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

Merkel Cell Polyomavirus and Merkel Cell Carcinoma

Viruses are the cause of approximately 15% of all human cancers. Both RNA and DNA human tumor viruses have been identified, with Merkel cell polyomavirus being the most recent one to be linked to cancer. This virus is associated with about 80% of Merkel cell carcinomas, a rare, but aggressive cutaneous malignancy. Despite its name, the cells of origin of this tumor may not be Merkel cells. This review provides an update on the structure and life cycle, cell tropism and epidemiology of the virus and its oncogenic properties. Putative strategies to prevent viral infection or treat virus-positive Merkel cell carcinoma patients are discussed.

Polyomavirus-Positive Merkel Cell Carcinoma Derived from a Trichoblastoma Suggests an Epithelial Origin of this Merkel Cell Carcinoma

Merkel cell carcinoma (MCC), an aggressive neuroendocrine carcinoma of the skin, is to date the only human cancer known to be frequently caused by a polyomavirus. However, it is a matter of debate which cells are targeted by the Merkel cell polyomavirus (MCPyV) to give rise to the phenotypically multifaceted MCC cells. To assess the lineage of origin of MCPyV-positive MCC, genetic analysis of a very rare tumor combining benign trichoblastoma and MCPyV-positive MCC was conducted by massive parallel sequencing. Although MCPyV was found to be integrated only in the MCC part, six somatic mutations were shared by both tumor components. The mutational overlap between the trichoblastoma and MCPyV-positive MCC parts of the combined tumor implies that MCPyV integration occurred in an epithelial tumor cell before MCC development. Therefore, our report demonstrates that MCPyV-positive MCC can derive from the epithelial lineage.

The Equine Hoof: Laminitis, Progenitor (Stem) Cells, and Therapy Development

The equine hoof capsule, composed of modified epidermis and dermis, is vital for protecting the third phalanx from forces of locomotion. There are descriptions of laminitis, defined as inflammation of sensitive hoof tissues but recognized as pathologic changes with or without inflammatory mediators, in the earliest records of domesticated horses. Laminitis can range from mild to serious, and signs can be acute, chronic, or transition from acute, severe inflammation to permanently abnormal tissue. Damage within the intricate dermal and epidermal connections of the primary and secondary lamellae is often associated with lifelong changes in hoof growth, repair, and conformation. Decades of research contribute to contemporary standards of care that include systemic and local therapies as well as mechanical hoof support. Despite this, consistent mechanisms to restore healthy tissue formation following a laminitic insult are lacking. Endogenous and exogenous progenitor cell contributions to healthy tissue formation is established for most tissues. There is comparably little information about equine hoof progenitor cells. Equine hoof anatomy, laminitis, and progenitor cells are covered in this review. The potential of progenitor cells to advance in vitro equine hoof tissue models and translate to clinical therapies may significantly improve prevention and treatment of a devastating condition that has afflicted equine companions throughout history.

Hair Follicle Stem Cell Isolation and Expansion

Stem cells are widely used for numerous clinical applications including limbal stem cell deficiency. Stem cell derived from the bulge region of the hair follicle have the ability to differentiate into a variety of cell types including interfollicular epidermis, hair follicle structures, sebaceous glands and corneal epithelial cells when provided the appropriate cues. Hair follicle stem cells are being studied as a valuable source of autologous stem cells to treat disease. The protocol described below details the isolation and expansion of these cells for eventual clinical application. We used a dual-reporter mouse model to visualize both isolation and eventual differentiation of these cells in a limbal stem cell-deficient mouse model.

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

Adherens junctions (AJs) and desmosomes connect the actin and keratin filament networks of adjacent cells into a mechanical unit. Whereas AJs function in mechanosensing and in transducing mechanical forces between the plasma membrane and the actomyosin cytoskeleton, desmosomes and intermediate filaments (IFs) provide mechanical stability required to maintain tissue architecture and integrity when the tissues are exposed to mechanical stress. Desmosomes are essential for stable intercellular cohesion, whereas keratins determine cell mechanics but are not involved in generating tension. Here, we summarize the current knowledge of the role of IFs and desmosomes in tissue mechanics and discuss whether the desmosome-keratin scaffold might be actively involved in mechanosensing and in the conversion of chemical signals into mechanical strength.

Comparative study of cytokeratin and langerin expression in keratinized cystic lesions of the oral and maxillofacial regions

Dermoid cysts (DMCs) and epidermoid cysts (EDMCs) usually arise in soft tissues, whereas orthokeratinized odontogenic cysts (OOCs) and keratocystic odontogenic tumors (KCOTs) develop in the jaw. In this study, we performed immunohistochemical analysis of cytokeratins (CKs) to examine differences in the lining epithelium of DMCs, EDMCs, OOCs, and KCOTs. In addition, we carried out immunohistochemical examination of langerin to clarify the biological characteristics of the orthokeratinized lining epithelium of DMCs, EDMCs, and OOCs. Seven DMCs, 30 EDMCs, 11 OOCs, and 28 KCOTs were examined immunohistochemically using antibodies against CK10, 13, 14, 16, 17, 19, and langerin. Immunoreactivities for CKs and langerin in oral DMCs and EDMCs were similar to those of lesions affecting the skin. Positive reactivity for CK13 and 17 was evident in OOCs, but not in DMCs/EDMCs. CK10 was significantly positive in all layers except for the basal layer in OOCs, but was negative in KCOTs. CK17 was positive in all layers in KCOTs, and in all layers except for the basal layer in both OOCs and dentigerous cysts. CK19 was negative in OOCs. Langerhans cells were found mainly in OOCs, but were hardly evident in KCOTs. These results suggest that DMCs/EDMCs, OOCs and KCOTs are independent diseases.

