Keratin 17 expression in the hard epithelial context of the hair and nail, and its relevance for the pachyonychia congenita phenotype

The development of hair follicles and nail

The hair follicle and nail unit develop and regenerate through epithelial-mesenchymal interactions. Here, we review some of the key signals and molecular interactions that regulate mammalian hair follicle and nail formation during embryonic development and how these interactions are reutilized to promote their regeneration during adult homeostasis and in response to skin wounding. Finally, we highlight the role of some of these signals in mediating human hair follicle and nail conditions.

The role of URO17® in diagnosis and follow up of bladder cancer patients

OBJECTIVE

to evaluate the role of urinary URO17® biomarker in the detection of urothelial tumors in haematuria patients and the detection of recurrence in non-muscle invasive bladder urothelial tumors.

MATERIALS AND METHODS

Our study was formed of two cohorts of patients, group I represents patients presenting with haematuria (n = 98), while group II represents patients with known non-muscle invasive bladder cancers on their scheduled follow up cystoscopic investigation (n = 51). For both groups, patients were asked to provide urine samples before cystoscopy, either primary as part of the haematuria investigation or as a scheduled follow-up. Urine samples were sent anonymously for standard urine cytology and URO17® biomarker immunostaining. Results were compared to cystoscopic findings using Chi-square analysis and Fisher's exact test (P < 0.05).

RESULTS

Group I was formed of 98 patients, with an average age of 60 years. URO17® showed 100% sensitivity and 96.15% specificity with a negative predictive value (NPV) of 100 and a positive predictive value (PPV) of 95.83. The results showed statistical significance with P value < 0.001. Group II was formed of 51 patients, with an average age of 75 years. URO17® was shown to have a sensitivity of 85.71% and NPV of 95.45. Eleven patients of group II were on scheduled BacillusCalmette-Guerin (BCG) and another 5 received Mitomycin C (MMC). The overall results of both groups combined (n = 149) showed statistical significance between flexible cystoscopy results and the results of urinary URO17® and urine cytology.

CONCLUSION

URO17® has a potential to be a reliable test for diagnosis and follow up of urothelial cancer patients and a screening tool adjunct to flexible cystoscopy.

TRIAL REGISTRATION

Not applicable as the current study is not a clinical trial, as per according to the National Institutes of Health, "studies that involve a comparison of methods and that do not evaluate the effect of the interventions on the participant do not meet the NIH clinical trial definition."

Coexistence of pachyonychia congenita and hidradenitis suppurativa: more than a coincidence

BACKGROUND

The coexistence of pachyonychia congenita (PC) and hidradenitis suppurativa (HS) has been described in case reports. However, the pathomechanism underlying this association and its true prevalence are unknown.

OBJECTIVES

To determine the genetic defect underlying the coexistence of PC and HS in a large kindred, to delineate a pathophysiological signalling defect jointly leading to both phenotypes, and to estimate the prevalence of HS in PC.

METHODS

We used direct sequencing and a NOTCH luciferase reporter assay to characterize the pathophysiological basis of the familial coexistence of HS and PC. A questionnaire was distributed to patients with PC registered with the International Pachyonychia Congenita Research Registry (IPCRR) to assess the prevalence of HS among patients with PC.

RESULTS

Direct sequencing of DNA samples obtained from family members displaying both PC and HS demonstrated a missense variant (c.275A>G) in KRT17, encoding keratin 17. Abnormal NOTCH signalling has been suggested to contribute to HS pathogenesis. Accordingly, the KRT17 c.275A>G variant resulted in a significant decrease in NOTCH activity. To ascertain the clinical importance of the association of HS with PC, we distributed a questionnaire to all patients with PC registered with the IPCRR. Seventy-two of 278 responders reported HS-associated clinical features (25·9%). Disease-causing mutations in KRT17 were most prevalent among patients with a dual phenotype of PC and HS (43%).

CONCLUSIONS

The coexistence of HS and KRT17-associated PC is more common than previously thought. Impaired NOTCH signalling as a result of KRT17 mutations may predispose patients with PC to HS. What is already known about this topic? The coexistence of pachyonychia congenita (PC) and hidradenitis suppurativa (HS) has been described in case reports. However, the pathomechanism underlying this association and its true prevalence are unknown. What does this study add? A dual phenotype consisting of PC and HS was found to be associated with a pathogenic variant in KRT17. This variant was found to affect NOTCH signalling, which has been previously implicated in HS pathogenesis. HS was found to be associated with PC in a large cohort of patients with PC, especially in patients carrying KRT17 variants, suggesting that KRT17 variants causing PC may also predispose to HS. What is the translational message? These findings suggest that patients with PC have a higher prevalence of HS than previously thought, and hence physicians should have a higher level of suspicion of HS diagnosis in patients with PC.

Stress Keratin 17 Expression in Head and Neck Cancer Contributes to Immune Evasion and Resistance to Immune-Checkpoint Blockade

PURPOSE

We investigated whether in human head and neck squamous cell carcinoma (HNSCC) high levels of expression of stress keratin 17 (K17) are associated with poor survival and resistance to immunotherapy.

EXPERIMENTAL DESIGN

We investigated the role of K17 in regulating both the tumor microenvironment and immune responsiveness of HNSCC using a syngeneic mouse HNSCC model, MOC2. MOC2 gives rise to immunologically cold tumors that are resistant to immune-checkpoint blockade (ICB). We engineered multiple, independent K17 knockout (KO) MOC2 cell lines and monitored their growth and response to ICB. We also measured K17 expression in human HNSCC of patients undergoing ICB.

RESULTS

MOC2 tumors were found to express K17 at high levels. When knocked out for K17 (K17KO MOC2), these cells formed tumors that grew slowly or spontaneously regressed and had a high CD8+ T-cell infiltrate in immunocompetent syngeneic C57BL/6 mice compared with parental MOC2 tumors. This phenotype was reversed when we depleted mice for T cells. Whereas parental MOC2 tumors were resistant to ICB treatment, K17KO MOC2 tumors that did not spontaneously regress were eliminated upon ICB treatment. In a cohort of patients with HNSCC receiving pembrolizumab, high K17 expression correlated with poor response. Single-cell RNA-sequencing analysis revealed broad differences in the immune landscape of K17KO MOC2 tumors compared with parental MOC2 tumors, including differences in multiple lymphoid and myeloid cell types.

