Pigmented human skin equivalent--as a model of the mechanisms of control of cell-cell and cell-matrix interactions

Development of a mammalian neurosensory full-thickness skin equivalent and its application to screen sensitizing stimuli

Human skin equivalents (HSEs) are an increasingly popular research tool due to limitations associated with animal testing for dermatological research. They recapitulate many aspects of skin structure and function, however, many only contain two basic cell types to model dermal and epidermal compartments, which limits their application. We describe advances in the field skin tissue modeling to produce a construct containing sensory-like neurons that is responsive to known noxious stimuli. Through incorporation of mammalian sensory-like neurons, we were able to recapitulate aspects of the neuroinflammatory response including secretion of substance P and a range of pro-inflammatory cytokines in response to a well-characterized neurosensitizing agent: capsaicin. We observed that neuronal cell bodies reside in the upper dermal compartment with neurites extending toward the keratinocytes of the stratum basale where they exist in close proximity to one another. These data suggest that we are able to model aspects of the neuroinflammatory response that occurs during exposure to dermatological stimuli including therapeutics and cosmetics. We propose that this skin construct can be considered a platform technology with a wide range of applications including screening of actives, therapeutics, modeling of inflammatory skin diseases, and fundamental approaches to probe underlying cell and molecular mechanisms.

Laminin-511-E8 promotes efficient in vitro expansion of human limbal melanocytes

Limbal melanocytes, located in the basal epithelial layer of the corneoscleral limbus, represent essential components of the corneal epithelial stem cell niche, but, due to difficulties in their isolation and cultivation, their biological roles and potential for stem cell-based tissue engineering approaches have not been comprehensively studied. Here, we established a protocol for the efficient isolation and cultivation of pure populations of human limbal melanocytes, which could be expanded at high yield by using recombinant laminin (LN)-511-E8 as culture substrate. Co-cultivation of limbal melanocytes with limbal epithelial stem/progenitor cells on fibrin hydrogels pre-incubated with LN-511-E8 resulted in multilayered stratified epithelial constructs within ten days. By reproducing physiological cell–cell and cell–matrix interactions of the native niche environment, these biomimetic co-culture systems provide a promising experimental model for investigating the functional roles of melanocytes in the limbal stem cell niche and their suitability for developing advanced epithelial grafts for ocular surface surface reconstruction.

Structural and Biomechanical Characterization of a Scaffold-Free Skin Equivalent Model via Biophysical Methods

AIMS

Among in vitro skin models, the scaffold-free skin equivalent (SFSE), without exogenous material, is interesting for pharmacotoxicological studies. Our aim was to adapt in vivo biophysical methods to study the structure, thickness, and extracellular matrix of our in vitro model without any chemical fixation needed as for histology.

METHODS

We evaluated 3 batches of SFSE and characterized them by histology, transmission electron microscopy (TEM), and immunofluorescence. In parallel, we investigated 3 biophysical methods classically used for in vivo evaluation, optical coherence tomography (OCT), and laser scanning microscopy (LSM) imaging devices as well as the cutometer suction to study the biomechanical properties.

RESULTS

OCT allowed the evaluation of SFSE total thickness and its different compartments. LSM has a greater resolution enabling an evaluation at the cell scale and the orientation of collagen fibers. The viscoelasticity measurement by cutometry was possible on our thin skin model and might be linked with mature collagen bundles visible in TEM and LSM and with elastic fibers seen in immunofluorescence.

CONCLUSION

Our data demonstrated the simplicity and sensitivity of these different in vivo biophysical devices on our thin skin model. These noninvasive tools allow to study the morphology and the biomechanics of in vitro models.