Canonical and non-canonical Wnt signaling control the regeneration of amputated rodent vibrissae follicles

Although mammals are notoriously poor at regeneration compared with many lower-order species, the hair follicle, particular to mammals, is capable of regeneration following partial amputation. The detailed internal mechanism of this phenomenon is still unclear. Development and regrowth of the hair follicle depends on dermal-epidermal interaction within the hair follicle. Previous studies have shown that Wnt/β-catenin, Shh, Bmp, PDGF, TGF and Notch signals all take part in the development and growth of the hair follicle, and the Wnt/β-catenin signaling additionally plays an indispensable role in hair follicle morphogenesis and regrowth. In this study, we investigated the localization, as well as, protein levels of Wnt/β-catenin signaling molecules during amputated whisker follicle regeneration.

Expression of Wnt/β-catenin signaling, stem-cell markers and proliferating cell markers in rat whisker hair follicles

The rat whisker hair follicle (HF) is a model for studying the reconstruction of the HF or dermal papilla (DP), and involves the Wnt/β-catenin signaling pathway, which is a key pathway in HF development and HF cycling after birth. It has been reported that Wnt/catenin signaling plays an indispensable role in human or rat pelages development and postnatal growth. However, the distribution of some Wnt/β-catenin signaling pathway factors and their relationship with the epithelial stem cell markers in whisker follicles has not been characterized. In this study, we investigated the immunolocalization of Wnt/catenin signaling pathway members, including Wnt10b, Wnt10a, Wnt5a, β-catenin, and downstream lymphoid enhancer-binding factor 1 (LEF1) and transcription factor 3 (TCF3), as well as, HF stem-cell markers CD34, CK15 and proliferating cell nuclear antigen (PCNA) protein, in rat anagen phase whisker follicles. β-catenin, Wnt5a, Wnt10b, Wnt10a, LEF1, and TCF3 were expressed in the outer root sheath (ORS), inner root sheath, matrix and hair shaft of anagen follicles. β-catenin, Wnt10b, LEF1, and TCF3 were highly expressed and Wnt5a and Wnt10a weakly expressed in DP and dermal sheath (DS) regions. The expression of α-smooth muscle actin was strong in the lower DS and it was also detected in some DP cells. CD34, CK15 and PCNA were all expressed in the ORS; and CD34 and PCNA were also detected in the matrix, however CD34 was extensively expressed in DP and DS regions. Our studies located the position of Wnts, downstream LEF1 and TCF3 and stem cell marker proteins, which provide new information in understanding the role of the Wnt singaling pathway in whisker follicles' growth.

Epidermal healing in burns: autologous keratinocyte transplantation as a standard procedure: update and perspective

Background:

Treatment of burned patients is a tricky clinical problem not only because of the extent of the physiologic abnormalities but also because of the limited area of normal skin available.

Methods:

Literature indexed in the National Center (PubMed) has been reviewed using combinations of key words (burns, children, skin graft, tissue engineering, and keratinocyte grafts). Articles investigating the association between burns and graft therapeutic modalities have been considered. Further literature has been obtained by analysis of references listed in reviewed articles.

Results:

Severe burns are conventionally treated with split-thickness skin autografts. However, there are usually not enough skin donor sites. For years, the question of how covering the wound surface became one of the major challenges in clinical research area and several procedures were proposed. The microskin graft is one of the oldest methods to cover extensive burns. This technique of skin expansion is efficient, but results remain inconsistent. An alternative is to graft cultured human epidermal keratinocytes. However, because of several complications and labor-intensive process of preparing grafts, the initial optimism for cultured epithelial autograft has gradually declined. In an effort to solve these drawbacks, isolated epithelial cells from selecting donor site were introduced in skin transplantation.

Conclusions:

Cell suspensions transplanted directly to the wound is an attractive process, removing the need for attachment to a membrane before transfer and avoiding one potential source of inefficiency. Choosing an optimal donor site containing cells with high proliferative capacity is essential for graft success in burns.

Study of proliferation and 3D epidermal reconstruction from foreskin, auricular and trunk keratinocytes in children

OBJECTIVE

Severe burns in children are conventionally treated with split-thickness skin autografts or epidermal sheets. An alternative approach is to graft isolated keratinocytes. We evaluated foreskin and other anatomic sites as donor sources for autologous keratinocyte graft in children. We studied in vitro capacities of isolated keratinocytes to divide and reconstitute epidermal tissue.

METHODS

Keratinocytes were isolated from foreskin, auricular skin, chest and abdominal skin by enzymatic digestion. Living cell recovery, in vitro proliferation, epidermal reconstruction capacities and differentiation status were analyzed.

RESULTS

In vitro studies revealed the higher yield of living keratinocyte recovery from foreskin and higher potential in terms of proliferative capacity, regeneration and differentiation. Cultured keratinocytes from foreskin express lower amounts of differentiation markers than those isolated from trunk and ear. Histological analysis of reconstituted human epidermis derived from foreskin and inguinal keratinocytes showed a structured multilayered epithelium, whereas those obtained from ear pinna-derived keratinocytes were unstructured.