CONCLUSIONS

We demonstrate that K17 expression in HNSCC contributes to immune evasion and resistance to ICB treatment by broadly altering immune landscapes of tumors.

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.

Discovery and validation of novel biomarkers for detection of cervical cancer

AIMS

To investigate novel biomarker for diagnosis of cervical cancer, we analyzed the datasets in Gene Expression Omnibus (GEO) and confirmed the candidate biomarker in patient sample.

MATERIALS AND METHODS

We collected major datasets of cervical cancer in GEO, and analyzed the differential expression of normal and cancer samples online with GEO2R and tested the differences, then focus on the GSE63514 to screen the target genes in different histological grades by using the R-Bioconductor package and R-heatmap. Then human specimens from the cervix in different histological grades were used to confirm the top 8 genes expression by immunohistochemical staining using Ki67 as a standard control.

RESULTS

We identified genes differentially expressed in normal and cervical cancer, 274 upregulated genes and 206 downregulated genes. After intersection with GSE63514, we found the obvious tendency in different histological grades. Then we screened the top 24 genes, and confirmed the top 8 genes in human cervix tissues. Immunohistochemical (IHC) results confirmed that keratin 17 (KRT17) was not expressed in normal cervical tissues and was over-expressed in cervical cancer. Cysteine-rich secretory protein-2 (CRISP2) was less expressed in high-grade squamous intraepithelial lesions (HSILs) than in other histological grades.

CONCLUSION

For the good repeatability and consistency of KRT17 and CRISP2, they may be good candidate biomarkers. Combined analysis of KRT17, CRISP2 expression at both genetic and protein levels can determine different histological grades of cervical squamous cell carcinoma. Such combined analysis is capable of improving diagnostic accuracy of cervical cancer.

The expression of equine keratins K42 and K124 is restricted to the hoof epidermal lamellae of Equus caballus

The equine hoof inner epithelium is folded into primary and secondary epidermal lamellae which increase the dermo-epidermal junction surface area of the hoof and can be affected by laminitis, a common disease of equids. Two keratin proteins (K), K42 and K124, are the most abundant keratins in the hoof lamellar tissue of Equus caballus. We hypothesize that these keratins are lamellar tissue-specific and could serve as differentiation- and disease-specific markers. Our objective was to characterize the expression of K42 and K124 in equine stratified epithelia and to generate monoclonal antibodies against K42 and K124. By RT-PCR analysis, keratin gene (KRT) KRT42 and KRT124 expression was present in lamellar tissue, but not cornea, haired skin, or hoof coronet. In situ hybridization studies showed that KRT124 localized to the suprabasal and, to a lesser extent, basal cells of the lamellae, was absent from haired skin and hoof coronet, and abruptly transitions from KRT124-negative coronet to KRT124-positive proximal lamellae. A monoclonal antibody generated against full-length recombinant equine K42 detected a lamellar keratin of the appropriate size, but also cross-reacted with other epidermal keratins. Three monoclonal antibodies generated against N- and C-terminal K124 peptides detected a band of the appropriate size in lamellar tissue and did not cross-react with proteins from haired skin, corneal limbus, hoof coronet, tongue, glabrous skin, oral mucosa, or chestnut on immunoblots. K124 localized to lamellar cells by indirect immunofluorescence. This is the first study to demonstrate the localization and expression of a hoof lamellar-specific keratin, K124, and to validate anti-K124 monoclonal antibodies.

Development of Hair Fibres

The growth of hairs occurs during the anagen phase of the follicle cycle. Hair growth begins with basement membrane-bound stem cells (mother cells) around the dermal papilla neck which continuously bud off daughter cells which further divide as a transient amplifying population. Division ceases as cell line differentiation begins, which entails changes in cell junctions, cell shape and position, and cell-line specific cytoplasmic expression of keratin and trichohyalin. As the differentiating cells migrate up the bulb, nuclear function ceases in cortex, cuticle and inner root sheath (IRS) layers. Past the top of the bulb, cell shape/position changes cease, and there is a period of keratin and keratin-associated protein (KAP) synthesis in fibre cell lines, with increases, in particular of KAP species. A gradual keratinization process begins in the cortex at this point and then non-keratin cell components are increasingly broken down. Terminal cornification, or hardening, is associated with water loss and precipitation of keratin. In the upper follicle, the hair, now in its mature form, detaches from the IRS, which is then extracted of material and becomes fragmented to release the fibre. Finally, the sebaceous and sudoriferous (if present) glands coat the fibre in lipid-rich material and the fibre emerges from the skin. This chapter follows the origin of the hair growth in the lower bulb and traces the development of the various cell lines.

Stem Cell Markers (Cytokeratin 17 and Cytokeratin 19) in Scarring and Nonscarring Alopecia

BACKGROUND

Alopecia is one of the most important hair follicle (HF) disorders, which is divided into scarring (cicatricial) and nonscarring (noncicatricial) types.

OBJECTIVE

The aim of this study is to investigate the expression of stem cell (SC) markers such as cytokeratin (CK) 17 and CK19 in scarring and nonscarring alopecia.

MATERIALS AND METHODS

Thirty patients with scalp alopecia (15 with scarring alopecia and 15 without) together with ten healthy volunteers were included in this study. Biopsies were taken from all participants and stained for CK17 and CK19 using immunohistochemistry.

RESULTS

There was a statistically significant difference between the nonscarring group and the control group with regard to CK17 expression in the outer layers of the HFs (P = 0.00) and CK19 staining of the inner layers of the HFs (P = 0.008). There was a statistically significant difference between the scarring and the control groups regarding CK17 expression in the outer (P = 0.00) and the inner layers (P = 0.00) of the HFs and CK19 expression in the inner layers of the HFs (P = 0.00). CK17 expression in the outer layers (P = 0.02) and the inner layers of the HFs (P = 0.00) together with CK19 expression in the inner layers of the HFs (P = 0.00) showed statistically significant differences between scarring and nonscarring alopecia groups.

