Pullulan hydrogels improve mesenchymal stem cell delivery into high-oxidative-stress wounds

Frontier and hot topics in the application of hydrogel in the biomedical field: a bibliometric analysis based on CiteSpace

Hydrogels are formed of crosslinked polymer chains arranged in three-dimensional (3D) networks. These chains have good water-containing capacity and are soft and malleable. Hydrogels have good biocompatibility due to their significant water content, flexible structure, and numerous holes. These characteristics make them analogous to biological tissues. Despite the publication of 8700 literature related to hydrogel biomedical applications in the past 52 years (1973 ~ 2024), studies on the use of hydrogels in biomedicine are few. To gain a comprehensive understanding of their current development status, research trends, and prospects in the biomedical application field, it is imperative to conduct a thorough retrospective analysis. In this study, we employ bibliometric analysis and CiteSpace software to quantitatively and visually analyze articles published in this field. Firstly, we provide a quantitative analysis of authorship and institutional publications over the past 52 years to elucidate the fundamental development status regarding hydrogel biomedical applications. Secondly, we did visual studies on terms that are high-frequency, explosive, keyword clustering, and so on, to understand the directionality and evolution of the main research hotspots during each period. Notably, our findings emphasize that fabricating hydrogels into wound healing-promoting dressings emerges as a prominent hotspot within the application field. We anticipate that this paper will inspire researchers with novel ideas for advancing hydrogel applications in biomedicine.

A structure-functionality insight into the bioactivity of microbial polysaccharides toward biomedical applications: A review

Microbial polysaccharides (MPs) are biopolymers secreted by microorganisms such as bacteria and fungi during their metabolic processes. Compared to polysaccharides derived from plants and animals, MPs have advantages such as wide sources, high production efficiency, and less susceptibility to natural environmental influences. The most attractive feature of MPs lies in their diverse biological activities, such as antioxidative, anti-tumor, antibacterial, and immunomodulatory activities, which have demonstrated immense potential for applications in functional foods, cosmetics, and biomedicine. These bioactivities are precisely regulated by their sophisticated molecular structure. However, the mechanisms underlying this precise regulation are not yet fully understood and continue to evolve. This article presents a comprehensive review of the most representative species of MPs, including their fermentation and purification processes and their biomedical applications in recent years. In particular, this work presents an in-depth analysis into the structure-activity relationships of MPs across multiple molecular levels. Additionally, this review discusses the challenges and prospects of investigating the structure-activity relationships, providing valuable insights into the broad and high-value utilization of MPs.

Bringing innovative wound care polymer materials to the market: Challenges, developments, and new trends

The development of innovative products for treating acute and chronic wounds has become a significant topic in healthcare, resulting in numerous products and innovations over time. The growing number of patients with comorbidities and chronic diseases, which may significantly alter, delay, or inhibit normal wound healing, has introduced considerable new challenges into the wound management scenario. Researchers in academia have quickly identified promising solutions, and many advanced wound healing materials have recently been designed; however, their successful translation to the market remains highly complex and unlikely without the contribution of industry experts. This review article condenses the main aspects of wound healing applications that will serve as a practical guide for researchers working in academia and industry devoted to designing, evaluating, validating, and translating polymer wound care materials to the market. The article highlights the current challenges in wound management, describes the state-of-the-art products already on the market and trending polymer materials, describes the regulation pathways for approval, discusses current wound healing models, and offers a perspective on new technologies that could soon reach consumers. We envision that this comprehensive review will significantly contribute to highlighting the importance of networking and exchanges between academia and healthcare companies. Only through the joint of these two actors, where innovation, manufacturing, regulatory insights, and financial resources act in harmony, can wound care products be developed efficiently to reach patients quickly and affordably.

Fundamental properties of smart hydrogels for tissue engineering applications: A review

Tissue engineering is an advanced and potential biomedical approach to treat patients suffering from lost or failed an organ or tissue to repair and regenerate damaged tissues that increase life expectancy. The biopolymers have been used to fabricate smart hydrogels to repair damaged tissue as they imitate the extracellular matrix (ECM) with intricate structural and functional characteristics. These hydrogels offer desired and controllable qualities, such as tunable mechanical stiffness and strength, inherent adaptability and biocompatibility, swellability, and biodegradability, all crucial for tissue engineering. Smart hydrogels provide a superior cellular environment for tissue engineering, enabling the generation of cutting-edge synthetic tissues due to their special qualities, such as stimuli sensitivity and reactivity. Numerous review articles have presented the exceptional potential of hydrogels for various biomedical applications, including drug delivery, regenerative medicine, and tissue engineering. Still, it is essential to write a comprehensive review article on smart hydrogels that successfully addresses the essential challenging issues in tissue engineering. Hence, the recent development on smart hydrogel for state-of-the-art tissue engineering conferred progress, highlighting significant challenges and future perspectives. This review discusses recent advances in smart hydrogels fabricated from biological macromolecules and their use for advanced tissue engineering. It also provides critical insight, emphasizing future research directions and progress in tissue engineering.

Cas9-mediated knockout of Ndrg2 enhances the regenerative potential of dendritic cells for wound healing

Chronic wounds impose a significant healthcare burden to a broad patient population. Cell-based therapies, while having shown benefits for the treatment of chronic wounds, have not yet achieved widespread adoption into clinical practice. We developed a CRISPR/Cas9 approach to precisely edit murine dendritic cells to enhance their therapeutic potential for healing chronic wounds. Using single-cell RNA sequencing of tolerogenic dendritic cells, we identified N-myc downregulated gene 2 (Ndrg2), which marks a specific population of dendritic cell progenitors, as a promising target for CRISPR knockout. Ndrg2-knockout alters the transcriptomic profile of dendritic cells and preserves an immature cell state with a strong pro-angiogenic and regenerative capacity. We then incorporated our CRISPR-based cell engineering within a therapeutic hydrogel for in vivo cell delivery and developed an effective translational approach for dendritic cell-based immunotherapy that accelerated healing of full-thickness wounds in both non-diabetic and diabetic mouse models. These findings could open the door to future clinical trials using safe gene editing in dendritic cells for treating various types of chronic wounds.