Next generation human skin constructs as advanced tools for drug development

Many diseases, as well as side effects of drugs, manifest themselves through skin symptoms. Skin is a complex tissue that hosts various specialized cell types and performs many roles including physical barrier, immune and sensory functions. Therefore, modeling skin in vitro presents technical challenges for tissue engineering. Since the first attempts at engineering human epidermis in 1970s, there has been a growing interest in generating full-thickness skin constructs mimicking physiological functions by incorporating various skin components, such as vasculature and melanocytes for pigmentation. Development of biomimetic in vitro human skin models with these physiological functions provides a new tool for drug discovery, disease modeling, regenerative medicine and basic research for skin biology. This goal, however, has long been delayed by the limited availability of different cell types, the challenges in establishing co-culture conditions, and the ability to recapitulate the 3D anatomy of the skin. Recent breakthroughs in induced pluripotent stem cell (iPSC) technology and microfabrication techniques such as 3D-printing have allowed for building more reliable and complex in vitro skin models for pharmaceutical screening. In this review, we focus on the current developments and prevailing challenges in generating skin constructs with vasculature, skin appendages such as hair follicles, pigmentation, immune response, innervation, and hypodermis. Furthermore, we discuss the promising advances that iPSC technology offers in order to generate in vitro models of genetic skin diseases, such as epidermolysis bullosa and psoriasis. We also discuss how future integration of the next generation human skin constructs onto microfluidic platforms along with other tissues could revolutionize the early stages of drug development by creating reliable evaluation of patient-specific effects of pharmaceutical agents. Impact statement Skin is a complex tissue that hosts various specialized cell types and performs many roles including barrier, immune, and sensory functions. For human-relevant drug testing, there has been a growing interest in building more physiological skin constructs by incorporating different skin components, such as vasculature, appendages, pigment, innervation, and adipose tissue. This paper provides an overview of the strategies to build complex human skin constructs that can faithfully recapitulate human skin and thus can be used in drug development targeting skin diseases. In particular, we discuss recent developments and remaining challenges in incorporating various skin components, availability of iPSC-derived skin cell types and in vitro skin disease models. In addition, we provide insights on the future integration of these complex skin models with other organs on microfluidic platforms as well as potential readout technologies for high-throughput drug screening.

Paraconsistent analysis network applied in the treatment of Raman spectroscopy data to support medical diagnosis of skin cancer

Paraconsistent logic (PL) is a type of non-classical logic that accepts contradiction as a fundamental concept and has produced valuable results in the analysis of uncertainties. In this work, algorithms based on a type of PL-paraconsistent annotated logic of two values (PAL2v)-are interconnected into a network of paraconsistent analysis (PANnet). PANnet was applied to a dataset comprising 146 Raman spectra of skin tissue biopsy fragments of which 30 spectra were determined to represent normal skin tissue (N), 96 were determined to represent tissue with basal cell carcinoma, and 19 were determined to be tissue with melanoma (MEL). In this database, paraconsistent analysis was able to correctly discriminate 136 out of a total of 145 fragments, obtaining a 93.793 % correct diagnostic accuracy. The application of PAL2v in the analysis of Raman spectroscopy signals produces better discrimination of cells than conventional statistical processes and presents a good graphical overview through its associated lattice structure. The technique of PAL2v-based data processing can be fundamental in the development of a computational tool dedicated to support the diagnosis of skin cancer using Raman spectroscopy.

A quantitative study of nanoparticle skin penetration with interactive segmentation

In the last decade, the application of nanotechnology techniques has expanded within diverse areas such as pharmacology, medicine, and optical science. Despite such wide-ranging possibilities for implementation into practice, the mechanisms behind nanoparticle skin absorption remain unknown. Moreover, the main mode of investigation has been qualitative analysis. Using interactive segmentation, this study suggests a method of objectively and quantitatively analyzing the mechanisms underlying the skin absorption of nanoparticles. Silica nanoparticles (SNPs) were assessed using transmission electron microscopy and applied to the human skin equivalent model. Captured fluorescence images of this model were used to evaluate degrees of skin penetration. These images underwent interactive segmentation and image processing in addition to statistical quantitative analyses of calculated image parameters including the mean, integrated density, skewness, kurtosis, and area fraction. In images from both groups, the distribution area and intensity of fluorescent silica gradually increased in proportion to time. Since statistical significance was achieved after 2 days in the negative charge group and after 4 days in the positive charge group, there is a periodic difference. Furthermore, the quantity of silica per unit area showed a dramatic change after 6 days in the negative charge group. Although this quantitative result is identical to results obtained by qualitative assessment, it is meaningful in that it was proven by statistical analysis with quantitation by using image processing. The present study suggests that the surface charge of SNPs could play an important role in the percutaneous absorption of NPs. These findings can help achieve a better understanding of the percutaneous transport of NPs. In addition, these results provide important guidance for the design of NPs for biomedical applications.

