-EGF), and its soluble form (sHB-EGF) is released by ectodomain shedding from proHB-EGF. To examine the significance of proHB-EGF processing in vivo, we generated mutant mice by targeted gene replacement, expressing either an uncleavable form (HB^uc) or a transmembrane domain–truncated form (HB^Δtm) of the molecule. HB^uc/uc mice developed severe heart failure and enlarged heart valves, phenotypes similar to those in proHB-EGF null mice. On the other hand, mice carrying HB^Δtm exhibited severe hyperplasia in both skin and heart. These results indicate that ectodomain shedding of proHB-EGF is essential for HB-EGF function in vivo, and that this process requires strict control.

Tight junction-related structures in the absence of a lumen: Occludin, claudins and tight junction plaque proteins in densely packed cell formations of stratified epithelia and squamous cell carcinomas

Tight junctions (TJs), hallmark structures of one-layered epithelia and of endothelia, are of central biological importance as intramembranous "fences" and as hydrophobic "barriers" between lumina represented by liquid- or gas-filled spaces on the one hand and the mesenchymal space on the other. They have long been thought to be absent from stratified epithelia. Recently, however, constitutive TJ proteins and TJ-related structures have also been identified in squamous stratified epithelia, including the epidermis, where they occur in special positions, most prominently in the uppermost living epidermal cell layer, the stratum granulosum. Much to our surprise, however, we have now also discovered several major TJ proteins (claudins 1 and 4, occludin, cingulin, symplekin, protein ZO-1) and TJ-related structures in specific positions of formations of epithelium-derived tissues that lack any lumen and do not border on luminal or body surfaces. Using immunohistochemistry and electron microscopy we have localized TJ proteins and structures in peripheral cells of the Hassall's corpuscles of human and bovine thymi as well as in specific central formations of tumor nests in squamous cell carcinomas, including the so-called "horn pearls". Such structures have even been found in carcinoma metastases. In carcinomas, they often seem to separate certain tumor regions from others or from stroma. The structural significance and the possible functional relevance of the locally restricted synthesis of TJ proteins and of the formations of TJ-related structures are discussed. It is proposed to include the determination of the presence or absence of such proteins and structures in the diagnostic program of tumor pathology.

K6irs1, K6irs2, K6irs3, and K6irs4 represent the inner-root-sheath-specific type II epithelial keratins of the human hair follicle

In this study we report on the cloning of two novel human type II keratin cDNAs, K6irs3 and K6irs4, which were specifically expressed in the inner root sheath of the hair follicle. Together with the genes of two previously described type II inner root sheath keratins, K6irs1 and K6irs2, the K6irs3 and K6irs4 genes were subclustered in the type II keratin/hair keratin gene domain on chromosome 12q13. Evolutionary tree analysis using all known type II epithelial and hair keratins revealed that the K6irs1-4 formed a branch separate from the other epithelial and hair keratins. RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles, which also included the K6irs2 keratin, demonstrated that both K6irs2 and K6irs3 were specifically expressed in the inner root sheath cuticle, but showed a different onset of expression in this compartment. Whereas the K6irs3 expression began in the lowermost bulb region, that of K6irs2 was delayed up to the height of the apex of the dermal papilla. In contrast, the K6irs4 keratin was specifically expressed in the Huxley layer. Moreover, K6irs4 was ideally suited to further investigate the occurrence of Flügelzellen, i.e., Huxley cells, characterized by horizontal cell extensions that pass through the Henle layer, abut upon the companion layer, and form desmosomal connections with the surrounding cells. Previously, we detected Flügelzellen only in the region along the differentiated Henle layer. Using the Huxley-cell-specific K6irs4 antiserum, we now demonstrate this cell type to be clearly apposed to the entire Henle layer. We provide evidence that Flügelzellen penetrate the Henle layer actively and may play a role in conferring plasticity and resilience to the otherwise rigid upper Henle layer.