CONCLUSION

Our studies highlight the potential of foreskin tissue for autograft applications in boys. A suitable alternative donor site for autologous cell transplantation in female paediatric burn patients remains an open question in our department. We tested the hypothesis that in vitro studies and RHE reconstructive capacities of cells from different body sites can be helpful to select an optimal site for keratinocyte isolation before considering graft protocols for girls.

Effect of intense pulsed light treatment on human skin in vitro: analysis of immediate effects on dermal papillae and hair follicle stem cells

BACKGROUND

Hair follicles house a permanent pool of epithelial stem cells. Intense pulsed light (IPL) sources have been successfully used for hair removal, but long-term hair reduction may require several treatments. Many questions remain regarding the impact of IPL treatment on the structure of the hair follicle, more specifically on hair follicular stem cells and dermal papilla cells, a group of specialized cells that orchestrate hair growth.

OBJECTIVES

To characterize the destruction of human hair follicles and surrounding tissues following IPL treatment, with more attention paid to the bulge and the bulb regions.

METHODS

Human scalp specimens of Fitzpatrick skin phototype II were exposed ex vivo to IPL pulses and were then processed for histological analysis, immunodetection of stem cell-associated keratin 19, and revelation of the endogenous alkaline phosphatase activity expressed in dermal papilla cells.

RESULTS

Histological analysis confirmed that pigmented structures, such as the melanin-rich matrix cells of the bulb in anagen follicles and the hair shaft, are principally targeted by IPL treatment, while white hairs and epidermis remained unaffected. Damage caused by heat sometimes extended over the dermal papilla cells, while stem cells were mostly spared.

CONCLUSIONS

IPL epilation principally targets pigmented structures. Our results suggest that, under the tested conditions, collateral damage does not deplete stem cells. Damage at the dermal papilla was observed only with high-energy treatment modalities. Extrapolated to frequently treated hairs, these observations explain why some hairs grow back after a single IPL treatment.

The immune system of mouse vibrissae follicles: cellular composition and indications of immune privilege

Although vibrissae hair follicles (VHFs) have long been a key research model in the life sciences, their immune system (IS) is essentially unknown. Therefore, we have characterized basic parameters of the VHF-IS of C57BL/6J mice by quantitative (immuno-)histomorphometry. Murine anagen VHF harbour few CD4+ and CD8+ T cells in the distal mesenchyme and sinuses but hardly any gamma-delta T cells in their distal epithelium. MHC class II+ Langerhans cells are seeded in the VHF infundibulum, which is also surrounded by MHC class II+ and CD11b+ cells (macrophages). The number of Langerhans cells then declines sharply in the VHF bulge, and the VHF bulb lacks MHC class II+ cells. Mast cells densely populate the VHF connective tissue sheath, where they strikingly cluster around the bulge. Both the bulge and the bulb of VHF display signs of immune privilege, that is, low MHC class I and MHC class II expression and local immunoinhibitor expression (CD200, TGFβ1). This immunophenotyping study fills an important gap in the immunobiology of murine skin and identifies differences between the IS of VHF, mouse pelage and human terminal HFs. This facilitates utilizing murine VHF as a versatile organ culture model for general immunology and immune privilege research in situ.

Beyond expectations: novel insights into epidermal keratin function and regulation

The epidermis is a stratified epithelium that relies on its cytoskeleton and cell junctions to protect the body against mechanical injury, dehydration, and infections. Keratin intermediate filament proteins are involved in many of these functions by forming cell-specific cytoskeletal scaffolds crucial for the maintenance of cell and tissue integrity. In response to various stresses, the expression and organization of keratins are altered at transcriptional and posttranslational levels to restore tissue homeostasis. Failure to restore tissue homeostasis in the presence of keratin gene mutations results in acute and chronic skin disorders for which currently no rational therapies are available. Here, we review the recent progress on the role of keratins in cytoarchitecture, adhesion, signaling, and inflammation. By focusing on epidermal keratins, we illustrate the contribution of keratin isotypes to differentiated epithelial functions.

Stem cells of the skin and cornea: their clinical applications in regenerative medicine

PURPOSE OF REVIEW

The use of stem cells is of great interest for the treatment of various pathologies and ultimately for the restoration of organ function. Progress pointing towards future treatments of skin and corneal epithelial stem cell defects are reviewed, including the transplantation of living tissue-engineered substitutes.

RECENT FINDINGS

This article focuses on substitutes optimized for permanent replacement of skin and cornea. New skin substitutes for burn care are currently under development. More complex tissue-engineered skin substitutes in which stroma, adipose tissue, capillaries, and neurons are combined with the epithelium are being developed. Some dermal/epidermal substitutes have been applied to the treatment of patients. Cultured corneal epithelial cells have been characterized and more complete corneal substitutes are being designed. Long-term clinical results on the transplantation of cultured corneal stem cells for the treatment of limbal stem cell deficiency have been reported.

SUMMARY

Advances in tissue engineering for the development of substitutes that will benefit patients suffering from skin or corneal stem cell deficiencies are reviewed. These products are often a combination of cells, scaffolds and other factors. Key considerations in the development of corneal and skin substitutes for clinical applications are discussed.

