Claudie Paquet

Regeneration of Skin and Cornea by Tissue Engineering

Progress in tissue engineering has led to the development of technologies allowing the reconstruction of autologous tissues from the patients own cells. Thus, tissue-engineered epithelial substitutes produced from cultured skin epithelial cells undergo long-term regeneration after grafting, indicating that functional stem cells were preserved during culture and following grafting. However, these cultured epithelial sheets reconstruct only the upper layer of the skin and lack the mechanical properties associated to the connective tissue of the dermis. We have designed a reconstructed skin entirely made from human cutaneous cells comprising both the dermis and the epidermis, as well as a well-organized basement membrane by a method named the self-assembly approach. In this chapter, protocols to generate reconstructed skin and corneal epithelium suitable for grafting are described in details. The methods include extraction and culture of human skin keratinocytes, human skin fibroblasts as well as rabbit and human corneal epithelial cells, and a complete description of the skin reconstructed by the self-assembly approach and of corneal epithelium reconstructed over a fibrin gel.

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 donors 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.

The Feeder layer-mediated extended lifetime of cultured human skin keratinocytes is associated with altered levels of the transcription factors Sp1 and Sp3

Primary cultured epithelial cells that are used for basic research are often cultivated on plastic whereas those used for clinical purposes are usually cultured in the presence of a feeder layer. Here, we examined the influence of a feeder layer on the expression, affinity and DNA binding ability of the transcription factors, Sp1 and Sp3 in primary cultures of human skin keratinocytes. Co‐culturing both newborn and adult skin keratinocytes with lethally irradiated 3T3 cells as a feeder layer contributed to maintain the cells morphological and growth characteristics and delayed terminal differentiation in vitro. 3T3 also stabilized the DNA binding properties of Sp1 without altering its transcription. Stimulation of Sp1/Sp3 expression appears to be mediated through cell–cell interactions and by factors secreted by 3T3. Thus, we propose that the feeder layer delay terminal differentiation of primary cultured skin keratinocytes by preventing extinction of transcription factors, like Sp1 and Sp3, which play pivotal functions in the cell cycle. J.Cell.Physiol.

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.

Considerations in the choice of a skin donor site for harvesting keratinocytes containing a high proportion of stem cells for culture in vitro

The treatment of severely burned patients has benefited from the grafting of skin substitutes obtained by expansion of epithelial cells in culture. The aim of this study was to evaluate whether the anatomic site chosen for harvesting skin had an impact on the quality of the derived cell cultures. Considering that hair follicles contain epithelial stem cells, we compared hairy skin sites featuring different densities and sizes of hair follicles for their capacity to generate high quality keratinocyte cultures. Three anatomic sites from adult subjects were compared: scalp, chest skin and p-auricular (comprising pre-auricular and post-auricular) skin. Keratin (K) 19 was used as a marker to evaluate the proportion of stem cells. Keratinocytes were isolated using the two-step thermolysin and trypsin cell extraction method, and cultured in vitro. The proportion of K19-positive cells harvested from p-auricular skin was about twice that of the scalp. This K19-positive cell content also remained higher during the first subcultures. In contrast to these in vitro results, the number of K19-positive cells estimated in situ on skin sections was about double in scalp as in p-auricular skin. Chest skin had the lowest number of K19-positive cells. These results indicate that in addition to the choice of an adult anatomic site featuring a high number of stem cells in situ, the quality of the cultures greatly depends on the ability to extract stem cells from the skin biopsy.

La médecine régénératrice : les cellules souches, les interactions cellulaires et matricielles dans la reconstruction cutanée et cornéenne par génie tissulaire

Considering that there is a shortage of organ donor, the aim of tissue engineering is to develop substitutes for the replacement of wounded or diseased tissues. Autologous tissue is evidently a preferable transplant material for long-term graft persistence because of the unavoidable rejection reaction occuring against allogeneic transplant. For the production of such substitutes, it is essential to control the culture conditions for post-natal human stem cells. Furthermore, histological organization and functionality of reconstructed tissues must approach those of native organs. For self-renewing tissues such as skin and cornea, tissue engineering strategies must include the preservation of stem cells during the in vitro process as well as after grafting to ensure the long-term regeneration of the transplants. We described a tissue engineering method named the self-assembly approach allowing the production of autologous living organs from human cells without any exogenous biomaterial. This approach is based on the capacity of mesenchymal cells to create in vitro their own extracellular matrix and then reform a tissue. Thereafter, various techniques allow the reorganization of such tissues in more complex organ such as valve leaflets, blood vessels, skin or cornea. These tissues offer the hope of new alternatives for organ transplantation in the future. In this review, the importance of preserving stem cells during in vitro expansion and controlling cell differentiation as well as tissue organization to ensure quality and functionality of tissue-engineered organs will be discussed, while focusing on skin and cornea.

Contribution of Sp1 to Telomerase Expression and Activity in Skin Keratinocytes Cultured With a Feeder Layer.

The growth of primary keratinocytes is improved by culturing them with a feeder layer. The aim of this study was to assess whether the feeder layer increases the lifespan of cultured epithelial cells by maintaining or improving telomerase activity and expression. The addition of an irradiated fibroblast feeder layer of either human or mouse origin (i3T3) helped maintain telomerase activity as well as expression of the transcription factor Sp1 in cultured keratinocytes. In contrast, senescence occurred earlier, together with a reduction of Sp1 expression and telomerase activity, in keratinocytes cultured without a feeder layer. Telomerase activity was consistently higher in keratinocytes grown on the three different feeder layers tested relative to cells grown without them. Suppression of Sp1 expression by RNA inhibition (RNAi) reduced both telomerase expression and activity in keratinocytes and also abolished their long‐term growth capacity suggesting that Sp1 is a key regulator of both telomerase gene expression and cell cycle progression of primary cultured human skin keratinocytes. The results of the present study therefore suggest that the beneficial influence of the feeder layer relies on its ability to preserve telomerase activity in cultured human keratinocytes through the maintenance of stable levels of Sp1 expression. J. Cell. Physiol. 230: 308–317, 2015.