CONCLUSIONS

The presence of SC markers (CK17 and CK19) in the HFs was affected in both scarring and nonscarring alopecia, but the defect in scarring alopecia is more evident than that of nonscarring alopecia. The persistence of SC markers in some types of scarring alopecia could give a hope for the recovery of these lesions. Further studies are recommended to clarify the benefit from using HF SCs in the treatment of alopecia.

Inhibition of Wnt/β-catenin pathway promotes regenerative repair of cutaneous and cartilage injury

Wound healing in mammals is a fibrotic process. The mechanisms driving fibrotic (as opposed to regenerative) repair are poorly understood. Herein we report that therapeutic Wnt inhibition with topical application of small-molecule Wnt inhibitors can reduce fibrosis and promote regenerative cutaneous wound repair. In the naturally stented model of ear punch injury, we found that Wnt/β-catenin pathway is activated most notably in the dermis of the wound bed early (d 2) after injury and subsides to baseline levels by d10. Topical application of either of 2 mechanistically distinct small-molecule Wnt pathway inhibitors (a tankyrase inhibitor, XAV-939, and the U.S. Food and Drug Administration-approved casein kinase activator, pyrvinium) in C57Bl/6J mice resulted in significantly increased rates of wound closure (72.3 ± 14.7% with XAV-939; and 52.1 ± 20.9% with pyrvinium) compared with contralateral controls (38.1 ± 23.0 and 40.4.± 16.7%, respectively). Histologically, Wnt inhibition reduced fibrosis as measured by α-smooth muscle actin positive myofibroblasts and collagen type I α1 synthesis. Wnt inhibition also restored skin architecture including adnexal structures in ear wounds and dermal-epidermal junction with rete pegs in excisional wounds. Additionally, in ear punch injury Wnt inhibitor treatment enabled regeneration of auricular cartilage. Our study shows that pharmacologic Wnt inhibition holds therapeutic utility for regenerative repair of cutaneous wounds.

Keratin-17 Promotes p27KIP1 Nuclear Export and Degradation and Offers Potential Prognostic Utility

Keratins that are overexpressed selectively in human carcinomas may offer diagnostic and prognostic utility. In this study, we show that high expression of keratin-17 (K17) predicts poor outcome in patients with cervical cancer, at early or late stages of disease, surpassing in accuracy either tumor staging or loss of p27(KIP1) as a negative prognostic marker in this setting. We investigated the mechanistic basis for the biologic impact of K17 through loss- and gain-of-function experiments in human cervix, breast, and pancreatic cancer cells. Specifically, we determined that K17 functions as an oncoprotein by regulating the subcellular localization and degradation of p27(KIP1). We found that K17 was released from intermediate filaments and translocated into the nucleus via a nuclear localization signal (NLS), specific among keratins, where it bound p27(KIP1) during G1 phase of the cell cycle. p27(KIP1) lacks a nuclear export signal (NES) and requires an adaptor for CRM1 binding for nuclear export. In K17, we defined and validated a leucine-rich NES that mediated CRM1 binding for export. Cervical cancer cells expressing K17 mutations in its NLS or NES signals exhibited an increase in levels of nuclear p27(KIP1), whereas cells expressing wild-type K17 exhibited a depletion in total endogenous p27(KIP1). In clinical specimens of cervical cancer, we confirmed that the expressions of K17 and p27(KIP1) were inversely correlated, both across tumors and within individual tumors. Overall, our findings establish that K17 functions specially among keratins as an oncoprotein by controlling the ability of p27(KIP1) to influence cervical cancer pathogenesis.

Comparative anatomy of mouse and human nail units

Recent studies of mice with hair defects have resulted in major contributions to the understanding of hair disorders. To use mouse models as a tool to study nail diseases, a basic understanding of the similarities and differences between the human and mouse nail unit is required. In this study we compare the human and mouse nail unit at the macroscopic and microscopic level and use immunohistochemistry to determine the keratin expression patterns in the mouse nail unit. Both species have a proximal nail fold, cuticle, nail matrix, nail bed, nail plate, and hyponychium. Distinguishing features are the shape of the nail and the presence of an extended hyponychium in the mouse. Expression patterns of most keratins are similar. These findings indicate that the mouse nail unit shares major characteristics with the human nail unit and overall represents a very similar structure, useful for the investigation of nail diseases and nail biology.

Immunolocalization of junctional proteins in human hairs indicates that the membrane complex stabilizes the inner root sheath while desmosomes contact the companion layer through specific keratins

The inner root sheath (IRS) sustains and addresses the hair shaft outside the follicle. Ultrastructural analysis of immunolabeling for beta-catenin, plakophilin-1, desmoglein-4 and keratin-17 in human hairs has indicated that adherens junctions and desmosomes initially connect cells in mature IRS and the companion layer. Beta-catenin immunolabeling for adherens junctions is only seen in sparse regions of differentiating Huxley cells, Flugelzellen cells and Henle cells, but disappears in cornified cells of the IRS. Desmoglein-4 and plakophilin-1 immunolabeling are observed in differentiating and cornified desmosomes of the Huxley and Henle layers and in the membrane complex joining these cells. Desmoglein-4 and plakophilin-1 are more frequently immunolocalized in the intracellular side of the junctions, but some labeling is also present in the delta-layer of the membrane complex. The labeling indicates a prevalent intracellular redistribution of desmoglein-4 and plakophilin-1 when the final cornification of the IRS occurs. Intense keratin-17 immunolabeling is observed in tonofilaments of the companion layer joining the plakophilin-1 rich desmosomes of the Henle layer. This suggests that this elastic type of keratin is present at desmosome junctions during the movements of the companion layer along the slippage plane of the hair shaft.