Characterization of pigmented dermo-epidermal skin substitutes in a long-term in vivo assay

In our laboratory, we have been using human pigmented dermo-epidermal skin substitutes for short-term experiments since several years. Little is known, however, about the long-term biology of such constructs after transplantation. We constructed human, melanocyte-containing dermo-epidermal skin substitutes of different (light and dark) pigmentation types and studied them in a long-term animal experiment. Developmental and maturational stages of the epidermal and dermal compartment as well as signs of homoeostasis were analysed 15 weeks after transplantation. Keratinocytes, melanocytes and fibroblasts from human skin biopsies were isolated and assembled into dermo-epidermal skin substitutes. These were transplanted onto immuno-incompetent rats and investigated 15 weeks after transplantation. Chromameter evaluation showed a consistent skin colour between 3 and 4 months after transplantation. Melanocytes resided in the epidermal basal layer in physiological numbers and melanin accumulated in keratinocytes in a supranuclear position. Skin substitutes showed a mature epidermis in a homoeostatic state and the presence of dermal components such as Fibrillin and Tropoelastin suggested advanced maturation. Overall, pigmented dermo-epidermal skin substitutes show a promising development towards achieving near-normal skin characteristics and epidermal and dermal tissue homoeostasis. In particular, melanocytes function correctly over several months whilst remaining in a physiological, epidermal position and yield a pigmentation resembling original donor skin colour.

Abrogating effect of N-linked carbohydrate modifiers on the stem cell factor and endothelin-1-stimulated epidermal pigmentation in human epidermal equivalents

BACKGROUND

We previously demonstrated that the hyperpigmentation that occurs in UVB-melanosis as well as in solar lentigos is associated with the increased production of melanogenic cytokines, such as endothelin (EDN)-1 and stem cell factor (SCF), by keratinocytes in those areas of the skin.

OBJECTIVE

We developed a model for these hyperpigmentary disorders in EDN1+SCF stimulated human epidermal equivalents (HEEs) and characterized the effects of the N-linked carbohydrate core synthesis inhibitor glucosamine or N-linked carbohydrate processing inhibitors deoxynojirimycin or monensin on the stimulated HEE pigmentation.

METHODS

Those effects were assessed by melanin analysis, real-time RT-PCR and Western blotting.

RESULTS

The addition of these N-linked carbohydrate modifiers (NCMs) markedly abolished the EDN1+SCF-elicited increase in HEE pigmentation over 14 days. Real-time RT-PCR and Western blotting of these NCM-treated HEEs unexpectedly revealed that the EDN1+SCF-stimulated steady-state levels of tyrosinase (TYR), TYR-related protein-1, dopachrome tautomerase and PMEL17 as well as microphthalmia-associated transcription factor (MITF) were significantly attenuated at the transcriptional and translational levels without any cytotoxic effects on keratinocytes and melanocytes in the HEEs. Pre-treatment of cultured normal human melanocytes with the NCMs interrupted the EDN1+SCF-induced stimulation of steady-state levels of MITF at the transcriptional and translational levels and TYR activity without any direct inhibitory effect on the catalytic activity of TYR in vitro.

CONCLUSION

This study provides evidence that NCMs have a potential to attenuate the EDN1+SCF-stimulated pigmentation of HEEs by abrogating the increased steady-state levels of MITF mRNA, which results in the attenuation of the increased steady-state levels of these melanocyte-specific proteins.

Artificial skin in perspective: concepts and applications

Skin, the largest organ of the human body, is organized into an elaborate layered structure consisting mainly of the outermost epidermis and the underlying dermis. A subcutaneous adipose-storing hypodermis layer and various appendages such as hair follicles, sweat glands, sebaceous glands, nerves, lymphatics, and blood vessels are also present in the skin. These multiple components of the skin ensure survival by carrying out critical functions such as protection, thermoregulation, excretion, absorption, metabolic functions, sensation, evaporation management, and aesthetics. The study of how these biological functions are performed is critical to our understanding of basic skin biology such as regulation of pigmentation and wound repair. Impairment of any of these functions may lead to pathogenic alterations, including skin cancers. Therefore, the development of genetically controlled and well characterized skin models can have important implications, not only for scientists and physicians, but also for manufacturers, consumers, governing regulatory boards and animal welfare organizations. As cells making up human skin tissue grow within an organized three-dimensional (3D) matrix surrounded by neighboring cells, standard monolayer (2D) cell cultures do not recapitulate the physiological architecture of the skin. Several types of human skin recombinants, also called artificial skin, that provide this critical 3D structure have now been reconstructed in vitro. This review contemplates the use of these organotypic skin models in different applications, including substitutes to animal testing.