Transcriptional dissection of human limbal niche compartments by massive parallel sequencing

Corneal epithelium is maintained throughout life by well-orchestrated proliferation of limbal epithelial stem cells (LESCs), followed by migration and maturation centripetally towards the ocular surface. Disturbance of LESCs can potentially lead to a blinding condition, which can be reversed by reconstitution of a functional LESC pool. The current clinical procedures are effective to some degree, however, deeper knowledge of the molecular interplay within the limbal niche is necessary to achieve a fully satisfactory patient outcome. The present study was thus undertaken to carry out a comprehensive transcriptome analysis of four distinct human limbal compartments, including basal limbal crypts (BLCs), superficial limbal crypts (SLCs), cornea, and the supporting stroma, with the aid of laser capture microdissection and deep RNA sequencing. The tissue harvest pipeline was rigorously optimized so that the exposure to cold ischemia would be less than five minutes. The global gene ontology analysis confirmed existence of primitive cells in BLCs, migratory and activated cells in SLCs, and differentiated cells in cornea. Interestingly, many significantly upregulated genes in SLCs mapped to processes involved in regulation of vasculature, such as sFLT1. In contrast, BLCs exhibited many genes mapping to neurogenic processes and processes related to cell development. The primitive nature of BLCs was, furthermore, confirmed by the KEGG pathway analysis, and some potential regulators of LESCs were revealed, such as Lrig1 and SOX9. The analysis also yielded comprehensive lists of uniquely expressed genes in both BLCs and cornea, which may be useful to identify possible biomarkers. In conclusion, the current investigation provides new insight into the relationship between distinct cell populations within the limbal niche, identifies candidates to be verified for novel biological functions, and yields a wealth of information for prospective data mining.

Foreskin-isolated keratinocytes provide successful extemporaneous autologous paediatric skin grafts

Severe burns in children are conventionally treated with split-thickness skin autografts or epidermal sheets. However, neither early complete healing nor quality of epithelialization is satisfactory. An alternative approach is to graft isolated keratinocytes. We evaluated paediatric foreskin and auricular skin as donor sources, autologous keratinocyte transplantation, and compared the graft efficiency to the in vitro capacities of isolated keratinocytes to divide and reconstitute epidermal tissue. Keratinocytes were isolated from surgical samples by enzymatic digestion. Living cell recovery, in vitro proliferation and epidermal reconstruction capacities were evaluated. Differentiation status was analysed, using qRT-PCR and immunolabelling. Eleven children were grafted with foreskin-derived (boys) or auricular (girls) keratinocyte suspensions dripped onto deep severe burns. The aesthetic and functional quality of epithelialization was monitored in a standardized way. Foreskin keratinocyte graft in male children provides for the re-epithelialization of partial deep severe burns and accelerates wound healing, thus allowing successful wound closure, and improves the quality of scars. In accordance, in vitro studies have revealed a high yield of living keratinocyte recovery from foreskin and their potential in terms of regeneration and differentiation. We report a successful method for grafting paediatric males presenting large severe burns through direct spreading of autologous foreskin keratinocytes. This alternative method is easy to implement, improves the quality of skin and minimizes associated donor site morbidity. In vitro studies have highlighted the potential of foreskin tissue for graft applications and could help in tissue selection with the prospect of grafting burns for girls.

Human epithelial stem cells persist within tissue-engineered skin produced by the self-assembly approach

To adequately and permanently restore organ function after grafting, human tissue-engineered skin substitutes (TESs) must ultimately contain and preserve functional epithelial stem cells (SCs). It is therefore essential that a maximum of SCs be preserved during each in vitro step leading to the production of TESs such as the culture process and the elaboration of a skin cell bank by cryopreservation. To investigate the presence and functionality of epithelial SCs within the human TESs made by the self-assembly approach, slow-cycling cells were identified using 5'-bromo-2'-deoxyuridine (BrdU) in the three-dimensional construct. A subset of basal epithelial cells retained the BrdU label and was positive for the SC-associated marker keratin 19 within TESs after a chase of 21 days in culture post-BrdU labeling. Moreover, keratinocytes harvested from TESs gave rise to SC-like colonies in secondary monolayer subcultures, indicating that SCs were preserved within TESs. To evaluate the effect of cryopreservation with dimethyl sulfoxide and storage in liquid nitrogen on SCs, human epithelial cells were extracted from skin samples, amplified in culture, and used to produce TESs, before cryopreservation as well as after thawing. We found that the proportion and the growth potential of epithelial SCs in monolayer culture and in TESs remained constant before and after cryopreservation. Further, the functionality of these substitutes was demonstrated by successfully grafting human TESs on athymic mice for 6 months. We conclude that human epithelial skin SCs are adequately preserved upon human tissue reconstruction. Thus, these TESs produced by the self-assembly approach are suitable for clinical applications.

Detailed histological structure of human hair follicle bulge region at different ages: a visible niche for nesting adult stem cells

