Keratinocytes can differentiate into eccrine sweat ducts in vitro: involvement of epidermal growth factor and fetal bovine serum

Ontogeny of Skin Stem Cells and Molecular Underpinnings

Skin stem cells (SCs) play a pivotal role in supporting tissue homeostasis. Several types of SCs are responsible for maintaining and regenerating skin tissue. These include bulge SCs and others residing in the interfollicular epidermis, infundibulum, isthmus, sebaceous glands, and sweat glands. The emergence of skin SCs commences during embryogenesis, where multipotent SCs arise from various precursor populations. These early events set the foundation for the diverse pool of SCs that will reside in the adult skin, ready to respond to tissue repair and regeneration demands. A network of molecular cues regulates skin SC behavior, balancing quiescence, self-renewal, and differentiation. The disruption of this delicate equilibrium can lead to SC exhaustion, impaired wound healing, and pathological conditions such as skin cancer. The present review explores the intricate mechanisms governing the development, activation, and differentiation of skin SCs, shedding light on the molecular signaling pathways that drive their fate decisions and skin homeostasis. Unraveling the complexities of these molecular drivers not only enhances our fundamental knowledge of skin biology but also holds promise for developing novel strategies to modulate skin SC fate for regenerative medicine applications, ultimately benefiting patients with skin disorders and injuries.

Bioactive materials for in vivo sweat gland regeneration

Loss of sweat glands (SwGs) commonly associated with extensive skin defects is a leading cause of hyperthermia and heat stroke. In vivo tissue engineering possesses the potential to take use of the body natural ability to regenerate SwGs, making it more conducive to clinical translation. Despite recent advances in regenerative medicine, reconstructing SwG tissue with the same structure and function as native tissue remains challenging. Elucidating the SwG generation mechanism and developing biomaterials for in vivo tissue engineering is essential for understanding and developing in vivo SwG regenerative strategies. Here, we outline the cell biology associated with functional wound healing and the characteristics of bioactive materials. We critically summarize the recent progress in bioactive material-based cell modulation approaches for in vivo SwG regeneration, including the recruitment of endogenous cells to the skin lesion for SwG regeneration and in vivo cellular reprogramming for SwG regeneration. We discussed the re-establishment of microenvironment via bioactive material-mediated regulators. Besides, we offer promising perspectives for directing in situ SwG regeneration via bioactive material-based cell-free strategy, which is a simple and effective approach to regenerate SwG tissue with both fidelity of structure and function. Finally, we discuss the opportunities and challenges of in vivo SwG regeneration in detail. The molecular mechanisms and cell fate modulation of in vivo SwG regeneration will provide further insights into the regeneration of patient-specific SwGs and the development of potential intervention strategies for gland-derived diseases.

Regional and time course differences in sweat cortisol, glucose, and select cytokine concentrations during exercise

INTRODUCTION

The use of sweat as a biofluid for non-invasive sampling and diagnostics is a popular area of research. However, concentrations of cortisol, glucose, and cytokines have not been described across anatomical regions or as time progresses throughout exercise.

PURPOSE

To determine regional and time course differences in sweat cortisol, glucose, and select cytokines (EGF, IFN-γ, IL-1β, IL-1α, IL-1ra, TNF-α, IL-6, IL-8, and IL-10).

METHODS

Sweat was collected with absorbent patches from eight subjects (24-44 y; 80.2 ± 10.2 kg) on the forehead (FH), right dorsal forearm (RDF), right scapula (RS), and right triceps (RT) at 0-25 min, 30-55 min, and 60-85 min during 90 min of cycling (~ 82% HRmax) in a heated chamber (32 °C, 50% rh). ANOVA was used to determine the effect of site and time on outcomes. Data are reported as LS means ± SE.

RESULTS

There was a significant effect of location on sweat analyte concentrations with FH having higher values than most other regions for cortisol (FH: 1.15 ± 0.08 ng/mL > RDF: 0.62 ± 0.09 ng/mL and RT: 0.65 ± 0.12 ng/mL, P = 0.02), IL-1ra (P < 0.0001), and IL-8 (P < 0.0001), but lower concentrations for glucose (P = 0.01), IL-1α (P < 0.0001), and IL-10 (P = 0.02). Sweat IL-1β concentration was higher on the RS than RT (P < 0.0001). Sweat cortisol concentration increased (25 min: 0.34 ± 0.10 ng/mL < 55 min: 0.89 ± 0.07 ng/mL < 85 min: 1.27 ± 0.07 ng/mL; P < 0.0001), while EGF (P < 0.0001), IL-1ra (P < 0.0001), and IL-6 (P = 0.02) concentrations decreased over time.