The onychomatricoma: additional histologic criteria and immunohistochemical study

Onychomatricoma (OM) is a tumor of the nail matrix typified histologically by multiple distal fibroepithelial projections and a thick keratogenous zone forming multiple V-shaped invaginations at the level of epithelial ridges, with the formation of a thick nail plate. In its proximal portion, the thickness of the nail looks like a spur originating from the ventral part of the nail plate. In its distal part, beyond the lunula, the nail plate is globally thickened and filled with cavities containing serous fluid. Often, however, the pathologist is not provided with the nail plate. The diagnosis then rests on the presence of a fibroepithelial tumor. In this article the histologic criteria of OM without nail plate are refined and OM is characterized immunohistochemically using three tumors fixed in liquid nitrogen and examined separately from the nail plate. On longitudinal section OM without nail plate appears as a unique pedunculated fibroepithelial tumor i.e., the multiple distal epithelial digitations arranged along a transversal plane are not seen. The feature is reminiscent of fibrokeratoma. When OM is visualized in longitudinal section, 3 main criteria differentiate OM from fibrokeratoma: the presence of epithelial-lined invaginations around optical cavities, a stroma organized in 2 layers, and the absence of horny corn. Patterns of expression of cytokeratins and integrins in OM are identical to that observed in the normal nail matrix. Involucrin finds expression from the basal layer through to the top of the epithelium, where it is more marked and where transglutaminase 1 is restricted. Merkel cells detected by CK 20 are increased in number and sometimes disposed in clusters. The fibrous component of OM is composed of 2 layers: a superficial stroma made of numerous fines fibrils of collagen IV intermingled with collagen I, and deep stroma made principally of collagen I. Antibody AE13, specific to trichocytic keratins Ha 1-4, represent a good potential marker of OM. Its V-shaped expression in epithelium ridges offers early identification of the keratogenous zone of OM, on tumors separated from their nail plates and limited to their fibroepithelial components.

A novel epithelial keratin, hK6irs1, is expressed differentially in all layers of the inner root sheath, including specialized huxley cells (Flügelzellen) of the human hair follicle

In this study we have characterized a novel human type II keratin, hK6irs1, which is specifically expressed in the inner root sheath of the hair follicle. This keratin represents the ortholog of the recently described mouse inner root sheath keratin mK6irs. The two keratins were highly related and migrated at the same height as keratin 6 in two-dimensional gel electrophoresis. Both RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles demonstrated hK6irs1 expression in the Henle and Huxley layers as well as in the cuticle of the inner root sheath. In all three layers, the expression of hK6irs1 mRNA and protein began simultaneously in adjacent cells of the lowermost bulb above the germinative cell pool. Higher up in the follicle, the detection limits for both hK6irs1 mRNA and protein precisely coincided with the asynchronous onset of abrupt terminal differentiation of the Henle layer, inner root sheath cuticle, and Huxley layer. Mainly above the level of terminal Henle cell differentiation, both indirect immunofluorescence and immunoelectron microscopy revealed the occurrence of distinct Huxley cells that developed pseudopodal hK6irs1-positive extensions passing through the fully keratinized Henle layer. These outwardly protruding foot processes abutted upon cells of the companion layer, with which they were connected by numerous desmosomes. These specialized Huxley cells have previously been termed "Flügelzellen", which means "winged cells", with reference to their characteristic foot processes. We provide evidence that, together with Henle cells, Flügelzellen ensure the maintenance of a continuous desmosomal anchorage of the companion layer along the entire inner root sheath. This tightly connected companion layer/inner root sheath unit provides an optimal molding and guidance of the growing hair shaft.

Cytokeratins as markers of follicular differentiation: an immunohistochemical study of trichoblastoma and basal cell carcinoma

Trichoblastoma(s) (TB) are benign neoplasms of follicular differentiation frequently found in nevus sebaceus. Many morphologic features are shared with nodular basal cell carcinoma(s) (BCC), sometimes rendering the differential diagnosis difficult. Because both neoplasms can simulate components of mature hair follicles histologically, we attempted to corroborate this by immunohistochemical examination of cytokeratins and hair keratins differentially expressed in the hair follicle. Trichoblastoma(s) and BCC showed homogenous expression of CK14 and CK17. The innermost cells of the tumor nodules in all TB and in 72% of BCC were positive for CK6hf. Using a specific CK15 antibody, 38% of TB showed a focal labeling and all BCC remained negative; 70% of TB and 22% of BCC expressed CK19. CK8 was expressed by numerous Merkel cells present in all TB but in none of the BCC examined. All type I and II hair keratins tested, (especially hHa1, hHa5, and hHa8) remained negative in all tumors examined. Trichoblastoma(s) and BCC show consistent expression of CK6hf, CK14, and CK17; variable expression of CK15 and CK19; and absence of hair keratins. This indicates a differentiation toward the outer root sheath epithelium or the companion layer and not toward the inner root sheath, matrix, or cortex.