Experimental models and high-throughput diagnostics for tissue regeneration

During wound healing, cells recreate functional structures to regenerate the injured tissue. Understanding the healing process is essential for the development of new concepts and the design of novel biomimetic approaches for delivery of cells, genes and growth factors to accelerate tissue regeneration. To this end, realistic experimental models and high-throughput diagnostics are necessary to understand the molecular mechanisms of healing and reveal the genetic networks that determine tissue repair versus regeneration. Following a brief overview of the biology of wound healing, this review covers the in vitro and in vivo models that are employed at present to study the healing process. Discussion then covers the application of high-throughput genomic and proteomic technologies in epithelial development, living skin substitutes and wound healing. Finally, this review provides a perspective on novel technologies that should be developed to facilitate the understanding of wound healing complications and the design of therapeutics that target the underlying deficiencies.

Nipple-areolar pigmentation: histology and potential for reconstitution in breast reconstruction

The makeup of nipple-areolar skin, in terms of its melanin and melanocyte content has not previously been established. This histological information is required if pigmentation of the reconstructed nipple-areola is to be successful in post-mastectomy breast reconstruction. We describe examination of 200 parallel sections of nipple-areolar skin of 20 women using histochemical (Masson-Fontana) and immunohistochemical (Mel-5) techniques, evaluated using quantitative image analysis. The amount of melanin present per length of basement membrane was 2.14 times higher in areolar skin than breast skin. The ratio of melanocytes to keratinocytes was 1:9.7 in areolar skin vs. 1:14.7 in breast skin. We also describe a cell culture and skin construct method using autologous human serum without toxic growth promoting additives, which could be used in the clinical setting of nipple-areolar reconstruction.

Alternative model to human skin organ culture: A preliminary study with Leibovitz L15 medium

Organ culture has been used to maintain the three-dimensional structure of the skin and the interaction between melanocytes and keratinocytes, which is essential for melanin production. In the current study we aimed to evaluate the general morphology, viability, and distribution of human melanocytes in a system that uses Leibovitz L15 medium at room temperature. By comparison with human skin explants maintained in Dulbecco's minimum Eagle's medium at 37 degrees C, we found that the skin was better preserved with Leibovitz L15 after 7 days in culture. The addition of 10% fetal bovine serum to this medium did not promote any change. Dividing cells labeled with Ki-67 were visualized at the basal and suprabasal epidermal layers. Retinoic acid was tested at 1 microg/mL and we recorded a reduction of the corneal layer after 48 hours and a complete detachment of the epidermis after 7 days, probably due to a toxic effect in the medium. Melanin and melanocytes were detected by ammoniacal silver and Dopa stainings under light microscopy. We observed that cells were viable throughout the culture period and melanin was distributed in melanocytes and keratinocytes. In conclusion, we suggest that the use of Leibovitz L15 medium at room temperature can be a viable alternative to the normal organ culture of human skin, which is an important system to study the activity and reaction of melanocytes to dermatological products and cosmetics.

Comparison of Long-Term Survival of Pigmented Epidermal Reconstructs Cultured In Vitro vs. Xenografted on Nude Mice

Epidermal reconstructs incorporating pigment cells have been used in vitro over the last decade to study the physiology of the epidermal melanin unit. However, the major limitation of this technology is the duration of the assays, which need to be completed within 2-3 weeks to obviate the problem of epidermal senescence and excessive terminal differentiation. This becomes a major problem for studying long-term biological phenomena in photoprotection and epidermal skin cancers. We report here a simplified surgical technique in immunotolerant mice allowing long-term studies. The creation of a vascularized mouse skin flap is the key point of the surgical procedure. Long-term pigmentation of the xenografts seemed macroscopically successful, but surprisingly microscopy at 11 and 16 weeks postgrafting showed mostly dermal pigment aggregates and rare Melan-A positive dermal and epidermal pigment cells. In the same reconstructs maintained in vitro, dermal pigment and dermal pigment cells were never noted. It could be speculated that in our model, the colonization of the xenografted dead human dermis by murine cells influences melanocyte survival.

Bednár tumor associated with dermal melanocytosis: melanocytic colonization or neuroectodermal multidirectional differentiation?

BACKGROUND

Neuroectodermal differentiation or melanocytic colonization are the opposing theories of histogenesis for the Bednár tumor or pigmented dermatofibrosarcoma protuberans (DFSP).