The touch dome defines an epidermal niche specialized for mechanosensory signaling

In mammalian skin, Merkel cells are mechanoreceptor cells that are required for the perception of gentle touch. Recent evidence indicates that mature Merkel cells descend from the proliferative layer of skin epidermis; however, the stem cell niche for Merkel cell homeostasis has not been reported. Here, we provide genetic evidence for maintenance of mature Merkel cells during homeostasis by Krt17+ stem cells located in epidermal touch domes of hairy skin and in the tips of the rete ridges of glabrous skin. Lineage tracing analysis indicated that the entire pool of mature Merkel cells is turned over every 7-8 weeks in the adult epidermis and that Krt17+ stem cells also maintain squamous differentiation in the touch dome and in glabrous skin. Finally, selective genetic ablation of Krt17+ touch-dome keratinocytes indicates that these cells, and not mature Merkel cells, are primarily responsible for maintaining innervation of the Merkel cell-neurite complex.

Morphogenesis of chimeric hair follicles in engineered skin substitutes with human keratinocytes and murine dermal papilla cells

Engineered skin substitutes (ESS) have been used successfully to treat life-threatening burns, but lack cutaneous appendages. To address this deficiency, dermal constructs were prepared using collagen-glycosaminoglycan scaffolds populated with murine dermal papilla cells expressing green fluorescent protein (mDPC-GFP), human dermal papilla cells (hDPC) and/or human fibroblasts (hF). Subsequently, human epidermal keratinocytes (hK) or hK genetically modified to overexpress stabilized β-catenin (hK') were used to prepare ESS epithelium. After 10 days incubation at air-liquid interface, ESS were grafted to athymic mice and were evaluated for 6 weeks. Neofollicles were observed in ESS containing mDPC-GFP, but not hDPC or hF, independent of whether or not the hK were genetically modified. Based on detection of GFP fluorescence, mDPC were localized to the dermal papillae of the well-defined follicular structures of grafted ESS. In addition, statistically significant increases in LEF1, WNT10A and WNT10B were found in ESS with neofollicles. These results demonstrate a model for generation of chimeric hair in ESS.

Ectopic production of hair keratin constitutes Rushton's hyaline bodies in association with hematogenous deposits

A Rushton's hyaline body (HB) is a concretion occasionally found in odontogenic cysts. Unspecified substances produced by the lining epithelium or derived from blood components have been suggested as possible causes of HB formation, but the origin of HBs is still elusive. This study aimed to clarify the origin of HBs. Ten specimens with HBs were obtained from 400 odontogenic cysts. HBs were stained by orcein and Congo red. Immunohistochemical examination revealed that HBs were positive for hair keratin and keratin 17. Hair keratin was concentrated in HBs, and cells with hair keratin expression were hardly seen, while cells with keratin 17 expression were observed near HBs. HBs were also positive for hemoglobin alpha chain. The presence of hair keratin in HBs was confirmed by Western blot analysis. The present study suggests that HBs are formed as a consequence of two independent events: unusual alteration of epithelial differentiation so as to provide hair keratin, and hemorrhage so as to provide erythrocytic substances. Although the ectopic production of hair keratin appears more essential, our results reconcile the long-standing debate between two theories, the keratin theory versus the hematogenous theory, concluding that both substances are required for the genesis of HBs, and also suggesting that they might be novel non-pathological amyloidogenic proteins.

Detection of a novel missense mutations in atrichia with papular lesions

BACKGROUND

Atrichia with papular lesions (APL) is a rare inherited disease characterized by early onset of total hair loss, followed by papular lesions over the extensor areas of the body. Recently, mutations in the human hairless (HR) gene have been implicated in its pathogenesis. The identification of mutations in the HR gene is important for differentiating between APL and alopecia universalis (AU).

OBJECTIVE

We compared the HR genes of patients with presumed AU who showed minimal or no response to treatment with the HR genes of healthy controls.

METHODS

The subjects were 11 patients with presumed AU who had not responded to treatments. Fifty healthy people were included as controls for molecular analysis. To screen for mutations, polymerase chain reaction was performed.

RESULTS

DNA analysis identified a novel heterozygous G-to-A transition at nucleotide position 191 in exon 5. The mutation was not found in the controls, other AU patients, or any unaffected family members except for the patients' mother and maternal grandfather, who were heterozygous HR gene carriers.

CONCLUSION

Our study identifies a novel missense mutation in exon 5 of the HR gene in a Korean APL patient previously diagnosed as AU.

The autoimmune regulator (AIRE), which is defective in autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy patients, is expressed in human epidermal and follicular keratinocytes and associates with the intermediate filament protein cytokeratin 17

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome, which is caused by mutation of the autoimmune regulator (AIRE) gene, is a highly variable disease characterized by multiple endocrine failure, chronic mucocutaneous candidiasis, and various ectodermal defects. AIRE is a transcriptional regulator classically expressed in medullary thymic epithelial cells, monocytes, macrophages, and dendritic cells. Previous studies have suggested that AIRE can shuttle between the nucleus and cytoplasm of cells, although its cytoplasmic functions are poorly characterized. Through mass spectrometry analysis of proteins co-immunoprecipitating with cytoplasmic AIRE, we identified a novel association of AIRE with the intermediate filament protein cytokeratin 17 (K17) in the THP-1 monocyte cell line. We confirmed AIRE expression in HaCaT epidermal keratinocytes, as well as its interaction with K17. Confocal microscopy of human fetal and adult scalp hair follicles demonstrated a cytoplasmic pattern of AIRE staining that moderately colocalized with K17. The cytoplasmic association of AIRE with the intermediate filament network in human epidermal and follicular keratinocytes may provide a new path to understanding the ectodermal abnormalities associated with the APECED syndrome.

Adult epidermal Notch activity induces dermal accumulation of T cells and neural crest derivatives through upregulation of jagged 1

Notch signalling regulates epidermal differentiation and tumour formation via non-cell autonomous mechanisms that are incompletely understood. This study shows that epidermal Notch activation via a 4-hydroxy-tamoxifen-inducible transgene caused epidermal thickening, focal detachment from the underlying dermis and hair clumping. In addition, there was dermal accumulation of T lymphocytes and stromal cells, some of which localised to the blisters at the epidermal-dermal boundary. The T cell infiltrate was responsible for hair clumping but not for other Notch phenotypes. Notch-induced stromal cells were heterogeneous, expressing markers of neural crest, melanocytes, smooth muscle and peripheral nerve. Although Slug1 expression was expanded in the epidermis, the stromal cells did not arise through epithelial-mesenchymal transition. Epidermal Notch activation resulted in upregulation of jagged 1 in both epidermis and dermis. When Notch was activated in the absence of epidermal jagged 1, jagged 1 was not upregulated in the dermis, and epidermal thickening, blister formation, accumulation of T cells and stromal cells were inhibited. Gene expression profiling revealed that epidermal Notch activation resulted in upregulation of several growth factors and cytokines, including TNFα, the expression of which was dependent on epidermal jagged 1. We conclude that jagged 1 is a key mediator of non-cell autonomous Notch signalling in skin.