CONCLUSION

Sweat analyte concentrations varied with time of sampling and anatomical region, which is essential information to consider when conducting future work in this area.

CLINICAL TRIAL IDENTIFIER

NCT04240951 registered January 27, 2020.

Innovative Approaches and Advances for Hair Follicle Regeneration

Pathological hair loss (also known as alopecia) and shortage of hair follicle (HF) donors have posed an urgent requirement for HF regeneration. With the revelation of mechanisms in tissue engineering, the proliferation of HFs in vitro has achieved more promising trust for the treatments of alopecia and other skin impairments. Theoretically, HF organoids have great potential to develop into native HFs and attachments such as sweat glands after transplantation. However, since the rich extracellular matrix (ECM) deficiency, the induction characteristics of skin-derived cells gradually fade away along with their trichogenic capacity after continuous cell passaging in vitro. Therefore, ECM-mimicking support is an essential prelude before HF transplantation is implemented. This review summarizes the status of providing various epidermal and dermal cells with a three-dimensional (3D) scaffold to support the cell homeostasis and better mimic in vivo environments for the sake of HF regeneration. HF-relevant cells including dermal papilla cells (DPCs), hair follicle stem cells (HFSCs), and mesenchymal stem cells (MSCs) are able to be induced to form HF organoids in the vitro culture system. The niche microenvironment simulated by different forms of biomaterial scaffold can offer the cells a network of ordered growth environment to alleviate inductivity loss and promote the expression of functional proteins. The scaffolds often play the role of ECM substrates and bring about epithelial-mesenchymal interaction (EMI) through coculture to ensure the functional preservation of HF cells during in vitro passage. Functional HF organoids can be formed either before or after transplantation into the dermis layer. Here, we review and emphasize the importance of 3D culture in HF regeneration in vitro. Finally, the latest progress in treatment trials and critical analysis of the properties and benefits of different emerging biomaterials for HF regeneration along with the main challenges and prospects of HF regenerative approaches are discussed.

3D-bioprinted microenvironments for sweat gland regeneration

The development of 3D bioprinting in recent years has provided new insights into the creation of in vitro microenvironments for promoting stem cell-based regeneration. Sweat glands (SGs) are mainly responsible for thermoregulation and are a highly differentiated organ with limited regenerative ability. Recent studies have focused on stem cell-based therapies as strategies for repairing SGs after deep dermal injury. In this review, we highlight the recent trend in 3D bioprinted native-like microenvironments and emphasize recent advances in functional SG regeneration using this technology. Furthermore, we discuss five possible regulatory mechanisms in terms of biochemical factors and structural and mechanical cues from 3D bioprinted microenvironments, as well as the most promising regulation from neighbor cells and the vascular microenvironment.

Eccrine Sweat Gland and Its Regeneration: Current Status and Future Directions

Eccrine sweat glands (ESGs) play an important role in temperature regulation by secreting sweat. Insufficiency or dysfunction of ESGs in a hot environment or during exercise can lead to hyperthermia, heat exhaustion, heatstroke, and even death, but the ability of ESGs to repair and regenerate themselves is very weak and limited. Repairing the damaged ESGs and regenerating the lost or dysfunctional ESGs poses a challenge for dermatologists and bum surgeons. To promote and accelerate research on the repair and regeneration of ESGs, we summarized the development, structure and function of ESGs, and current strategies to repair and regenerate ESGs based on stem cells, scaffolds, and possible signaling pathways involved.

Advances in generation of three-dimensional skin equivalents: pre-clinical studies to clinical therapies

The inability of two-dimensional cell culture systems to adequately map the structure and function of complex organs like skin necessitates the development of three-dimensional (3D) skin models. A diverse range of 3D skin equivalents have been developed over the last few decades for studying complex properties of skin as well as for drug discovery and clinical applications for skin regeneration in chronic wounds, such as diabetic foot ulcers, where the normal mechanism of wound healing is compromised. These 3D skin substitutes also serve as a suitable alternative to animal models in industrial applications and fundamental research. With the emergence of tissue engineering, new scaffolds and matrices have been integrated into 3D cell culture systems, along with gene therapy approaches, to increase the efficacy of transplanted cells in skin regeneration. This review summarizes recent approaches to the development of skin equivalents as well as different models for studying skin diseases and properties and current therapeutic applications of skin substitutes.