OBSERVATION

A 31-year-old African-American woman presented with a 2-cm blue-black shoulder nodule of 1-year duration. Punch biopsy revealed a CD34+, Factor XIIIa-DFSP, harboring numerous, pigmented spindle S100+, Mart-1+ and HMB-45+ cells. Subsequent wide excision demonstrated pigmented dendritic and spindled cells widely scattered throughout the dermis of the 3-cm excisional margins and punch biopsy specimens of normal skin from both shoulders. This latter process was interpreted as dermal melanocytosis (nevus of Ito). The dermal pigmented spindle cells were Mart-1+ and CD34-, and were associated with non-pigmented CD34+, cytologically banal spindle cells, which were more numerous in the excisional margins than the contralateral shoulder.

CONCLUSION

Reported herein is a singular case of Bednár tumor associated with dermal melanocytosis. Although the coexistence of these processes implicates colonization of the DFSP by constituent dermal melanocytes, the mixed immunophenotype (CD34+ or Mart-1+ cells) of dispersed dermal spindle cells hints at the possibility of a common cell of origin: the putative neuromesenchymal cell. In effect, the Bednár tumor could represent one part of a spectrum of neural crest-derived dermal tumors that includes dermal melanocytosis, cellular blue nevus and conventional DFSP.

Coordination of cell growth in cocultures by a genetic proliferation control system

Advanced cell culture systems such as organotypic cultures or stem cell expansion often involve cocultivation of two or more different cell types. A typical situation is the use of a feeder cell line that supports survival, growth or controls the differentiation status of primary cells. Unequal proliferation capacities result in unbalanced cell growth, such as the undesired expansion of immortalized cells at the expense of the primary cells. We utilized a genetic regulatory system to coordinate cell growth in cocultures. Proliferation control of a feeder cell line by a recombinant transcription factor was employed to prevent feeder cells from overgrowing cocultured stem cells, thereby avoiding premature media exhaustion. Relevant characteristic cell-surface proteins present on the parental feeder cell line were preserved. No special equipment was required and cell-type specific growth could be adjusted even during coculture. When compared to a conventional gamma-irradiation procedure, genetic proliferation control was superior with respect to ease of use, robustness, cell adherence, morphology and viability. The results demonstrate the applicability of genetic proliferation control to coordinate growth of different cell types in cocultures.

Possible mechanisms of hypopigmentation in lichen sclerosus

Lichen sclerosus (LS) shares with vitiligo a milky-white appearance. By biopsy, pathognomonic dermal sclerosis readily distinguishes LS from vitiligo and other causes of leukoderma. To determine what the mechanism of hypopigmentation is in LS, we examined samples from LS cases for alterations in melanin content (Fontana-Masson stain) and melanocyte number (HMB-45 [PMEL-17/gp100], Mel-5 [TRP-1], Mart-1 [Melan A]) and compared these findings with those in controls of normal skin, acute scars, vitiligo, and lichen planus (LP; a common inflammatory cause of hyperpigmentation). The degree and extent of melanization found in LS overlapped with that in acute scars showing predominantly hypomelanized keratinocytes, with that in LP containing regions with numerous melanophages, and with that in vitiligo exhibiting focal regions of keratinocytes devoid of melanin pigment. By hematoxylin-eosin staining and immunocytochemistry for Mel-5 and Mart-1, LS had a lower mean count of melanocytes than acute scars, LP, and normal skin per 200 basal keratinocytes. In addition, a few LS cases had a significant loss of melanocytes comparable to that of vitiligo. Surprisingly, Mart-1 identified rare melanocytes in 67% of vitiligo cases and a significantly larger pool of melanocytes in LS and controls other than those labeled by Mel-5. Furthermore, LP and evolving lesions of LS contained the highest Mart-1 counts. HMB-45-immunoreactive melanocytes were found in the majority of acute scars and in LP and late-stage LS lesions at significantly lower levels than Mel-5- and Mart-1- labeled melanocytes, but they were not found in vitiligo or normal skin. We propose that several mechanisms may play a role in the production of leucoderma in LS: 1) decreased melanin production; 2) block in transfer of melanosomes to keratinocytes; and 3) melanocyte loss. The latter finding may be the pathogenic connection (lichenoid dermatitis of LS triggering an autoimmune reaction to melanocytes) that underlies the documented association of LS with vitiligo.