Defining the complex epithelia that comprise the canine claw with molecular markers of differentiation

Canine claws are complex epithelial structures resembling the mammalian hair fibre, and human nail plate, in terms of tissue-specific differentiation. They are composed of several distinct epithelial cell lineages undergoing either hard or soft keratinization. The claw plate has three distinct regions: stratum externum, stratum medium (SM) and stratum internum and the underside and tip are cushioned by a soft keratinizing epithelium, the sole. We have examined keratin expression in the canine claw and associated epithelia. Digits from German shepherd dogs were decalcified, processed and sectioned by sledge microtome. Sections were stained with haematoxylin and eosin or treated with specific antibodies to various keratins (immunohistochemistry). Proteins were extracted from claw components and analysed by SDS-PAGE and Western blotting. The keratinized canine claw plate expressed hair-specific keratins (type I, K25-K38 and type II, K71-K86) but only the inner region of the SM contained K6- and K16-positive tubules, soft epithelia running through the hard keratinized claw plate. The soft keratinaceous sole epithelium expressed keratins K5, K6, K14, K16 and K17 and contained cells with abundant envelopes. The canine claw had two slippage zones, the inner claw bed, between the claw plate and ungula process, which expressed K17 and the region between the inner and outer claw sheath, equivalent to the hair follicle companion layer, which expressed K6, K77, K16 and K17. In conclusion, several different cell types have been defined in the canine claw presenting a complex mechanism of cellular differentiation.

The keratins of the human beard hair medulla: the riddle in the middle

We have investigated the expression of 52 of the 54 keratins in beard hair medulla. We found that not only 12 hair keratins but, unexpectedly, also 12 epithelial keratins are potentially expressed in medulla cells. The latter comprise keratins also present in outer- and inner-root sheaths and in the companion layer. Keratins K5, K14, K17, K25, K27, K28, and K75 define a "pre-medulla," composed of cells apposed to the upper dermal papilla. Besides K6, K16, K7, K19, and K80, all pre-medullary epithelial keratins continue to be expressed in the medulla proper, along with the 12 hair keratins. Besides this unique feature of cellular keratin co-expression, the keratin pattern itself is highly variable in individual medulla cells. Remarkably, both epithelial and hair keratins behave highly promiscuously with regard to heterodimer- and IF formation, which also includes keratin chain interactions in IF bundles. We also identified cortex cells within the medullary column. These exhibit all the properties of genuine cortex cells, including a particular type of keratin heterogeneity of their compact IF bundles. In both keratin expression profile and keratin number, medulla cells are distinct from all other cells of the hair follicle or from any other epithelium.

Hedgehog signaling, keratin 6 induction, and sebaceous gland morphogenesis: implications for pachyonychia congenita and related conditions

Keratins 6a and b (K6a, K6b) belong to a subset of keratin genes with constitutive expression in epithelial appendages, and inducible expression in additional epithelia, when subjected to environmental challenges or disease. Mutations in K6a or K6b cause a broad spectrum of epithelial lesions that differentially affect nail, hair, and glands in humans. Some lesions reflect a loss of the structural support function shared by K6, other keratins, and intermediate filament proteins. The formation of sebaceous gland-derived epithelial cysts does not fit this paradigm, raising the question of the unique functions of different K6 isoforms in this setting. Here, we exploit a mouse model of constitutively expressed Gli2, a Hedgehog (Hh) signal effector, to show that K6a expression correlates with duct fate in sebaceous glands (SGs). Whether in the setting of Gli2 transgenic mice skin, which develops a prominent SG duct and additional pairs of highly branched SGs, or in wild-type mouse skin, K6a expression consistently coincides with Hh signaling in ductal tissue. Gli2 expression modestly transactivates a K6a promoter-driven reporter in heterologous systems. Our findings thus identify K6 as a marker of duct fate in SGs, partly in response to Hh signaling, with implications for the pathological expansion of SGs that arises in the context of certain keratin-based diseases and related disorders.

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.

Keratin expression provides novel insight into the morphogenesis and function of the companion layer in hair follicles

Hair follicles cycle between stages of growth (anagen) and metabolic quiescence (telogen) throughout life. In mature follicles, transition from telogen back into anagen involves the activation, proliferation, and differentiation of epithelial stem cells located in the bulge, a specialization of the outer root sheath. Recent studies identified keratin 6a (K6a) transcripts as enriched in bulge epithelial stem cells in mouse skin. We used messenger RNA probes, antibodies, a LacZ reporter mouse model, and whole-mount staining assays to investigate the regulation of mK6a during mouse postnatal hair cycling, and compare it to mK75, a companion layer (Cl) marker. We find that mK75 regulation parallels that of inner root sheath (IRS) markers, with expression onset at anagen IIIa above the new hair bulb and subsequent spreading towards the bulge. Although also occurring in the Cl, mK6a expression begins at anagen IIIb in differentiating cells located proximal to the bulge, and subsequently spreads towards the hair bulb. mK6a and mK75 thus exhibit temporally distinct, and spatially opposed, expression patterns in the Cl during postnatal anagen. These findings provide novel insight into the morphogenesis and properties of the Cl, and raise the distinct possibility that it is an integral part of the IRS compartment.