Removal of Methylene Blue Dye from Aqueous Solutions by Pullulan Polysaccharide/Polyacrylamide/Activated Carbon Complex Hydrogel Adsorption

In this study, composite hydrogels were prepared using a simple synthetic technique to adsorb methylene blue (MB) from water. The hydrogel comprised potassium persulfate (KPS) as the initiator, N,N'-methylene bisacrylamide as the crosslinking agent, and sodium hydroxide (NaOH) as the activator. It was employed to adsorb MB at different concentrations from water. The morphology and properties of PUL/PAM/GO composites were characterized through thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. Moreover, the adsorption properties, adsorption isotherms, adsorption kinetics, adsorption thermodynamics, and swelling properties of the hydrogel for MB were investigated. The optimal ratio of PUL to AC was obtained as 6:1 by fixing the amount of PUL and loading AC of different masses. The maximum adsorption capacity was obtained as 591.4 mg/g. It also exhibited certain mechanical strength. The adsorption of MB conforms to pseudo-first-order kinetics and Langmuir isotherms. In this study, an environment-friendly, cheap, simple, and efficient way was presented for the composite hydrogel in the direction of water treatment.

Innovative Cell and Platelet Rich Plasma Therapies for Diabetic Foot Ulcer Treatment: The Allogeneic Approach

Cutaneous chronic wounds are a major global health burden in continuous growth, because of population aging and the higher incidence of chronic diseases, such as diabetes. Different treatments have been proposed: biological, surgical, and physical. However, most of these treatments are palliative and none of them can be considered fully satisfactory. During a spontaneous wound healing, endogenous regeneration mechanisms and resident cell activity are triggered by the released platelet content. Activated stem and progenitor cells are key factors for ulcer healing, and they can be either recruited to the wound site from the tissue itself (resident cells) or from elsewhere. Transplant of skin substitutes, and of stem cells derived from tissues such as bone marrow or adipose tissue, together with platelet-rich plasma (PRP) treatments have been proposed as therapeutic options, and they represent the today most promising tools to promote ulcer healing in diabetes. Although stem cells can directly participate to skin repair, they primarily contribute to the tissue remodeling by releasing biomolecules and microvesicles able to stimulate the endogenous regeneration mechanisms. Stem cells and PRP can be obtained from patients as autologous preparations. However, in the diabetic condition, poor cell number, reduced cell activity or impaired PRP efficacy may limit their use. Administration of allogeneic preparations from healthy and/or younger donors is regarded with increasing interest to overcome such limitation. This review summarizes the results obtained when these innovative treatments were adopted in preclinical animal models of diabetes and in diabetic patients, with a focus on allogeneic preparations.

Pullulan-Collagen Hydrogel Wound Dressing Promotes Dermal Remodeling and Wound Healing Compared to Commercially Available Collagen Dressings

Biological scaffolds such as hydrogels provide an ideal, physio‐mimetic of native extracellular matrix (ECM) that can improve wound healing outcomes after cutaneous injury. While most studies have focused on the benefits of hydrogels in accelerating wound healing, there are minimal data directly comparing different hydrogel material compositions. In this study, we utilized a splinted excisional wound model that recapitulates human‐like wound healing in mice and treated wounds with three different collagen hydrogel dressings. We assessed the feasibility of applying each dressing and performed histologic and histopathologic analysis on the explanted scar tissues to assess variations in collagen architecture and alignment, as well as the tissue response. Our data indicate that the material properties of hydrogel dressings can significantly influence healing time, cellular response, and resulting architecture of healed scars. Specifically, our pullulan‐collagen hydrogel dressing accelerated wound closure and promoted healed tissue with less dense, more randomly aligned, and shorter collagen fibres. Further understanding of how hydrogel properties affect the healing and resulting scar architecture of wounds may lead to novel insights and further optimization of the material properties of wound dressings.

Acrylonitrile and Pullulan Based Nanofiber Mats as Easily Accessible Scaffolds for 3D Skin Cell Models Containing Primary Cells

(1) Background: Three-dimensional (3D) collagen I-based skin models are commonly used in drug development and substance testing but have major drawbacks such as batch-to-batch variations and ethical concerns. Recently, synthetic nanofibrous scaffolds created by electrospinning have received increasing interest as potential alternatives due to their morphological similarities to native collagen fibrils in size and orientation. The overall objective of this proof-of-concept study was to demonstrate the suitability of two synthetic polymers in creating electrospun scaffolds for 3D skin cell models. (2) Methods: Electrospun nanofiber mats were produced with (i) poly(acrylonitrile-co-methyl acrylate) (P(AN-MA)) and (ii) a blend of pullulan (Pul), poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) (Pul/PVA/PAA) and characterized by scanning electron microscopy (SEM) and diffuse reflectance infrared Fourier transform (DRIFT) spectra. Primary skin fibroblasts and keratinocytes were seeded onto the nanofiber mats and analyzed for phenotypic characteristics (phalloidin staining), viability (Presto Blue HS assay), proliferation (Ki-67 staining), distribution (H/E staining), responsiveness to biological stimuli (qPCR), and formation of skin-like structures (H/E staining). (3) Results: P(AN-MA) mats were more loosely packed than the Pul/PVA/PAA mats, concomitant with larger fiber diameter (340 nm ± 120 nm vs. 250 nm ± 120 nm, p < 0.0001). After sterilization and exposure to cell culture media for 28 days, P(AN-MA) mats showed significant adsorption of fetal calf serum (FCS) from the media into the fibers (DRIFT spectra) and increased fiber diameter (590 nm ± 290 nm, p < 0.0001). Skin fibroblasts were viable over time on both nanofiber mats, but suitable cell infiltration only occurred in the P(AN-MA) nanofiber mats. On P(AN-MA) mats, fibroblasts showed their characteristic spindle-like shape, produced a dermis-like structure, and responded well to TGFβ stimulation, with a significant increase in the mRNA expression of PAI1, COL1A1, and αSMA (all p < 0.05). Primary keratinocytes seeded on top of the dermis equivalent proliferated and formed a stratified epidermis-like structure. (4) Conclusion: P(AN-MA) and Pul/PVA/PAA are both biocompatible materials suitable for nanofiber mat production. P(AN-MA) mats hold greater potential as future 3D skin models due to enhanced cell compatibility (i.e., adsorption of FCS proteins), cell infiltration (i.e., increased pore size due to swelling behavior), and cell phenotype preservation. Thus, our proof-of-concept study shows an easy and robust process of producing electrospun scaffolds for 3D skin cell models made of P(AN-MA) nanofibers without the need for bioactive molecule attachments.