In the bulge region of the hair follicle, a densely and concentrically packed cell mass is encircled by the arrector pili muscle (APM), which offers a specilized microenvironment (niche) for housing heterogeneous adult stem cells. However, the detailed histological architecture and the cellular composition of the bulge region warrants intensive study and may have implications for the regulation of hair follicle growth regulation. This study was designed to define the gene-expression profiles of putative stem cells and lineage-specific precursors in the mid-portions of plucked hair follicles prepared according to the presence of detectable autofluorescence. The structure was also characterized by using a consecutive sectioning technique. The bulge region of the hair follicle with autofluorescence was precisely excised by employing a micro-dissection procedure. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was performed to identify the gene expression profiles specific for epithelial, melanocyte and stromal stem cells in the bulge region of the hair follicle visualized by autofluorescence. The morphology and its age-dependent changes of bulge region of the hair follicles with autofluorescence segment were also examined in 9 scalp skin specimens collected from patients aged 30 weeks to 75 years, by serial sectioning and immuno-staining. Gene expression profile analysis revealed that there were cells with mRNA transcripts of Dct(Hi)Tyrase(Lo)-Tyrp1(Lo)MC1R(Lo)MITF(Lo)/K15(Hi)/NPNT(Hi) in the bulge region of the hair follicle with autofluorescence segments, which differed from the patterns in hair bulbs. Small cell-protrusions that sprouted from the outer root sheath (ORS) were clearly observed at the APM inserting level in serial sections of hair follicles by immunohistological staining, which were characteristically replete with K15+/K19+expressing cells. Likewise, the muscle bundles of APM positive for smooth muscle actin intimately encircled these cell-protrusions, and the occurrence frequency of the cell-protrusions was increased in fetal scalp skin compared with adult scalp skin. This study provided the evidence that the cell-protrusions occurring at the ORS relative to the APM insertion are more likely to be characteristic of the visible niches that are filled with abundant stem cells. The occurrence frequency of these cell-protrusions was significantly increased in fetal scalp skin samples (128%) as compared with the scalp skins of younger (49.4%) and older (25.4%) adults (P<0.01), but difference in the frequency between the two adult groups were not significant. These results indicated that these cell-protrusions function as a niche house for the myriad stem cells and/or precursors to meet the needs of the development of hair follicles in an embryo. The micro-dissection used in this study was simple and reliable in excising the bulge region of the hair follicle with autofluorescence segments dependent on their autofluorescence is of value for the study of stem cell culture.

Harvesting the potential of the human umbilical cord: isolation and characterisation of four cell types for tissue engineering applications

The human umbilical cord (UC) has attracted interest as a source of cells for many research applications. UC solid tissues contain four cell types: epithelial, stromal, smooth muscle and endothelial cells. We have developed a unique protocol for the sequential extraction of all four cell types from a single UC, allowing tissue reconstruction using multiple cell types from the same source. By combining perfusion, immersion and explant techniques, all four cell types have been successfully expanded in monolayer cultures. We have also characterised epithelial and Wharton's jelly cells (WJC) by immunolabelling of specific proteins. Epithelial cell yields averaged at 2.3 × 10(5) cells per centimetre UC, and the cells expressed an unusual combination of keratins typical of simple, mucous and stratified epithelia. Stromal cells in the Wharton's jelly expressed desmin, α-smooth muscle actin, elastin, keratins (K12, K16, K18 and K19), vimentin and collagens. Expression patterns in cultured cells resembled those found in situ except for basement membrane components and type III collagen. These stromal cells featured a sustained proliferation rate up to passage 12 after thawing. The mesenchymal stem cell (MSC) character of the WJC was confirmed by their expression of typical MSC surface markers and by adipogenic and osteogenic differentiation assays. To emphasise and demonstrate their potential for regenerative medicine, UC cell types were successfully used to produce human tissue-engineered constructs. Both bilayered stromal/epithelial and vascular substitutes were produced, establishing the versatility and importance of these cells for research and therapeutic applications.

Re-assessing K15 as an epidermal stem cell marker

The intermediate filament keratin 15 (K15) is present in variable amounts in various stratified epithelia, but has also been reported to be a stem cell marker in the hair follicle. Using peptide specific antibodies, we evaluated the temporal and spatial distribution pattern of K15 expression/localization during normal epidermal development and initiation of hair follicle formation, and in the injured mature epidermis (e.g., during acute injury and repair and in tumorigenesis). During development, K15 expression is first localized to a subset of epidermal basal cells and the overlying periderm at E12.5, but its expression is seen throughout the basal layer by E15.5 and beyond. In hair follicle morphogenesis, initial peg formation occurs in a K15-null area at E14.5 and as peg elongation proceeds through to the mature hair follicle, K15 expression follows the leading edge with positive cells restricted to the outer root sheath. In an epidermal injury model, K15 is first up-regulated and associated with both the basal and suprabasal layers of the interfollicular epidermis then expression becomes sporadic and down-regulated before a basal layer-specific association is re-established in the repaired epidermis. During tumorigenesis, K15 is first mis-expressed, and is ultimately down-regulated. Our data suggest that K15 protein expression may reflect not only expression in a stem or progenitor cell subpopulation, but also reflects the activity and responsiveness of basal-like cells to loss of homeostasis of the epidermal differentiation program. Thus, the data suggest caution in using K15 alone to delineate epidermal stem cells, and underscore the need for further investigation of K15 and other markers in epidermal cell subpopulations.

From hair to cornea: toward the therapeutic use of hair follicle-derived stem cells in the treatment of limbal stem cell deficiency

Limbal stem cell deficiency (LSCD) leads to severe ocular surface abnormalities that can result in the loss of vision. The most successful therapy currently being used is transplantation of limbal epithelial cell sheets cultivated from a limbal biopsy obtained from the patient's healthy, contralateral eye or cadaveric tissue. In this study, we investigated the therapeutic potential of murine vibrissae hair follicle bulge-derived stem cells (HFSCs) as an autologous stem cell (SC) source for ocular surface reconstruction in patients bilaterally affected by LSCD. This study is an expansion of our previously published work showing transdifferentiation of HFSCs into cells of a corneal epithelial phenotype in an in vitro system. In this study, we used a transgenic mouse model, K12(rtTA/rtTA) /tetO-cre/ROSA(mTmG) , which allows for HFSCs to change color, from red to green, once differentiation to corneal epithelial cells occurs and Krt12, the corneal epithelial-specific differentiation marker, is expressed. HFSCs were isolated from transgenic mice, amplified by clonal expansion on a 3T3 feeder layer, and transplanted on a fibrin carrier to the eye of LSCD wild-type mice (n = 31). The HFSC transplant was able to reconstruct the ocular surface in 80% of the transplanted animals; differentiating into cells with a corneal epithelial phenotype, expressing Krt12, and repopulating the corneal SC pool while suppressing vascularization and conjunctival ingrowth. These data highlight the therapeutic properties of using HFSC to treat LSCD in a mouse model while demonstrating a strong translational potential and points to the niche as a key factor for determining stem cell differentiation.