Clinical and Pathological Features of Pachyonychia Congenita

Pachyonychia congenita (PC) is a rare genodermatosis affecting the nails, skin, oral mucosae, larynx, hair, and teeth. Pathogenic mutations in keratins K6a or K16 are associated with the PC-1 phenotype whereas K6b and K17 mutations are associated with the PC-2 phenotype. Analysis of clinical, pathological, and genetic data from the literature and two research registries reveal that >97% of PC cases exhibit fingernail and toenail thickening, and painful plantar keratoderma. Prospective evaluation of 57 PC patients from 41 families revealed variable clinical findings: hyperhidrosis (79%), oral leukokeratosis (75%), follicular keratosis (65%), palmar keratoderma (60%), cutaneous cysts (35%), hoarseness or laryngeal involvement (16%), coarse or twisted hair (26%), early primary tooth loss (14%), and presence of natal or prenatal teeth (2%). Stratification of these data by keratin mutation confirmed the increased incidence of cyst formation and natal teeth among PC-2 patients, although cysts were more commonly seen in PC-1 than previously reported (25%-33%). Previously unreported clinical features of PC include development of painful oral and nipple lesions during breastfeeding, copious production of waxy material in ears, and inability to walk without an ambulatory aid (50%). Possible pathogenic mechanisms are discussed with respect to the clinicopathologic and genetic correlations observed.

Treatment of pachyonychia congenita

There are currently no specific treatments for pachyonychia congenita (PC). Available treatments generally are directed at specific manifestations of the disorder, and an effective treatment plan must recognize that different patients are more or less troubled by different manifestations of the disease. Treatment for all aspects of PC has been less than completely satisfactory. Very few studies have compared different approaches to treatment, and fewer still have given longitudinal follow-up of efficacy and patient acceptance. This review is essentially a compilation of anecdotes. It was collected from physicians' reports in the literature, from direct communication with physicians currently following patients with PC and from patients who answered a questionnaire on the Pachyonychia Congenita Project web page (http://www.pachyonychia.org/Registry.html).

Exploiting the keratin 17 gene promoter to visualize live cells in epithelial appendages of mice

Keratin genes afford, given their large number (>50) and differential regulation, a unique opportunity to study the mechanisms underlying specification and differentiation in epithelia of higher metazoans. Moreover, the small size and regulation in cis of many keratin genes enable the use of their regulatory sequence to achieve targeted gene expression in mice. Here we show that 2 kilobases of 5' upstream region from the mouse keratin 17 gene (mK17) confers expression of green fluorescent protein (GFP) in major epithelial appendages of transgenic mice. Like that of mK17, onset of [mK17 5']-GFP reporter expression coincides with the appearance of ectoderm-derived epithelial appendages during embryonic development. In adult mice, [mK17 5']-GFP is appropriately regulated within hair, nail, glands, and oral papilla. Tracking of GFP fluorescence allows for the visualization of growth cycle-related changes in hair follicles, and the defects engendered by the hairless mutation, in live skin tissue. Deletion of an internal 48-bp interval, which encompasses a Gli-responsive element, from this promoter results in loss of GFP fluorescence in most appendages in vivo, suggesting that sonic hedgehog participates in K17 regulation. The compact mK17 gene promoter provides a novel tool for appendage-preferred gene expression and manipulation in transgenic mice.

Overcoming functional redundancy to elicit pachyonychia congenita-like nail lesions in transgenic mice

Mutations affecting the coding sequence of intermediate filament (IF) proteins account for >30 disorders, including numerous skin bullous diseases, myopathies, neuropathies, and even progeria. The manipulation of IF genes in mice has been widely successful for modeling key features of such clinically distinct disorders. A notable exception is pachyonychia congenita (PC), a disorder in which the nail and other epithelial appendages are profoundly aberrant. Most cases of PC are due to mutations in one of the following keratin-encoding genes: K6, K16, and K17. Yet null alleles obliterating the function of both K6 genes (K6alpha and K6beta) or the K17 gene, as well as the targeted expression of a dominant-negative K6alpha mutant, elicit only a subset of PC-specific epithelial lesions (excluding that of the nail in mice). We show that newborn mice null for K6alpha, K6beta, and K17 exhibit severe lysis restricted to the nail bed epithelium, where all three genes are robustly expressed, providing strong evidence that this region of the nail unit is initially targeted in PC. Our findings point to significant redundancy among the multiple keratins expressed in hair and nail, which can be related to the common ancestry, clustered organization, and sequence relatedness of specific keratin genes.

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.

Keratins and skin disorders

The association of keratin mutations with genetic skin fragility disorders is now one of the best-established examples of cytoskeleton disorders. It has served as a paradigm for many other diseases and has been highly informative for the study of intermediate filaments and their associated components, in helping to understand the functions of this large family of structural proteins. The keratin diseases have shown unequivocally that, at least in the case of the epidermal keratins, a major function of intermediate filaments is to provide physical resilience for epithelial cells. This review article reflects on the variety of phenotypes arising from mutations in keratins and the reasons for this variation.

In vivo alteration of the keratin 17 gene in hair follicles by oligonucleotide-directed gene targeting

Using intradermal injection of a chimeric RNA-DNA oligonucleotide (RDO) or a single-stranded oligonucleotide (ssODN) into murine skin, we attempted to make a dominant mutation (R94p) in the conserve alpha-helical domain of keratin 17 (K17), the same mutation found in pachyononychia congenichia type 2 (PC-2) patients with phenotypes ranging from twisted hair and multiple pilosebaceous cysts. Both K17A-RDO and -ssODN contained a single base mismatch (CGC to CCC) to alter the normal K17 sequence to cause an amino acid substitution (R94P). The complexes consisting of oligonucleotides and cationic liposomes were injected to C57B1/6 murine skin at 2 and 5 day after birth. Histological examination of skin biopsies at postnatal day 8 from several mice showed consistent twisted hair shafts or broken hair follicles at the sebaceous gland level and occasional rupture of the hair bulb or epidermal cyst-like changes. In the injected area, the number of full anagen hair follicles decrease by 50%. Injection of the control oligonucleotide, identical to K17A-RDO but containing no mismatch to the normal sequence, did not result in any detectable abnormality. The frequency of gene alteration was lower than 3%, according to the restriction fragment length polymorphism (RFLP) analysis of the genomic DNA isolated by dissection of hair follicles from slides. Although intradermal injection of K17A-RDO or K17-ssODN caused a dominant mutation in K17 affecting hair growth and morphology, these phenotypic changes were transient either due to the compensation of K17 by other keratins or the replacement of the mutated cells by normal surrounding cells during hair growth.