Biochemical and structural cues of 3D-printed matrix synergistically direct MSC differentiation for functional sweat gland regeneration

Mesenchymal stem cells (MSCs) encapsulation by three-dimensionally (3D) printed matrices were believed to provide a biomimetic microenvironment to drive differentiation into tissue-specific progeny, which made them a great therapeutic potential for regenerative medicine. Despite this potential, the underlying mechanisms of controlling cell fate in 3D microenvironments remained relatively unexplored. Here, we bioprinted a sweat gland (SG)-like matrix to direct the conversion of MSC into functional SGs and facilitated SGs recovery in mice. By extracellular matrix differential protein expression analysis, we identified that CTHRC1 was a critical biochemical regulator for SG specification. Our findings showed that Hmox1 could respond to the 3D structure activation and also be involved in MSC differentiation. Using inhibition and activation assay, CTHRC1 and Hmox1 synergistically boosted SG gene expression profile. Together, these findings indicated that biochemical and structural cues served as two critical impacts of 3D-printed matrix on MSC fate decision into the glandular lineage and functional SG recovery.

Sweat gland organoids contribute to cutaneous wound healing and sweat gland regeneration

Sweat glands perform a vital thermoregulatory function in mammals. Like other skin components, they originate from epidermal progenitors. However, they have low regenerative potential in response to injury. We have established a sweat gland culture and expansion method using 3D organoids cultures. The epithelial cells derived from sweat glands in dermis of adult mouse paw pads were embedded into Matrigel and formed sweat gland organoids (SGOs). These organoids maintained remarkable stem cell features and demonstrated differentiation capacity to give rise to either sweat gland cells (SGCs) or epidermal cells. Moreover, the bipotent SGO-derived cells could be induced into stratified epidermis structures at the air−liquid interface culture in a medium tailored for skin epidermal cells in vitro. The SGCs embedded in Matrigel tailored for sweat glands formed epithelial organoids, which expressed sweat-gland-specific markers, such as cytokeratin (CK) 18 and CK19, aquaporin (AQP) 5 and αATP. More importantly, they had potential of regeneration of epidermis and sweat gland when they were transplanted into the mouse back wound and claw pad with sweat gland injury, respectively. In summary, we established and optimized culture conditions for effective generation of mouse SGOs. These cells are candidates to restore impaired sweat gland tissue as well as to improve cutaneous skin regeneration.

Role of caveolin-1 in epidermal stem cells during burn wound healing in rats

Local transplantation of stem cells has therapeutic effects on skin damage but cannot provide satisfactory wound healing. Studies on the mechanisms underlying the therapeutic effects of stem cells on skin wound healing will be needed. Hence, in the present study, we explored the role of Caveolin-1 in epidermal stem cells (EpiSCs) in the modulation of wound healing. We first isolated EpiSCs from mouse skin tissues and established stable EpiSCs with overexpression of Caveolin-1 using a lentiviral construct. We then evaluated the epidermal growth factor (EGF)-induced cell proliferation ability using cell counting Kit-8 (CCK-8) assay and assessed EpiSC pluripotency by examining Nanog mRNA levels in EpiSCs. Furthermore, we treated mice with skin burn injury using EpiSCs with overexpression of Caveolin-1. Histological examinations were conducted to evaluate re-epithelialization, wound scores, cell proliferation and capillary density in wounds. We found that overexpression of Caveolin-1 in EpiSCs promoted EGF-induced cell proliferation ability and increased wound closure in a mouse model of skin burn injury. Histological evaluation demonstrated that overexpression of Caveolin-1 in EpiSCs promoted re-epithelialization in wounds, enhanced cellularity, and increased vasculature, as well as increased wound scores. Taken together, our results suggested that Caveolin-1 expression in the EpiSCs play a critical role in the regulation of EpiSC proliferation ability and alteration of EpiSC proliferation ability may be an effective approach in promoting EpiSC-based therapy in skin wound healing.

Gene-activated matrix/bone marrow-derived mesenchymal stem cells constructs regenerate sweat glands-like structure in vivo

It is a significant challenge to regenerate full-thickness skin defects with sweat glands. Various skin substitutes have been developed to resolve this issue with minimal success. In this study, to yield a novel construct for in situ regeneration of sweat glands, the collagen-chitosan porous scaffold was combined with Lipofectamine 2000/pDNA-EGF complexes to obtain the gene-activated scaffold (GAS), which was then seeded with bone marrow-derived mesenchymal stem cells (BM-MSCs). The porous scaffold functionalized as a reservoir for the incorporated gene complexes which were released in a sustained manner. The seeded BM-MSCs were transfected in situ by the released complexes and specially differentiated into sweat gland cells in vitro under the induction of the expressed epidermal growth factor (EGF). Application in vivo of the GAS/BM-MSCs constructs on the full-thickness skin defects of SD rats confirmed that GAS/BM-MSCs could accelerate the wound healing process and induce the in situ regeneration of the full-thickness skin with sweat gland-like structures. Analyzed by immunohistochemical staining, RT-qPCR and Western-blotting, the levels of the major sweat gland markers such as carcino-embryonic antigen (CEA), cytokeratin 8 (CK8) and cytokeratin 14 (CK14) were all up-regulated, indicating that GAS/BM-MSCs can facilitate the regeneration of sweat glands-like structure in vivo.