The catalog of human hair keratins. II. Expression of the six type II members in the hair follicle and the combined catalog of human type I and II keratins

The human type II hair keratin subfamily consists of six individual members and can be divided into two groups. The group A members hHb1, hHb3, and hHb6 are structurally related, whereas group C members hHb2, hHb4, and hHb5 are rather distinct. Specific antisera against the individual hair keratins were used to establish the two-dimensional catalog of human type II hair keratins. In this catalog, hHb5 showed up as a series of isoelectric variants, well separated from a lower, more acidic, and complex protein streak containing isoelectric variants of hair keratins hHb1, hHb2, hHb3, and hHb6. Both in situ hybridization and immunohistochemistry on anagen hair follicles showed that hHb5 and hHb2 defined early stages of hair differentiation in the matrix (hHb5) and cuticle (hHb5 and hHb2), respectively. Although cuticular differentiation proceeded without the expression of further type II hair keratins, cortex cells simultaneously expressed hHb1, hHb3, and hHb6 at an advanced stage of differentiation. In contrast, hHb4, which is undetectable in hair follicle extracts and sections, could be identified as the largest and most alkaline member of this subfamily in cytoskeletal extracts of dorsal tongue. This hair keratin was localized in the posterior compartment of the tongue filiform papillae. Comparative analysis of type II with the previously published type I hair keratin expression profiles suggested specific, but more likely, random keratin-pairing principles during trichocyte differentiation. Finally, by combining the previously published type I hair keratin catalog with the type II hair keratin catalog and integrating both into the existing catalog of human epithelial keratins, we present a two-dimensional compilation of the presently known human keratins.

A unique type I keratin intermediate filament gene family is abundantly expressed in the inner root sheaths of sheep and human hair follicles

A unique type I keratin intermediate filament group, comprising three highly related proteins and expressed in the inner root sheath of hair follicles, has been identified in both sheep and human. The first members from these species are named oIRSa1 and hIRSa1 and each encodes a protein of 450 amino acids, with compositional characteristics intermediate between those of previously described hair keratin and epidermal cytokeratin type I intermediate filaments. Detection of abundant mRNA transcripts derived from the sheep and human genes by cRNA in situ hybridization only in the inner root sheath and not in the medulla concurs with the findings of earlier ultrastructural analyses that have reported intermediate filaments only in the inner root sheath. Clustering of the IRSa keratin genes is apparent in the genomes of both species. The three hIRSa genes, known to reside on human chromosome 17, are closely linked to three further type I keratin intermediate filament genes of unknown function. This new gene complex, contained almost entirely within a 156 kb BAC (hRPK.142_H_19), is likely to lie near the type I intermediate filament cytokeratin and hair keratin gene loci at 17q12-q21. A phylogenetic analysis including all known human type I intermediate filament cytokeratins, hHa keratins, hIRSa, and hIRSa-linked keratins suggests that origin of the IRSa keratin intermediate filament linkage group preceded origin of most of the epidermal cytokeratins and all hair keratins during emergence of the keratin intermediate filament genes.

Analysis of mouse keratin 6a regulatory sequences in transgenic mice reveals constitutive, tissue-specific expression by a keratin 6a minigene

The analysis of keratin 6 expression is complicated by the presence of multiple isoforms that are expressed constitutively in a number of internal stratified epithelia, in palmoplantar epidermis, and in the companion cell layer of the hair follicle. In addition, keratin 6 expression is inducible in interfollicular epidermis and the outer root sheath of the follicle, in response to wounding stimuli, phorbol esters, or retinoic acid. In order to establish the critical regions involved in the regulation of keratin 6a (the dominant isoform in mice), we generated transgenic mice with two different-sized mouse keratin 6a constructs containing either 1.3 kb or 0.12 kb of 5' flanking sequence linked to the lacZ reporter gene. Both constructs also contained the first intron and the 3' flanking sequence of mouse keratin 6a. Ectopic expression of either transgene was not observed. Double-label immunofluorescence analyses demonstrated expression of the reporter gene in keratin 6 expressing tissues, including the hair follicle, tongue, footpad, and nail bed, showing that both transgenes retained keratinocyte-specific expression. Quantitative analysis of beta-galactosidase activity verified that both the 1.3 and 0.12 kb keratin 6a promoter constructs produced similar levels of the reporter. Notably, both constructs were constitutively expressed in the outer root sheath and interfollicular epidermis in the absence of any activating stimulus, suggesting that they lack the regulatory elements that normally silence transcription in these cells. This study has revealed that a keratin 6a minigene contains critical cis elements that mediate tissue-specific expression and that the elements regulating keratin 6 induction lie distal to the 1.3 kb promoter region.