Fig. 1
Schematic illustration of dedifferentiation of epidermal cells. Differentiated or differentiating epidermal cells respond to internal or external signals by undergoing dedifferentiation. In this context, stimulation of wound or growth factors induce reentry into cell-cycle, proliferation, and redifferentiation. We postulate that Wnt/β-catenin pathway and ERK pathway underlie epithelial cell dedifferentiation and that the reversion is a crucial or even necessary step in the endogenous regeneration when endogenous stem cells are lost or exhausted
To follow up the successful cell and animal experiments, we began a clinical trial of burn patients. We obtained autologus bone-marrow MSCs from patients and co-cultured them as previously described for transplantation onto skin wounds; repaired skin showed regeneration of a sweat-gland-like structure. We then confirmed sweat-gland-like structures in biopsies of treated wounds, and these sweat-gland-like structures showed normal sweating function (Fig. 2). The pH and levels of biochemical components, such as Na+, K+, Cl− and iCa2+, in sweat obtained from the transplantation area were similar to that in normal sweat. To date, 23 patients with severe burn have undergone this innovative sweat-gland regeneration process, with satisfactory sweating results. The follow-up in some typical cases over 2 years has confirmed continuous sweating function (Fig. 2). Another interesting finding is that the scar formation in induced MSC-treated wounds is less than that in controls (Figs. 2 and 3) [16].
Fig. 2
One-year follow-up of a burn case treated with induced mesenchymal stem cells (MSCs) and its control. Iodine-starch perspiration test confirmed a positive reaction in wounds implanted with sweat-gland-like cells (a) and a negative reaction in the control (b). Two-year follow-up in a burn case treated with induced MSCs and its control. Iodine-starch perspiration test confirmed a negative reaction in the control (c) but a positive reaction in wounds implanted with sweat-gland-like cells (d)
Fig. 3
Three-month follow-up in a burn case treated with induced MSCs. Scar formation in wounds treated with induced MSCs (a) is less than that in the control (b)
From these results, we are further studying whether these induced SGCs could be used as seed cells to establish a new generation of tissue-engineered skin with sweat glands. Because of the different pH and cell culture conditions needed for SGCs and tissue-engineered skin, a medium suitable for both is a key issue. Our preliminary studies confirmed that a new generation of tissue-engineered skin containing sweat glands could be established with this method.
Our studies are preliminary successes in sweat-gland regeneration and bring new hope for small-organ repair and regeneration through stem-cell dedifferentiation or transdifferentiation, but we still have a long way to go. Questions of mechanisms and clinical application still need to be answered. The first issue is whether the co-culture media and condition we used is the best niche for transdifferentiation of MSCs into SGCs. On histology, we found sweat-gland-like structures in biopsies from induced MSC-treated wounds, but the structures differed from the normal sweat-gland structures. Thus, whether these structures have the same perspiration mechanisms and function as normal sweat glands still needs to be studied. Another issue is whether our method can solve all perspiration anomalies in massive and deep skin burns. We observed perspiration results only in some small areas. As well, we wonder whether our innovative methods can be used for a new generation of tissue-engineered skin containing sweat glands. However, our preliminary work investigating engineering skin constructs with sweat glands provides new hope for burn patients.
3 Sweat Gland Regeneration and Its Translation Application: New Generation of Artificial Skins with Sweat Glands
Current available artificial skin products has offered great promise in the treatment of burns, donor sites, chronic skin ulcers (e.g., venous, pressure and diabetic foot ulcers), and various other dermatological conditions. Despite therapeutic efficacy of these skin substitutes shown in vivo, they have not yet replaced the current “gold standard” of an autologous skin graft and have even not achieved wide use in clinical treatment. This situation may be explained, at least in part, by the fact that no artificial skin product can completely replicate the anatomy, physiology or biological stability of nature skin. One of the main issues in structure and function loss is the shortage of skin appendages. From a therapeutic standpoint, the presence of skin appendages is of major clinical importance for the maintenance of skin homeostasis and physiological function. For example, sweat glands is likely to play a key role in regulating body temperature, which may offer the hope to improve comfort level and living quality of survivors from severe skin injury.
The regenerative role of stem cells residing in skin appendages is multiple; such cells can participate in the repair and regeneration of injured skin. However, with extensive exploration on the road of regenerating defective skin tissues, hardly have we seen the structural and functional formation of sweat gland, which derives from epithelial compartment during development. Recent work of Reichmann proposed that human sweat glands were an alternative source of keratinocytes to generate a stratified epidermis. However, sweat gland-derived epithelial cells switched their phenotype to keratinocytes and there were no sweat gland in the ultimately formed epidermis. These observations may confer us with the clue that proper epithelial-mesenchymal interaction which exists throughout sweat gland development is essential for its morphogenesis and maintenance of epithelial homeostasis in the engineered models. Actually, from a tissue engineering point of view, this limitation can be overcome by the development of bioengineered 3D models that mimic nature skin microenvironment, in which epidermal cells are grown at an air–liquid interface on a connective tissue substrate harboring viable fibroblasts.
Although a number of tissue engineering-based therapies for research purposes and for clinical applications have been vigorously investigated, this strategy for restoration of functional sweat glands in renewal skin remains unmentioned. By multiple steps of tissue engineering operation, we propose that an artificial skin incorporating sweat glands can be constructed in vitro for improving the quality of skin repair and sweat gland regeneration during wound healing process (Fig. 4). To ensure sweat glands can be integrated, 3D human skin reconstruct is engineered to recapitulate natural sweat gland growth matrix. The reconstruct consists of a “dermis” with fibroblasts embedded in a collagen-based matrix, which provides scaffolding, nutrient delivery, and potential for cell-to-cell interaction; an “epidermis”, which is comprised of sweat gland cells and epidermal growth factor (EGF)-loaded microspheres, which represent as both a slow-release depot for growth factors and a delivery vehicle for sweat gland cells [17]. Naturally derived materials-based microsphere-technology has attracted growing interest as promising tools in cell transplantation and tissue engineering applications. By this method, trypsinization could be avoided and the cells could be transported to grow in a more natural state. Moreover, more potent and constant effect is demonstrated to come from the growth factors incorporated into microspheres, which would otherwise rapidly diffuse or be readily enzymatically digested or deactivated. Based on our previous founding, EGF exhibited the potential of stimulating sweat gland regeneration. This is in consistency with the report that postnatal application of EGF could induce the formation of functional sweat glands on the paws of the tabby mouse, a model for X-linked anhydrotic ectodermal dysplasia characterized by lack of sweat glands and other tissues of ectodermal origin [18–26].
Fig. 4
Construction of artificial skin incorporating sweat glands. The reconstruct consists of a “dermis” with fibroblasts embedded in a collagen-based matrix and an “epidermis” with sweat gland cells and epidermal growth factor (EGF)-loaded microspheres complex. The efficacy of wound healing and sweat gland regeneration was examined by implanting the artificial skin into excisional wounds on both back and paws of hind legs in a murine model
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