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.

3D bioprinted extracellular matrix mimics facilitate directed differentiation of epithelial progenitors for sweat gland regeneration

Sweat glands perform a vital thermoregulatory function in mammals. Like other skin appendages, they originate from epidermal progenitors. However, they have low regenerative potential in response to injury, and whether adult epidermal progenitors could be specified to differentiate to a sweat gland cell lineage remains largely unexplored. We used bioprinting technology to create a functional in vitro cell-laden 3D extracellular matrix mimics (3D-ECM) with composite hydrogels based on gelatin and sodium alginate because of chemical and structural similarity to ECM components. To achieve specific cell differentiation, mouse plantar dermis and epidermal growth factor were synchronously incorporated into the 3D-ECM mimics to create an inductive niche for epidermal progenitor cells obtained from mice. The biological 3D construct could maintain cell viability, thereby facilitating cell spreading and matrix formation. In vitro data by immunofluorescence and gene expression assay of key cell-surface markers demonstrated that the bioprinted 3D-ECM could effectively create a restrictive niche for epidermal progenitors that ensures unilateral differentiation into sweat gland cells. Furthermore, direct delivery of bioprinted 3D-ECM into burned paws of mice resulted in functional restoration of sweat glands. This study represents the rational design to enhance the specific differentiation of epidermal lineages using 3D bioprinting and may have clinical and translational implications in regenerating sweat glands.

STATEMENT OF SIGNIFICANCE

Sweat gland regeneration after injury is of clinical importance but remains largely unsolved because of low regenerative potential and lack of a definite niche. Some studies have shown sweat gland regeneration with gene-based interventions or cell-based induction via embryonic components, but translation to clinic is challenging. The novelty and significance of the work lies in the fact that we design a 3D bioprinted extracellular matrix that provides the spatial inductive cues for enhancing specific differentiation of epidermal lineages to regenerate sweat glands, which is critical for treating deep burns or other wounds. Our studies are encouraging given the overwhelming advantages of our designed 3D bioprinting construct over other cell delivery technology in maintaining high cell proliferation; another interesting finding is that adult tissue components retain a gland lineage-inductive power as embryonic tissue, which can facilitate translation.

Sweat gland regeneration after burn injury: is stem cell therapy a new hope?

Stem cells are the seeds of tissue repair and regeneration and a promising source for novel therapies. The skin of patients with an extensive deep burn injury is repaired by a hypertrophic scar without regeneration of sweat glands and therefore loses the function of perspiration. Stem cell therapy provides the possibility of sweat gland regeneration. In particular, recent studies have reported the reprogramming of mesenchymal stromal cells into sweat gland-like (SGL) cells. We present an overview of recent researches into sweat gland regeneration with stem cells. Difficulties of sweat gland regeneration after deep burns have been elaborated. The advantage and disadvantage of several stem cell types in sweat gland regeneration have been discussed. Additionally, the possible mechanisms for reprogramming stem cells to SGL cells are summarized. A brief discussion on clinical application of stem cell-derived SGL cells is also presented. This review may possibly provide some implications for sweat gland regeneration.

Sweat gland progenitors in development, homeostasis, and wound repair

The human body is covered with several million sweat glands. These tiny coiled tubular skin appendages produce the sweat that is our primary source of cooling and hydration of the skin. Numerous studies have been published on their morphology and physiology. Until recently, however, little was known about how glandular skin maintains homeostasis and repairs itself after tissue injury. Here, we provide a brief overview of sweat gland biology, including newly identified reservoirs of stem cells in glandular skin and their activation in response to different types of injuries. Finally, we discuss how the genetics and biology of glandular skin has advanced our knowledge of human disorders associated with altered sweat gland activity.

Antigen expression of human eccrine sweat glands

BACKGROUND

The proliferating abilities of sweat glands are very limited, so researches on the repair and regeneration of sweat glands are important. First of all, we must find out reliable and specific antigen markers of sweat glands.

OBJECTIVE

To investigate the antigen expression of human eccrine sweat glands.

METHODS

The development of eccrine sweat glands was investigated by hematoxylin and eosin staining, and the antigen expression was detected by immunohistochemical techniques.