Optimizing an Antioxidant TEMPO Copolymer for Reactive Oxygen Species Scavenging and Anti-Inflammatory Effects in Vivo

Oxidative stress is broadly implicated in chronic, inflammatory diseases because it causes protein and lipid damage, cell death, and stimulation of inflammatory signaling. Supplementation of innate antioxidant mechanisms with drugs such as the superoxide dismutase (SOD) mimetic compound 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) is a promising strategy for reducing oxidative stress-driven pathologies. TEMPO is inexpensive to produce and has strong antioxidant activity, but it is limited as a drug due to rapid clearance from the body. It is also challenging to encapsulate into micellar nanoparticles or polymer microparticles, because it is a small, water soluble molecule that does not efficiently load into hydrophobic carrier systems. In this work, we pursued a polymeric form of TEMPO [poly(TEMPO)] to increase its molecular weight with the goal of improving in vivo bioavailability. High density of TEMPO on the poly(TEMPO) backbone limited water solubility and bioactivity of the product, a challenge that was overcome by tuning the density of TEMPO in the polymer by copolymerization with the hydrophilic monomer dimethylacrylamide (DMA). Using this strategy, we formed a series of poly(DMA-co-TEMPO) random copolymers. An optimal composition of 40 mol % TEMPO/60 mol % DMA was identified for water solubility and O2•- scavenging in vitro. In an air pouch model of acute local inflammation, the optimized copolymer outperformed both the free drug and a 100% poly(TEMPO) formulation in O2•- scavenging, retention, and reduction of TNFα levels. Additionally, the optimized copolymer reduced ROS levels after systemic injection in a footpad model of inflammation. These results demonstrate the benefit of polymerizing TEMPO for in vivo efficacy and could lead to a useful antioxidant polymer formulation for next-generation anti-inflammatory treatments.

Hydrogel Scaffolds to Deliver Cell Therapies for Wound Healing

Cutaneous wounds are a growing global health burden as a result of an aging population coupled with increasing incidence of diabetes, obesity, and cancer. Cell-based approaches have been used to treat wounds due to their secretory, immunomodulatory, and regenerative effects, and recent studies have highlighted that delivery of stem cells may provide the most benefits. Delivering these cells to wounds with direct injection has been associated with low viability, transient retention, and overall poor efficacy. The use of bioactive scaffolds provides a promising method to improve cell therapy delivery. Specifically, hydrogels provide a physiologic microenvironment for transplanted cells, including mechanical support and protection from native immune cells, and cell-hydrogel interactions may be tailored based on specific tissue properties. In this review, we describe the current and future directions of various cell therapies and usage of hydrogels to deliver these cells for wound healing applications.

Stem cells and growth factors-based delivery approaches for chronic wound repair and regeneration: A promise to heal from within

The sophisticated chain of cellular and molecular episodes during wound healing includes cell migration, cell proliferation, deposition of extracellular matrix, and remodelling and are onerous to replicate. Encapsulation of growth factors (GFs) and Stem cell-based (SCs) has been proclaimed to accelerate healing by transforming every phase associated with wound healing to enhance skin regeneration. Therapeutic application of mesenchymal stem cells (MSCs), embryonic stem cells (ESCs) and induced pluripotent stem cells (PSCs) provides aid in wound fixing, tissue integrity restoration and function of impaired tissue. Several scientific studies have established the essential role GFs in wound healing and their reduced degree in the chronic wound. The overall limitation includes half-life, unfriendly microhabitat abundant with protease, and inadequate delivery approaches results in decreased delivery of effective amounts in a suitable time-based fashion. Advancements in the area of reformative medicine as well as tissue engineering have offered techniques competent of dispensing SCs and GFs in site-oriented manner. The progress in nanotechnology-based approaches attracts researcher to study and evaluate the potential of this SCs and GFs based therapy in chronic wounds. These techniques embrace the polymeric regime viz., nano-formulations, hydrogels, liposomes, scaffolds, nanofibers, metallic nanoparticles, lipid-based nanoparticles and dendrimers that have established better retort through targeting tissues when GFs and SCs are transported via these humans made devices. Assumed the current problems, improvements in delivery approaches and difficulties offered by chronic wounds, we hope to show that encapsulation of SCs and GFs loaded nanoformulations therapies is the rational next step in improving wound care.