Production of an optimized tissue-engineered pig connective tissue for the reconstruction of the urinary tract

Nonurological autologous tissues are used for urethral reconstruction to correct urinary tract disorders but are still leading to complications. Other substitutes have been studied on small animal models without great success. For preclinical tests, we selected the porcine model for its similarity to the human urinary tract. Up to now, porcine skin fibroblasts were not able to synthesize enough extracellular matrix under standard conditions to sustain the formation of an adequate tissue for transplantation purposes. Therefore, our goal was to optimize the harvesting site and culture conditions to obtain a thick and easy to handle porcine fibroblast tissue. The oral mucosa was found to be the ideal harvesting site, and a culture temperature of 39°C enabled the formation of a good porcine fibroblast sheet. We successfully superimpose three fibroblast sheets that merged into a thick and resistant tissue where physiological extracellular matrix was produced. Mechanical resistance evaluation by uniaxial traction on the three-layer fibroblast constructs also demonstrated its suitable properties. The production of this porcine connective tissue offers an interesting option in the field of urological tissue engineering. Autologous experiments on a larger animal model are now possible and accessible, allowing the performance of long-term in vivo studies.

Antioxidant proteins and reactive oxygen species are decreased in a murine epidermal side population with stem cell-like characteristics

Reactive oxygen species (ROS) and antioxidants are essential to maintain a redox balance within tissues and cells. Intracellular ROS regulate key cellular functions such as proliferation, differentiation and apoptosis through cellular signaling, and response to injury. The redox environment is particularly important for stem/progenitor cells, as their self-renewal and differentiation has been shown to be redox sensitive. However, not much is known about ROS and antioxidant protein function in freshly isolated keratinocytes, notably the different keratinocyte subpopulations. Immunostaining of neonatal cutaneous sections revealed that antioxidant enzymes [catalase, SOD2, gluthatione peroxidase-1 (GPx)] and ROS are localized predominantly to the epidermis. We isolated keratinocyte subpopulations and found lower levels of SOD2, catalase and GPx, as well as decreased SOD and catalase activity in an epidermal side population with stem cell-like characteristics (EpSPs) compared to more differentiated (Non-SP) keratinocytes. EpSPs also exhibited less mitochondrial area, fewer peroxisomes and produced lower levels of ROS than Non-SPs. Finally, EpSPs were more resistant to UV radiation than their progeny. Together, our data indicate ROS and antioxidant levels are decreased in stem-like EpSPs.

Tissue-engineered skin preserving the potential of epithelial cells to differentiate into hair after grafting

The aim of this study was to evaluate whether tissue-engineered skin produced in vitro was able to sustain growth of hair follicles in vitro and after grafting. Different tissues were designed. Dissociated newborn mouse keratinocytes or newborn mouse hair buds (HBs) were added onto dermal constructs consisting of a tissue-engineered cell-derived matrix elaborated from either newborn mouse or adult human fibroblasts cultured with ascorbic acid. After 7-21 days of maturation at the air-liquid interface, no hair was noticed in vitro. Epidermal differentiation was observed in all tissue-engineered skin. However, human fibroblast-derived tissue-engineered dermis (hD) promoted a thicker epidermis than mouse fibroblast-derived tissue-engineered dermis (mD). In association with mD, HBs developed epithelial cyst-like inclusions presenting outer root sheath-like attributes. In contrast, epidermoid cyst-like inclusions lined by a stratified squamous epithelium were present in tissues composed of HBs and hD. After grafting, pilo-sebaceous units formed and hair grew in skin elaborated from HBs cultured 10-26 days submerged in culture medium in association with mD. However, the number of normal hair follicles decreased with longer culture time. This hair-forming capacity after grafting was not observed in tissues composed of hD overlaid with HBs. These results demonstrate that epithelial stem cells can be kept in vitro in a permissive tissue-engineered dermal environment without losing their potential to induce hair growth after grafting.

The morphology of the rat vibrissal follicle-sinus complex revealed by three-dimensional computer-aided reconstruction

The vibrissal follicle-sinus complex (FSC) is a sensory receptor of the mammalian integumentary system that is located around the mouth. The purpose of the present study was to identify the actual 3-dimensional structure of the rat vibrissal FSC. Rat skin tissue was serially sectioned at a thickness of 10 μm and then stained with Masson's trichrome. The serial sections were reconstructed 3-dimensionally using Reconstruct software. The rat vibrissal follicle is a spindle-shaped structure that is embedded within a blood sinus and enveloped within a thick collagenous capsule. The vibrissal FSC is innervated by the deep vibrissal and superficial vibrissal nerves. The deep vibrissal nerve, travelling in the basal-to-apical direction, penetrates the thick collagenous capsule of the vibrissal FSC. The sinus system can be divided into a superior portion, known as the ring sinus, and an inferior portion, known as the cavernous sinus. The ring sinus contains a C-shaped structure, the ringwulst, which is suspended from the mesenchymal sheath of the follicle. Collagenous trabeculae can be seen in the cavernous sinus but not in the ring sinus. The ring sinus encircles the follicle obliquely and asymmetrically. The ringwulst encircles the follicle incompletely, in a C-shaped fashion. This study has demonstrated the previously underappreciated 3-dimensional structure of the vibrissal FSC, which differs from previously reported descriptions, and provides data that will enhance the understanding of vibrissal function.