RESULTS

Human eccrine sweat glands expressed cytokeratin (CK) 7, CK8, CK14, CK18, CK19 and epithelial membrane antigen (EMA). Carcinoembryonic antigen (CEA) was only expressed in sweat glands in the adult skin. Developing and developed sweat glands all had some cells expressing Ki67 and p63 antigens. Epidermal growth factor (EGF) was mainly localized in the secretory cells and ductal cells. Some myoepithelial cells were also labeled with anti-EGF antibody. In the older fetus, positive staining for EGF was seen in the lumen of the secretory portion. EGF receptor (EGFR) was expressed in the ducts.

CONCLUSIONS

Human eccrine sweat glands express CK7, CK8, CK14, CK18, CK19, CEA, EMA, Ki67, p63, EGF and EGFR. In skin, CEA can be used as a specific immunological marker of sweat glands.

Therapeutic potential of stem cells in skin repair and regeneration

Stem cells are defined by their capacity of self-renewal and multilineage differentiation, which make them uniquely situated to treat a broad spectrum of human diseases. Based on a series of remarkable studies in several fields of regenerative medicine, their application is not too far from the clinical practice. Full-thickness burns and severe traumas can injure skin and its appendages, which protect animals from water loss, temperature change, radiation, trauma and infection. In adults, the normal outcome of repair of massive full-thickness burns is fibrosis and scarring without any appendages, such as hair follicles, sweat and sebaceous glands. Perfect skin regeneration has been considered impossible due to the limited regenerative capacity of epidermal keratinocytes, which are generally thought to be the key source of the epidermis and skin appendages. Currently, researches on stem cells, such as epidermal stem cells, dermal stem cells, mesenchymal stem cells from bone marrow, and embryonic stem cells, bring promise to functional repair of skin after severe burn injury, namely, complete regeneration of skin and its appendages. In this study, we present an overview of the most recent advances in skin repair and regeneration by using stem cells.

Postnatal expression and denervation induced up-regulation of aquaporin-5 protein in rat sweat gland

The evolution of aquaporin-5 (AQP5) expression during postnatal development has not been defined in the sweat gland. Previous studies have suggested that AQP isoforms in several peripheral targets are regulated by a neural mechanism. We have examined, in rat sweat glands, the expression of AQP5 during postnatal development and the effects of denervation on AQP5 expression. Both AQP5 mRNA and protein begin to be expressed at postnatal day 10, before sweat-secretory responsiveness first appears; this expression coincides with the occurrence of vasoactive intestinal peptide (VIP) immunoreactivity. Early noradrenergic and later cholinergic interaction between sweat glands and their innervation are disrupted by neonatal chemical sympathectomy or postnatal severance of the sciatic nerve. Examination of such denervated developing rats has shown that secretory responsiveness fails to arise later in the adults, and AQP5 immunostaining increases in the denervated glands, whereas gland morphogenesis and the occurrence of AQP5 expression proceed normally. Immunobloting has revealed an increase of AQP5 abundance after the denervated mature glands lose their secretory ability. These findings suggest that AQP5 protein is necessary for sweat secretion, and that the expression of AQP5 in rat sweat glands is independent of sympathetic innervation. Our data also indicate that factor(s) regulating the normal morphological development of sweat gland might be responsible for controlling AQP5 expression.

Adult bone-marrow-derived mesenchymal stem cells contribute to wound healing of skin appendages

Adult bone-marrow-derived mesenchymal stem cells (MSCs) are well-established as having the capacity to differentiate into cells with mesodermal, ectodermal, and endodermal characteristics and can leave their niche to home toward and engraft within foreign tissues. To investigate whether adult MSCs contribute to the repair of skin appendages after injury, BrdU-labeled MSCs were co-cultured with heat-shocked confluent sweat gland cells (SGCs) in vitro and later intravenously injected into full-thickness skin wounds in rats. When adult MSCs were co-cultured with heat-shocked SGCs, a subset of adult MSCs differentiated into SGCs, the percentage of differentiation being enhanced by epidermal growth factor and the injured microenviroment, but weakened by PD98059. The ERK (extracellular signal-regulated kinase) pathway, especially pERK, was involved in the phenotype conversion of human MSCs into human SGC. Labeled MSCs were noted in hair follicles, sebaceous glands, blood vessels, and dermis in full-thickness wounds, and the incorporated cells in hair follicles and sebaceous glands were also positive for pan-cytokeratin. After wound healing, some labeled MSCs returned to the bone marrow, whereas other were retained in the dermis. We conclude that adult MSCs have the capacity to dock at specific sites, to contribute to wound healing of skin appendages, and to home toward marrow, and that engraftment of bone-marrow-derived cells is a functional event.