Redox-Responsive Nanobiomaterials-Based Therapeutics for Neurodegenerative Diseases

Redox regulation has recently been proposed as a critical intracellular mechanism affecting cell survival, proliferation, and differentiation. Redox homeostasis has also been implicated in a variety of degenerative neurological disorders such as Parkinson's and Alzheimer's disease. In fact, it is hypothesized that markers of oxidative stress precede pathologic lesions in Alzheimer's disease and other neurodegenerative diseases. Several therapeutic approaches have been suggested so far to improve the endogenous defense against oxidative stress and its harmful effects. Among such approaches, the use of artificial antioxidant systems has gained increased popularity as an effective strategy. Nanoscale drug delivery systems loaded with enzymes, bioinspired catalytic nanoparticles and other nanomaterials have emerged as promising candidates. The development of degradable hydrogels scaffolds with antioxidant effects could also enable scientists to positively influence cell fate. This current review summarizes nanobiomaterial-based approaches for redox regulation and their potential applications as central nervous system neurodegenerative disease treatments.

Promotion of dermal regeneration using pullulan/gelatin porous skin substitute

Tissue-engineered dermal substitutes represent a promising approach to improve wound healing and provide more sufficient regeneration, compared with current clinical standards on care of large wounds, early excision, and grafting of autografts. However, inadequate regenerative capacity, impaired regeneration/degradation profile, and high cost of current commercial tissue-engineered dermal regeneration templates hinder their utilization, and the development of an efficient and cost-effective tissue-engineered dermal substitute remains a challenge. Inspired from our previously reported data on a pullulan/gelatin scaffold, here we present a new generation of a porous pullulan/gelatin scaffold (PG2) served as a dermal substitute with enhanced chemical and structural characteristics. PG2 shows excellent biocompatibility (viability, migration, and proliferation), assessed by in vitro incorporation of human dermal fibroblasts in comparison with the Integra® dermal regeneration template (Control). When applied on a mouse full-thickness excisional wound, PG2 shows rapid scaffold degradation, more granulation tissue, more collagen deposition, and more cellularity in comparison with Control at 20 days post surgery. The faster degradation is likely due to the enhanced recruitment of inflammatory macrophages to the scaffold from the wound bed, and that leads to earlier maturation of granulation tissue with less myofibroblastic cells. Collectively, our data reveal PG2's characteristics as an applicable dermal substitute with excellent dermal regeneration, which may attenuate scar formation.

A review: therapeutic potential of adipose-derived stem cells in cutaneous wound healing and regeneration

As the most important barrier for the human body, the skin often suffers from acute and chronic injuries, especially refractory wounds, which seriously affect the quality of life of patients. For these refractory wounds that cannot be cured by various surgical methods, stem cell transplantation becomes an effective research direction. As one of the adult stem cells, adipose-derived stem cells play an indispensable role in the repair of skin wounds more than other stem cells because of their advantages such as immune compatibility and freedom from ethical constraints. Here, we actively explore the role of adipose-derived stem cells in the repair of cutaneous wound and conclude that it can significantly promote cutaneous wound healing and regeneration. Based on a large number of animal and clinical trials, we believe that adipose-derived stem cells will have a greater breakthrough in the field of skin wound repair in the future, especially in chronic refractory wounds.

Biomaterials for Skin Substitutes

Patients with extensive burns rely on the use of tissue engineered skin due to a lack of sufficient donor tissue, but it is a challenge to identify reliable and economical scaffold materials and donor cell sources for the generation of a functional skin substitute. The current review attempts to evaluate the performance of the wide range of biomaterials available for generating skin substitutes, including both natural biopolymers and synthetic polymers, in terms of tissue response and potential for use in the operating room. Natural biopolymers display an improved cell response, while synthetic polymers provide better control over chemical composition and mechanical properties. It is suggested that not one material meets all the requirements for a skin substitute. Rather, a composite scaffold fabricated from both natural and synthetic biomaterials may allow for the generation of skin substitutes that meet all clinical requirements including a tailored wound size and type, the degree of burn, the patient age, and the available preparation technique. This review aims to be a valuable directory for researchers in the field to find the optimal material or combination of materials based on their specific application.

Delivery of monocyte lineage cells in a biomimetic scaffold enhances tissue repair

The monocyte lineage is essential to normal wound healing. Macrophage inhibition or knockout in mice results in impaired wound healing through reduced neovascularization, granulation tissue formation, and reepithelialization. Numerous studies have either depleted macrophages or reduced their activity in the context of wound healing. Here, we demonstrate that by increasing the number of macrophages or monocytes in the wound site above physiologic levels via pullulan-collagen composite dermal hydrogel scaffold delivery, the rate of wound healing can be significantly accelerated in both wild-type and diabetic mice, with no adverse effect on the quality of repair. Macrophages transplanted onto wounds differentiate into M1 and M2 phenotypes of different proportions at various time points, ultimately increasing angiogenesis. Given that monocytes can be readily isolated from peripheral blood without in vitro manipulation, these findings hold promise for translational medicine aimed at accelerating wound healing across a broad spectrum of diseases.

Paracrine Effects of Mesenchymal Stromal Cells Cultured in Three-Dimensional Settings on Tissue Repair

Mesenchymal stromal cells (MSCs) are a promising cell source for promoting tissue repair, due to their ability to release growth, angiogenic, and immunomodulatory factors. However, when injected as a suspension, these cells suffer from poor survival and localization, and suboptimal release of paracrine factors. While there have been attempts to overcome these limitations by modifying MSCs themselves, a more versatile solution is to grow them in three dimensions, as aggregates or embedded into biomaterials. Here we review the mechanisms by which 3D culture can influence the regenerative capacity of undifferentiated MSCs, focusing on recent examples from the literature. We further discuss how knowledge of these mechanisms can lead to strategic design of MSC therapies that overcome some of the challenges to their effective translation.