Keratin 17 promotes epithelial proliferation and tumor growth by polarizing the immune response in skin

Basaloid skin tumors, including basal cell carcinoma (BCC) and basaloid follicular hamartoma (BFH), are associated with aberrant Hedgehog (Hh) signaling1 and, in the case of BCC, an expanding set of genetic variants including keratin 5 (K5)2, an intermediate filament-forming protein. We show that genetic ablation of keratin 17 (K17) protein, which is induced in basaloid skin tumors3,4 and co-polymerizes with K5 in vivo5, delays BFH tumor initiation and growth in mice with constitutive Hh signaling in epidermis6,7. The delay is preceded by reduced inflammation and a polarization of inflammatory cytokines from a Th1/Th17- to a Th2-dominated profile. Absence of K17 also attenuates hyperplasia and inflammation in a model of acute dermatitis. Re-expression of K17 in Gli2^tg K17^−/− keratinocytes induces select Th1 chemokines with established roles in BCC. Our findings establish a novel immunomodulatory role for K17 in Hh-driven basaloid skin tumors that could impact additional tumor settings, psoriasis, and wound repair.

Tissue engineering of skin and cornea: Development of new models for in vitro studies

Human beings are greatly preoccupied with the unavoidable nature of aging. While the biological processes of senescence and aging are the subjects of intense investigations, the molecular mechanisms linking aging with disease and death are yet to be elucidated. Tissue engineering offers new models to study the various processes associated with aging. Using keratin 19 as a stem cell marker, our studies have revealed that stem cells are preserved in human skin reconstructed by tissue engineering and that the number of epithelial stem cells varies according to the donor's age. As with skin, human corneas can also be engineered in vitro. Among the epithelial cells used for reconstructing skin and corneas, significant age-dependent variations in the expression of the transcription factor Sp1 were observed. Culturing skin epithelial cells with a feeder layer extended their life span in culture, likely by preventing Sp1 degradation in epithelial cells, therefore demonstrating the pivotal role played by this transcription factor in cell proliferation. Finally, using the human tissue-engineered skin as a model, we linked Hsp27 activation with skin differentiation.

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.

Prolactin--a novel neuroendocrine regulator of human keratin expression in situ

The controls of human keratin expression in situ remain to be fully elucidated. Here, we have investigated the effects of the neurohormone prolactin (PRL) on keratin expression in a physiologically and clinically relevant test system: organ-cultured normal human hair follicles (HFs). Not only do HFs express a wide range of keratins, but they are also a source and target of PRL. Microarray analysis revealed that PRL differentially regulated a defined subset of keratins and keratin-associated proteins. Quantitative immunohistomorphometry and quantitative PCR confirmed that PRL up-regulated expression of keratins K5 and K14 and the epithelial stem cell-associated keratins K15 and K19 in organ-cultured HFs and/or isolated HF keratinocytes. PRL also up-regulated K15 promoter activity and K15 protein expression in situ, whereas it inhibited K6 and K31 expression. These regulatory effects were reversed by a pure competitive PRL receptor antagonist. Antagonist alone also modulated keratin expression, suggesting that "tonic stimulation" by endogenous PRL is required for normal expression levels of selected keratins. Therefore, our study identifies PRL as a major, clinically relevant, novel neuroendocrine regulator of both human keratin expression and human epithelial stem cell biology in situ.

Identification of epithelial stem cells in vivo and in vitro using keratin 19 and BrdU

Progress in the identification of skin stem cells and the improvement of culture methods open the possibility to use stem cells in regenerative medicine. Based on their quiescent nature, the development of label retention assays allowed the localization of skin stem cells in the bulge region of the pilosebaceous units and in the bottom of rete ridges in glabrous skin. The development of markers such as keratin 19 also permits their study in human tissues. In this chapter, protocols to identify skin stem cells based on their slow-cycling property and their expression of keratin 19 will be described in detail. The methods include the labeling of skin stem cells within mouse or rat tissues in vivo, the labeling of proliferative human cells in vitro using 5-bromo-2-deoxyuridine (BrdU), and the detection of keratin 19 and BrdU by immunofluorescence or immunoperoxidase staining.

Self-organization of keratin intermediate filaments into cross-linked networks

Keratins, the largest subgroup of intermediate filament (IF) proteins, form a network of 10-nm filaments built from type I/II heterodimers in epithelial cells. A major function of keratin IFs is to protect epithelial cells from mechanical stress. Like filamentous actin, keratin IFs must be cross-linked in vitro to achieve the high level of mechanical resilience characteristic of live cells. Keratins 5 and 14 (K5 and K14), the main pairing occurring in the basal progenitor layer of epidermis and related epithelia, can readily self-organize into large filament bundles in vitro and in vivo. Here, we show that filament self-organization is mediated by multivalent interactions involving distinct regions in K5 and K14 proteins. Self-organization is determined independently of polymerization into 10-nm filaments, but involves specific type I–type II keratin complementarity. We propose that self-organization is a key determinant of the structural support function of keratin IFs in vivo.