A novel mouse type I intermediate filament gene, keratin 17n (K17n), exhibits preferred expression in nail tissue

Inactivating the type I keratin 17 gene (mK17) causes severe but reversible hair loss in a strain-dependent fashion in mouse (McGowan et al, Genes Dev. 16:1412, 2002). Missense mutations in human K17 give rise to two dominantly inherited disorders apparented to ectodermal dysplasias, pachyonychia congenita (PC), and steatocystoma multiplex (SM). In contrast to the null phenotype in mouse, marked lesions are seen in the nail and nail bed and sebaceous glands of PC and SM patients, respectively. In an effort to understand the lack of nail involvement in mK17 null mice, we discovered that the gene located immediately 5' upstream from mK17 is functional and encodes a type I keratin protein highly analogous to mK17. mRNA and protein localization studies show that the expression of this novel gene is highly restricted and most prevalent in the nail bed and matrix, leading to its designation as mK17n (n stands for nail). Weak expression of mK17n also occurs in vibrissae follicles, in filiform and fungiform papillae of oral mucosa. These findings have direct implications for the mK17 null phenotype. Depending on the existence of a human ortholog or a functional equivalent, our findings may also provide a molecular explanation for several unusual aspects of hK17-based diseases.

Androgen regulation of the human hair follicle: the type I hair keratin hHa7 is a direct target gene in trichocytes

Previous work had shown that most members of the complex human hair keratin family were expressed in terminal scalp hairs. An exception to this rule was the type I hair keratin hHa7, which was only detected in some but not all vellus hairs of the human scalp (Langbein et al, 1999). Here we show that hHa7 exhibits constitutive expression in medullary cells of all types of male and female sexual hairs. Medullated beard, axillary, and pubic hairs arise during puberty from small, unmedullated vellus hairs under the influence of circulating androgens. This suggested an androgen-controlled expression of the hHa7 gene. Further evidence for this assumption was provided by the demonstration of androgen receptor (AR) expression in the nuclei of medullary cells of beard hairs. Moreover, homology search for the semipalindromic androgen receptor-binding element (ARE) consensus sequence GG(A)/(T)ACAnnnTGTTCT in the proximal hHa7 promoter revealed three putative ARE motifs. Electrophoretic mobility shift assays demonstrated the specific binding of AR to all three hHa7 AREs. Their function as AR-responsive elements, either individually or in concert within the hHa7 promoter, could be further confirmed by transfection studies with or without an AR expression vector in PtK2 and prostate PC3-Arwt cells, respectively in the presence or absence of a synthetic androgen. Our study detected hHa7 as the first gene in hair follicle trichocytes whose expression appears to be directly regulated by androgens. As such, hHa7 represents a marker for androgen action on hair follicles and might be a suitable tool for investigations of androgen-dependent hair disorders.

A Novel Mutation in the Second Half of the Keratin 17 1A Domain in a Large Pedigree with Delayed-Onset Pachyonychia Congenita Type 2

Pachyonychia congenita type 2 (PC-2), also known as Jackson-Lawler type PC, is an autosomal dominant disorder characterized by hypertrophic nail dystrophy associated with focal keratoderma and multiple pilosebaceous cysts. We report a large Chinese pedigree of typical delayed-onset PC-2 that includes 19 affected members. Direct sequencing of PCR products revealed a novel heterozygous 325A-->G mutation in the affected members. This mutation predicts the substitution of asparagine by aspartic acid in codon 109 (N109D) located in the second half of the keratin 17 1A domain, where similar mutation in keratin 5 is associated with the mild Weber-Cockayne form of epidermolysis bullosa simplex.

Type II epithelial keratin 6hf (K6hf) is expressed in the companion layer, matrix, and medulla in anagen-stage hair follicles

More than half of the known keratin genes (n approximately 50) are expressed in the hair follicle. An in-depth knowledge of their differential expression in this organ will help us to understand the mechanisms of its formation and cycling, and the etiology of inherited hair disorders. Keratin 6hf is a type II keratin recently shown to occur in the companion layer. We cloned the mouse ortholog and characterized its expression in skin and oral mucosa. The mK6hf gene is 9.1 kb long and located in the cluster of type II keratin genes on mouse chromosome 15, between the keratin 6 (mK6alpha/mK6beta) and hair keratin genes. In situ hybridization and protein immunolocalization showed that, in addition to the companion layer, mK6hf is expressed in the upper matrix and medulla of the anagen-stage hair. This distribution is seen for all types of mouse hairs and medullated human hairs. The distribution of keratin 6hf protein in the hair shaft mirrors that of keratin 17, and the observation of reduced levels of keratin 6hf in keratin 17 null hair argues for a direct interaction between them. mK6hf is also expressed in the nail bed epithelium and fungiform papillae of dorsal tongue epithelium. Our findings provide an additional marker for the hair matrix and medulla, and suggest that the cellular precursors for the medulla, cortex, and cuticle compartments are already spatially segregated within the hair matrix. They also have obvious implications for the epithelial alterations associated with defects in keratin 6 genes.

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.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

The molecular genetics of keratin disorders

Keratins are the type I and II intermediate filament proteins which form a cytoskeletal network within all epithelial cells. They are expressed in pairs in a tissue- and differentiation-specific fashion. Epidermolysis bullosa simplex (EBS) was the first human disorder to be associated with keratin mutations. The abnormal keratin filament aggregates observed in basal cell keratinocytes of some EBS patients are composed of keratins K5 and K14. Dominant mutations in the genes encoding these proteins were shown to disrupt the keratin filament cytoskeleton resulting in cells that are less resilient and blister with mild physical trauma. Identification of mutations in other keratin genes soon followed with attention focussed on disorders showing abnormal clumping of keratin filaments in specific cells. For example, in bullous congenital ichthyosiform erythroderma, clumping of filaments in the suprabasal cells led to the identification of mutations in the suprabasal keratins, K1 and K10. Mutations have now been identified in 18 keratins, all of which produce a fragile cell phenotype. These include ichthyosis bullosa of Siemens (K2e), epidermolytic palmoplantar keratoderma (K1, K9), pachyonychia congenita (K6a, K6b, K16, K17), white sponge nevus (K4, K13), Meesmann's corneal dystrophy (K3, K12), cryptogenic cirrhosis (K8, K18) and monilethrix (hHb6, hHb1).In general, these disorders are inherited as autosomal dominant traits and the mutations act in a dominant-negative manner. Therefore, treatment in the form of gene therapy is difficult, as the mutant gene needs to be inactivated. Ways of achieving this are actively being studied. Reliable mutation detection methods from genomic DNA are now available. This enables rapid screening of patients for keratin mutations. For some of the more severe phenotypes, prenatal diagnosis may be requested and this can now be performed from chorionic villus samples at an early stage of the pregnancy. This review article describes the discovery of, to date, mutations in 18 keratin genes associated with inherited human diseases.