Reactive Oxygen Species Shielding Hydrogel for the Delivery of Adherent and Nonadherent Therapeutic Cell Types<sup/>

Cell therapies suffer from poor survival post-transplant due to placement into hostile implant sites characterized by host immune response and innate production of high levels of reactive oxygen species (ROS). We hypothesized that cellular encapsulation within an injectable, antioxidant hydrogel would improve viability of cells exposed to high oxidative stress. To test this hypothesis, we applied a dual thermo- and ROS-responsive hydrogel comprising the ABC triblock polymer poly[(propylene sulfide)-block-(N,N-dimethyl acrylamide)-block-(N-isopropylacrylamide)] (PPS₁₃₅-b-PDMA₁₅₂-b-PNIPAAM₂₂₅, PDN). The PPS chemistry reacts irreversibly with ROS such as hydrogen peroxide (H₂O₂), imparting inherent antioxidant properties to the system. Here, PDN hydrogels were successfully integrated with type 1 collagen to form ROS-protective, composite hydrogels amenable to spreading and growth of adherent cell types such as mesenchymal stem cells (MSCs). It was also shown that, using a control hydrogel substituting nonreactive polycaprolactone in place of PPS, the ROS-reactive PPS chemistry is directly responsible for PDN hydrogel cytoprotection of both MSCs and insulin-producing β-cell pseudo-islets against H₂O₂ toxicity. In sum, these results establish the potential of cytoprotective, thermogelling PDN biomaterials for injectable delivery of cell therapies.

Adipose-Derived Stem Cell-Seeded Hydrogels Increase Endogenous Progenitor Cell Recruitment and Neovascularization in Wounds

Adipose-derived mesenchymal stem cells (ASCs) are appealing for cell-based wound therapies because of their accessibility and ease of harvest, but their utility is limited by poor cell survival within the harsh wound microenvironment. In prior work, our laboratory has demonstrated that seeding ASCs within a soft pullulan-collagen hydrogel enhances ASC survival and improves wound healing. To more fully understand the mechanism of this therapy, we examined whether ASC-seeded hydrogels were able to modulate the recruitment and/or functionality of endogenous progenitor cells. Employing a parabiosis model and fluorescence-activated cell sorting analysis, we demonstrate that application of ASC-seeded hydrogels to wounds, when compared with injected ASCs or a noncell control, increased the recruitment of provascular circulating bone marrow-derived mesenchymal progenitor cells (BM-MPCs). BM-MPCs comprised 23.0% of recruited circulating progenitor cells in wounds treated with ASC-seeded hydrogels versus 8.4% and 2.1% in those treated with controls, p < 0.05. Exploring the potential for functional modulation of BM-MPCs, we demonstrate a statistically significant increase in BM-MPC migration, proliferation, and tubulization when exposed to hydrogel-seeded ASC-conditioned medium versus control ASC-conditioned medium (73.8% vs. 51.4% scratch assay closure; 9.1% vs. 1.4% proliferation rate; 10.2 vs. 5.5 tubules/HPF; p < 0.05 for all assays). BM-MPC expression of genes related to cell stemness and angiogenesis was also significantly increased following exposure to hydrogel-seeded ASC-conditioned medium (p < 0.05). These data suggest that ASC-seeded hydrogels improve both progenitor cell recruitment and functionality to effect greater neovascularization.

Mesenchymal Stem Cells as a Prospective Therapy for the Diabetic Foot

The diabetic foot is a serious complication of diabetes. Mesenchymal stem cells are an abundant source of stem cells which occupy a special position in cell therapies, and recent studies have suggested that mesenchymal stem cells can play essential roles in treatments for the diabetic foot. Here, we discuss the advances that have been made in mesenchymal stem cell treatments for this condition. The roles and functional mechanisms of mesenchymal stem cells in the diabetic foot are also summarized, and insights into current and future studies are presented.

Surface-Selective Preferential Production of Reactive Oxygen Species on Piezoelectric Ceramics for Bacterial Killing

Reactive oxygen species (ROS) can be used to kill bacterial cells, and thus the selective generation of ROS from material surfaces is an emerging direction in antibacterial material discovery. We found the polarization of piezoelectric ceramic causes the two sides of the disk to become positively and negatively charged, which translate into cathode and anode surfaces in an aqueous solution. Because of the microelectrolysis of water, ROS are preferentially formed on the cathode surface. Consequently, the bacteria are selectively killed on the cathode surface. However, the cell experiment suggested that the level of ROS is safe for normal mammalian cells.

Suction assisted liposuction does not impair the regenerative potential of adipose derived stem cells

Background

Adipose-derived stem cells (ASCs) have been identified as a population of multipotent cells with promising applications in tissue engineering and regenerative medicine. ASCs are abundant in fat tissue, which can be safely harvested through the minimally invasive procedure of liposuction. However, there exist a variety of different harvesting methods, with unclear impact on ASC regenerative potential. The aim of this study was thus to compare the functionality of ASCs derived from the common technique of suction-assisted lipoaspiration (SAL) versus resection.

Methods

Human adipose tissue was obtained from paired abdominoplasty and SAL samples from three female donors, and was processed to isolate the stromal vascular fraction. Fluorescence-activated cell sorting was used to determine ASC yield, and cell viability was assayed. Adipogenic and osteogenic differentiation capacity were assessed in vitro using phenotypic staining and quantification of gene expression. Finally, ASCs were applied in an in vivo model of tissue repair to evaluate their regenerative potential.

Results

SAL specimens provided significantly fewer ASCs when compared to excised fat tissue, however, with equivalent viability. SAL-derived ASCs demonstrated greater expression of the adipogenic markers FABP-4 and LPL, although this did not result in a difference in adipogenic differentiation. There were no differences detected in osteogenic differentiation capacity as measured by alkaline phosphatase, mineralization or osteogenic gene expression. Both SAL- and resection-derived ASCs enhanced significantly cutaneous healing and vascularization in vivo, with no significant difference between the two groups.