An active role of the DeltaN isoform of p63 in regulating basal keratin genes K5 and K14 and directing epidermal cell fate

Background

One major defining characteristic of the basal keratinocytes of the stratified epithelium is the expression of the keratin genes K5 and K14. The temporal and spatial expression of these two genes is usually tightly and coordinately regulated at the transcriptional level. This ensures the obligate pairing of K5 and K14 proteins to generate an intermediate filament (IF) network that is essential for the structure and function of the proliferative keratinocytes. Our previous studies have shown that the basal-keratinocyte restricted transcription factor p63 is a direct regulator of K14 gene.

Methodology/Principal Findings

Here we provide evidence that p63, specifically the ΔN isoform also regulates the expression of the K5 gene by binding to a conserved enhancer within the 5′ upstream region. By using specific antibodies against ΔNp63, we show a concordance in the expression between basal keratins and ΔNp63 proteins but not the TAp63 isoforms during early embryonic skin development. We demonstrate, that contrary to a previous report, transgenic mice expressing ΔNp63 in lung epithelium exhibit squamous metaplasia with de novo induction of K5 and K14 as well as transdifferentiation to the epidermal cell lineage. Interestingly, the in vivo epidermal inductive properties of ΔNp63 do not require the C-terminal SAM domain. Finally, we show that ΔNp63 alone can restore the expression of the basal keratins and reinitiate the failed epidermal differentiation program in the skin of p63 null animals.

Significance

ΔNp63 is a critical mediator of keratinocyte stratification program and directly regulates the basal keratin genes.

Molecular profiling of conjunctival epithelial side-population stem cells: atypical cell surface markers and sources of a slow-cycling phenotype

PURPOSE

Side-population (SP) cells isolated from limbal and conjunctival epithelia derive from cells that are slow cycling in vivo, a known feature of tissue stem cells. The purpose of this study was to define the molecular signature of the conjunctival SP cells and identify markers and signaling pathways associated with the phenotype of these cells.

METHODS

Overnight cultures of freshly isolated human conjunctival epithelial cells stained with Hoechst 33342 were sorted by flow cytometry into SP and non-SP cohorts. Isolated RNA was processed for microarray analysis using a commercial oligonucleotide spotted array. Results were validated at the gene and protein levels by quantitative PCR and immunologic methods. Data mining methods were used to identify cellular processes relevant for stem cell function.

RESULTS

Comparative analyses of transcripts expression based on present and absent software calls across four replicate experiments identified 16,993 conjunctival epithelial transcripts including 10,266 unique known genes of approximately 24,000 represented in the array. Of those genes, 1254 and 363 were overexpressed (>2-fold) or underexpressed (<0.5-fold), respectively, in the SP. The overexpressed set included genes coding for proteins that have been associated with (1) embryonic development and/or stem cell self renewal (MSX, MEIS, ID, Hes1, and SIX homeodomain genes); (2) cell survival (e.g., CYP1A1 to degrade aromatic genotoxic compounds); (3) cycling rate (e.g., DUSPs and Pax6 to foster slow cycling); and (4) genes whose expression is not typical in epithelia (e.g., CD62E).

CONCLUSIONS

The molecular signature of conjunctival SP cells is consistent with a stem cell phenotype. Their gene expression patterns underpin slow cycling and plasticity, features associated with tissue stem cells. The results provide valuable insights for the preservation and/or expansion of epithelial stem cells.

Corneal limbal microenvironment can induce transdifferentiation of hair follicle stem cells into corneal epithelial-like cells

The aim of this study was to investigate the transdifferentiation potential of murine vibrissa hair follicle (HF) stem cells into corneal epithelial-like cells through modulation by corneal- or limbus-specific microenvironmental factors. Adult epithelial stem cells were isolated from the HF bulge region by mechanical dissection or fluorescence-activated cell sorting using antibodies to alpha6 integrin, enriched by clonal expansion, and subcultivated on various extracellular matrices (type IV collagen, laminin-1, laminin-5, fibronectin) and in different conditioned media derived from central and peripheral corneal fibroblasts, limbal stromal fibroblasts, and 3T3 fibroblasts. Cellular phenotype and differentiation were evaluated by light and electron microscopy, real-time reverse transcription-polymerase chain reaction, immunocytochemistry, and Western blotting, using antibodies against putative stem cell markers (K15, alpha6 integrin) and differentiation markers characteristic for corneal epithelium (K12, Pax6) or epidermis (K10). Using laminin-5, a major component of the corneo-limbal basement membrane zone, and conditioned medium from limbal stromal fibroblasts, clonally enriched HF stem and progenitor cells adhered rapidly and formed regularly arranged stratified cell sheets. Conditioned medium derived from limbal fibroblasts markedly upregulated expression of cornea-specific K12 and Pax6 on the mRNA and protein level, whereas expression of the epidermal keratinocyte marker K10 was strongly downregulated. These findings suggest that adult HF epithelial stem cells are capable of differentiating into corneal epithelial-like cells in vitro when exposed to a limbus-specific microenvironment. Therefore, the HF may be an easily accessible alternative therapeutic source of autologous adult stem cells for replacement of the corneal epithelium and restoration of visual function in patients with ocular surface disorders.

Hair follicle epithelial stem cells get their sox on

Sox family transcription factors have emerged as key regulators of multiple developmental processes, including fate determination and differentiation in multiple tissues. In this issue of Cell Stem Cell, Nowak et al. (2008) implicate Sox9 in the specification of early hair follicle stem cells and subsequent epithelial lineages.