Expression patterns of the transcription factor AP-2alpha during hair follicle morphogenesis and cycling

AP-2alpha is a member of a family of transcription factors expressed in cells of the epithelial and neural crest lineage. AP-2alpha plays an essential role in embryonic development and in regulation of epithelial gene transcription. To further characterize the role of AP-2alpha in skin biology, we assessed its expression in the skin of C57BL/6J mice during defined stages of hair follicle morphogenesis and cycling. During early hair follicle morphogenesis, AP-2alpha was upregulated in the epidermal placode, in the basal keratinocytes of the hair follicle bud, and then in the inner root sheath. The follicular papilla cells underwent a brief upregulation of AP-2alpha expression during the initiation of hair shaft formation and active hair follicle downward growth. Completion of hair follicle morphogenesis was associated with a marked reduction of AP-2alpha immunoreactivity in the lower portion of the hair follicle including both epithelial and mesenchymal compartments. In adolescent mouse skin, consistently strong AP-2alpha expression was found in the basal keratinocytes of the epidermis, in the hair follicle infundibulum, and in the sebocytes. In the follicular papilla, AP-2alpha was weakly expressed in telogen, significantly upregulated in early anagen, then gradually declined, and reappeared again in middle catagen. In the inner root sheaths, AP-2alpha expression was detected during early and middle anagen and during middle catagen stages. Prominent AP-2alpha expression was also seen in the zone of club hair formation. Therefore, AP-2alpha upregulation in both epithelial and mesenchymal hair follicle compartments was coordinated with initiation of major remodeling processes. Our findings support the use of the hair follicle as a model to explore the role of AP-2alpha in physiologic remodeling of developing organs and in reciprocal ectodermal-mesenchymal interactions.

Hair follicle stem cells

The workshop on Hair Follicle Stem Cells brought together investigators who have used a variety of approaches to try to understand the biology of follicular epithelial stem cells, and the role that these cells play in regulating the hair cycle. One of the main concepts to emerge from this workshop is that follicular epithelial stem cells are multipotent, capable of giving rise not only to all the cell types of the hair, but also to the epidermis and the sebaceous gland. Furthermore, such multipotent stem cells may represent the ultimate epidermal stem cell. Another example of epithelial stem cell and transit amplifying cell plasticity, was the demonstration that adult corneal epithelium, under the influence of embryonic skin dermis could form an epidermis as well as hair follicles. With regards to the location of follicular epithelial stem cells, immunohistochemical and ultrastructural data was presented, indicating that cells with stem cell attributes were localized to the prominent bulge region of developing human fetal hair follicles. Finally, a new notion was put forth concerning the roles that the bulge-located stem cells and the hair germ cells played with respect to the hair cycle.

Optimal management of hair loss (alopecia) in children

Hair loss in children encompasses a wide range of conditions that can be congenital or acquired. A congenital hair abnormality may be an isolated finding in an otherwise healthy child or may exist as a feature of a clinical syndrome. A thorough understanding of basic hair biology and normal hair development enables accurate assessment of the child with hair loss. Knowledge of the normal range and variation observed in children's hair additionally enhances this assessment. Social and cultural factors also influence these norms. The psychological and cosmetic importance of hair is immense in our society. The clinical presentation of pediatric hair disorders ranges from subtle to disfiguring. Management of hair disorders requires a holistic approach to the child and family. Young children usually lack self-awareness and it may be the parent who, projecting their own concerns onto the child, most acutely feels any associated anxiety. In addition, parents of a child with an inherited hair condition often feel guilt, and siblings can develop unsupported fears that they may be affected. Hair loss for the older child can lead to low self-esteem, depression and humiliation. Congenital and hereditary hypotrichosis and hair shaft abnormalities often have no effective treatment. There is a variety of treatment options for alopecia areata and telogen effluvium, but no single treatment is 100% effective. Tinea capitis is an infective condition of the hair that responds readily to the appropriate medical therapy. If no effective treatment for the hair loss exists, cosmetic camouflage with wigs is the best option.

Keratin 16 expression defines a subset of epithelial cells during skin morphogenesis and the hair cycle

The morphogenesis of skin epithelia and adult hair follicle cycling both require integrated signaling between the epithelium and underlying mesenchyme. Because of their unique regulation, keratin intermediate filaments represent useful markers for the analysis of determination and differentiation processes in complex epithelia, such as the skin. In this study, we analyzed the distribution of mouse type I keratin 16 during skin morphogenesis, in the adult hair cycle, and in challenged epidermis. In mature hair follicles, we find keratin 16 along with its type II keratin partner keratin 6 in the companion layer of the outer root sheath during anagen and in the club hair sheath during catagen and telogen. During embryonic development, the distribution of keratin 16 is uncoupled from its presumed polymerization partner, keratin 6. Keratin 16 initially localizes within early hair germs, but rapidly shifts to a subset of cells at the interface of basal and suprabasal cells above and around the hair germ. The presence of keratin 16 at the transition between mitotically active and differentiating cells is recapitulated in primary keratinocytes cultured in vitro and in phorbol 12-myristate 13-acetate-treated back skin in vivo. We propose that keratin 16 marks cells in an intermediate state of cellular properties in which keratinocytes retain the flexibility required for activities such as cell migration and even mitosis but are resilient enough to provide the structural integrity required of the early suprabasal layers in the context of development, adult hair cycling, and wound repair.