Conclusion

SAL provides viable ASCs with full capacity for multi-lineage differentiation and tissue regeneration, and is an effective method of obtaining ASCs for cell-based therapies.

Cellularized Bilayer Pullulan-Gelatin Hydrogel for Skin Regeneration

Skin substitutes significantly reduce the morbidity and mortality of patients with burn injuries and chronic wounds. However, current skin substitutes have disadvantages related to high costs and inadequate skin regeneration due to highly inflammatory wounds. Thus, new skin substitutes are needed. By combining two polymers, pullulan, an inexpensive polysaccharide with antioxidant properties, and gelatin, a derivative of collagen with high water absorbency, we created a novel inexpensive hydrogel-named PG-1 for "pullulan-gelatin first generation hydrogel"-suitable for skin substitutes. After incorporating human fibroblasts and keratinocytes onto PG-1 using centrifugation over 5 days, we created a cellularized bilayer skin substitute. Cellularized PG-1 was compared to acellular PG-1 and no hydrogel (control) in vivo in a mouse excisional skin biopsy model using newly developed dome inserts to house the skin substitutes and prevent mouse skin contraction during wound healing. PG-1 had an average pore size of 61.69 μm with an ideal elastic modulus, swelling behavior, and biodegradability for use as a hydrogel for skin substitutes. Excellent skin cell viability, proliferation, differentiation, and morphology were visualized through live/dead assays, 5-bromo-2'-deoxyuridine proliferation assays, and confocal microscopy. Trichrome and immunohistochemical staining of excisional wounds treated with the cellularized skin substitute revealed thicker newly formed skin with a higher proportion of actively proliferating cells and incorporation of human cells compared to acellular PG-1 or control. Excisional wounds treated with acellular or cellularized hydrogels showed significantly less macrophage infiltration and increased angiogenesis 14 days post skin biopsy compared to control. These results show that PG-1 has ideal mechanical characteristics and allows ideal cellular characteristics. In vivo evidence suggests that cellularized PG-1 promotes skin regeneration and may help promote wound healing in highly inflammatory wounds, such as burns and chronic wounds.

Mesenchymal stem cells and cutaneous wound healing: novel methods to increase cell delivery and therapeutic efficacy

Mesenchymal stem cells (MSCs) (also known as multipotent mesenchymal stromal cells) possess the capacity for self-renewal and multi-lineage differentiation, and their ability to enhance cutaneous wound healing has been well characterized. Acting via paracrine interactions, MSCs accelerate wound closure, increase angiogenesis, promote resolution of wound inflammation, favorably regulate extracellular matrix remodeling, and encourage regeneration of skin with normal architecture and function. A number of studies have employed novel methods to amplify the delivery and efficacy of MSCs. Non-traditional sources of MSCs, including Wharton’s jelly and medical waste material, have shown efficacy comparable to that of traditional sources, such as bone marrow and adipose tissue. The potential of alternative methods to both introduce MSCs into wounds and increase migration of MSCs into wound areas has also been demonstrated. Taking advantage of the associations between MSCs with M2 macrophages and microRNA, methods to enhance the immunomodulatory capacity of MSCs have shown success. New measures to enhance angiogenic capabilities have also exhibited effectiveness, often demonstrated by increased levels of proangiogenic vascular endothelial growth factor. Finally, hypoxia has been shown to have strong wound-healing potential in terms of increasing MSC efficacy. We have critically reviewed the results of the novel studies that show promise for the continued development of MSC-based wound-healing therapies and provide direction for continued research in this field.

Time-course and intensity-based classifications of oxidative stresses and their potential application in biomedical, comparative and environmental research

OBJECTIVE

We propose some clues for classification of oxidative stresses based on their intensity and time-course.

BACKGROUND

Oxidative stress is studied for more than three decades and it is clear that it may differ on the parameters of interest. But up to now there is no any system for formal discrimination between different types of the stress. Such approach can provide important benefits at description of experimental data.

METHOD

We briefly review information on oxidative stresses and show that the theoretical concept is actually poorly developed since introduction of the first definition in 1985 by H. Sies. We argue that the stresses can differ on their intensities and time-curses, but there was no theoretical basis for discrimination between them.

RESULTS

On the basis of these analyses, we propose two systems of classifications of oxidative stresses enabling their description taking into account their intensity and time-course. We analyze essential biomarkers of oxidative stress to be used for classification such as levels of modified by reactive oxygen species proteins, lipids, nucleic acids, and low molecular mass compounds. Finally, we describe potential applications of the proposed classifications to biomedical, comparative and environmental research.

CONCLUSION

The proposed classifications of oxidative stress may facilitate description of experimental data and their comparison between different organisms and methods of induction of oxidative stresses. Additionally this work may provide some clues to develop quantitative approaches for formal categorization of oxidative stresses.

APPLICATION

Most applications of the classifications proposed are theoretical and applied studies where oxidative stress takes place.

Challenges and Opportunities in Drug Delivery for Wound Healing

Significance: Chronic wounds remain a significant public health problem. Alterations in normal physiological processes caused by aging or diabetes lead to impaired tissue repair and the development of chronic and nonhealing wounds. Understanding the unique features of the wound environment will be required to develop new therapeutics that impact these disabling conditions. New drug-delivery systems (DDSs) may enhance current and future therapies for this challenging clinical problem. Recent Advances: Historically, physical barriers and biological degradation limited the efficacy of DDSs in wound healing. In aiming at improving and optimizing drug delivery, recent data suggest that combinations of delivery mechanisms, such as hydrogels, small molecules, RNA interference (RNAi), as well as growth factor and stem cell-based therapies (biologics), could offer exciting new opportunities for improving tissue repair. Critical Issues: The lack of effective therapeutic approaches to combat the significant disability associated with chronic wounds has become an area of increasing clinical concern. However, the unique challenges of the wound environment have limited the development of effective therapeutic options for clinical use. Future Directions: New platforms presented in this review may provide clinicians and scientists with an improved understanding of the alternatives for drug delivery in wound care, which may facilitate the development of new therapeutic approaches for patients.

Ultrasound-Assisted Liposuction Does Not Compromise the Regenerative Potential of Adipose-Derived Stem Cells

The regenerative abilities of adipose-derived mesenchymal stem cells (ASCs) harvested via a third-generation ultrasound-assisted liposuction (UAL) device versus ASCs obtained via standard suction-assisted lipoaspiration were evaluated. ASC yield and viability, and expression of most osteogenic, adipogenic, and key regenerative genes were equivalent between the two methods. Cells harvested via UAL showed comparable abilities to enhance cutaneous regeneration and appear suitable for cell therapy and tissue engineering applications.

Human mesenchymal stem cells (MSCs) have recently become a focus of regenerative medicine, both for their multilineage differentiation capacity and their excretion of proregenerative cytokines. Adipose-derived mesenchymal stem cells (ASCs) are of particular interest because of their abundance in fat tissue and the ease of harvest via liposuction. However, little is known about the impact of different liposuction methods on the functionality of ASCs. Here we evaluate the regenerative abilities of ASCs harvested via a third-generation ultrasound-assisted liposuction (UAL) device versus ASCs obtained via standard suction-assisted lipoaspiration (SAL). Lipoaspirates were sorted using fluorescent assisted cell sorting based on an established surface-marker profile (CD34+/CD31−/CD45−), to obtain viable ASCs. Yield and viability were compared and the differentiation capacities of the ASCs were assessed. Finally, the regenerative potential of ASCs was examined using an in vivo model of tissue regeneration. UAL- and SAL-derived samples demonstrated equivalent ASC yield and viability, and UAL ASCs were not impaired in their osteogenic, adipogenic, or chondrogenic differentiation capacity. Equally, quantitative real-time polymerase chain reaction showed comparable expression of most osteogenic, adipogenic, and key regenerative genes between both ASC groups. Cutaneous regeneration and neovascularization were significantly enhanced in mice treated with ASCs obtained by either UAL or SAL compared with controls, but there were no significant differences in healing between cell-therapy groups. We conclude that UAL is a successful method of obtaining fully functional ASCs for regenerative medicine purposes. Cells harvested with this alternative approach to liposuction are suitable for cell therapy and tissue engineering applications.

Significance

Adipose-derived mesenchymal stem cells (ASCs) are an appealing source of therapeutic progenitor cells because of their multipotency, diverse cytokine profile, and ease of harvest via liposuction. Alternative approaches to classical suction-assisted liposuction are gaining popularity; however, little evidence exists regarding the impact of different liposuction methods on the regenerative functionality of ASCs. Human ASC characteristics and regenerative capacity were assessed when harvested via ultrasound-assisted (UAL) versus standard suction-assisted liposuction. ASCs obtained via UAL were of equal quality when directly compared with the current gold standard harvest method. UAL is an adjunctive source of fully functional mesenchymal stem cells for applications in basic research and clinical therapy.

Stem Cells in Wound Healing: The Future of Regenerative Medicine? A Mini-Review

The increased risk of disease and decreased capacity to respond to tissue insult in the setting of aging results from complex changes in homeostatic mechanisms, including the regulation of oxidative stress and cellular heterogeneity. In aged skin, the healing capacity is markedly diminished resulting in a high risk for chronic wounds. Stem cell-based therapies have the potential to enhance cutaneous regeneration, largely through trophic and paracrine activity. Candidate cell populations for therapeutic application include adult mesenchymal stem cells, embryonic stem cells and induced pluripotent stem cells. Autologous cell-based approaches are ideal to minimize immune rejection but may be limited by the declining cellular function associated with aging. One strategy to overcome age-related impairments in various stem cell populations is to identify and enrich with functionally superior stem cell subsets via single cell transcriptomics. Another approach is to optimize cell delivery to the harsh environment of aged wounds via scaffold-based cell applications to enhance engraftment and paracrine activity of therapeutic stem cells. In this review, we shed light on challenges and recent advances surrounding stem cell therapies for wound healing and discuss limitations for their clinical adoption.

Pullulan: a new cytoadhesive for cell-mediated cartilage repair

Introduction

Local delivery of mesenchymal stem cells (MSCs) to the acutely injured or osteoarthritic joint retards cartilage destruction. However, in the absence of assistive materials the efficiency of engraftment of MSCs to either intact or fibrillated cartilage is low and localization is further reduced by natural movement of the joint surfaces. It is hypothesised that enhanced engraftment of the delivered MSCs at the cartilage surface will increase their reparative effect and that the application of a bioadhesive to the degraded cartilage surface will provide improved cell retention. Pullulan is a structurally flexible, non-immunogenic exopolysaccharide with wet-stick adhesive properties and has previously been used for drug delivery via the wet surfaces of the buccal cavity. In this study, the adhesive character of pullulan was exploited to enhance MSC retention on the damaged cartilage surface.

Methods

MSCs labeled with PKH26 were applied to pullulan-coated osteoarthritic cartilage explants to measure cell retention. Cytocompatability was assessed by measuring the effects of prolonged exposure to the bioadhesive on MSC viability and proliferation. The surface phenotype of the cells was assessed by flow cytometry and their multipotent nature by measuring osteogenic, adipogenic and chrondrogenic differentiation. Experiments were also carried out to determine expression of the C-type lectin Dectin-2 receptor.

Results

MSCs maintained a stable phenotype following exposure to pullulan in terms of metabolic activity, proliferation, differentiation and surface antigen expression. An increase in osteogenic activity and Dectin-2 receptor expression was seen in MSCs treated with pullulan. Markedly enhanced retention of MSCs was observed in explant culture of osteoarthritic cartilage.

Conclusions

Pullulan is a biocompatible and effective cytoadhesive material for tissue engraftment of MSCs. Prolonged exposure to pullulan has no negative impact on the phenotype, viability and differentiation potential of the cells. Pullulan dramatically improves the retention of MSCs at the fibrillated surface of osteoarthritic articular cartilage. Pullulan causes an upregulation in expression of the Dectin-2 C-type lectin transmembrane complex.

Capillary force seeding of hydrogels for adipose-derived stem cell delivery in wounds

Effective skin regeneration therapies require a successful interface between progenitor cells and biocompatible delivery systems. We previously demonstrated the efficiency of a biomimetic pullulan-collagen hydrogel scaffold for improving bone marrow-derived mesenchymal stem cell survival within ischemic skin wounds by creating a "stem cell niche" that enhances regenerative cytokine secretion. Adipose-derived mesenchymal stem cells (ASCs) represent an even more appealing source of stem cells because of their abundance and accessibility, and in this study we explored the utility of ASCs for hydrogel-based therapies. To optimize hydrogel cell seeding, a rapid, capillary force-based approach was developed and compared with previously established cell seeding methods. ASC viability and functionality following capillary hydrogel seeding were then analyzed in vitro and in vivo. In these experiments, ASCs were seeded more efficiently by capillary force than by traditional methods and remained viable and functional in this niche for up to 14 days. Additionally, hydrogel seeding of ASCs resulted in the enhanced expression of multiple stemness and angiogenesis-related genes, including Oct4, Vegf, Mcp-1, and Sdf-1. Moving in vivo, hydrogel delivery improved ASC survival, and application of both murine and human ASC-seeded hydrogels to splinted murine wounds resulted in accelerated wound closure and increased vascularity when compared with control wounds treated with unseeded hydrogels. In conclusion, capillary seeding of ASCs within a pullulan-collagen hydrogel bioscaffold provides a convenient and simple way to deliver therapeutic cells to wound environments. Moreover, ASC-seeded constructs display a significant potential to accelerate wound healing that can be easily translated to a clinical setting.

Diabetes impairs the angiogenic potential of adipose-derived stem cells by selectively depleting cellular subpopulations

Introduction

Pathophysiologic changes associated with diabetes impair new blood vessel formation and wound healing. Mesenchymal stem cells derived from adipose tissue (ASCs) have been used clinically to promote healing, although it remains unclear whether diabetes impairs their functional and therapeutic capacity.

Methods

In this study, we examined the impact of diabetes on the murine ASC niche as well as on the potential of isolated cells to promote neovascularization in vitro and in vivo. A novel single-cell analytical approach was used to interrogate ASC heterogeneity and subpopulation dynamics in this pathologic setting.

Results

Our results demonstrate that diabetes alters the ASC niche in situ and that diabetic ASCs are compromised in their ability to establish a vascular network both in vitro and in vivo. Moreover, these diabetic cells were ineffective in promoting soft tissue neovascularization and wound healing. Single-cell transcriptional analysis identified a subpopulation of cells which was diminished in both type 1 and type 2 models of diabetes. These cells were characterized by the high expression of genes known to be important for new blood vessel growth.

Conclusions

Perturbations in specific cellular subpopulations, visible only on a single-cell level, represent a previously unreported mechanism for the dysfunction of diabetic ASCs. These data suggest that the utility of autologous ASCs for cell-based therapies in patients with diabetes may be limited and that interventions to improve cell function before application are warranted.

Antioxidants cause rapid expansion of human adipose-derived mesenchymal stem cells via CDK and CDK inhibitor regulation

Background

Antioxidants have been shown to enhance the proliferation of adipose-derived mesenchymal stem cells (ADMSCs) in vitro, although the detailed mechanism(s) and potential side effects are not fully understood.

In this study, human ADMSCs cultured in ImF-A medium supplemented with antioxidants (N-acetyl-l-cysteine and ascorbic acid-2-phosphate) and fibroblast growth factor 2 (FGF-2) were compared with ADMSCs cultured with FGF-2 alone (ImF) or with FGF-2 under 5% pO₂ conditions (ImF-H).

Results

During log-phase growth, exposure to ImF-A resulted in a higher percentage of ADMSCs in the S phase of the cell cycle and a smaller percentage in G0/G1 phase. This resulted in a significantly reduced cell-doubling time and increased number of cells in the antioxidant-supplemented cultures compared with those supplemented with FGF-2 alone, an approximately 225% higher cell density after 7 days. Western blotting showed that the levels of the CDK inhibitors p21 and p27 decreased after ImF-A treatment, whereas CDK2, CDK4, and CDC2 levels clearly increased. In addition, ImF-A resulted in significant reduction in the expression of CD29, CD90, and CD105, whereas relative telomere length, osteogenesis, adipogenesis, and chondrogenesis were enhanced. The results were similar for ADMSCs treated with antioxidants and those under hypoxic conditions.

Conclusion

Antioxidant treatment promotes entry of ADMSCs into the S phase by suppressing cyclin-dependent kinase inhibitors and results in rapid cell proliferation similar to that observed under hypoxic conditions.

Surface tethered epidermal growth factor protects proliferating and differentiating multipotential stromal cells from FasL-induced apoptosis

Multipotential stromal cells or mesenchymal stem cells (MSCs) have been proposed as aids in regenerating bone and adipose tissues, as these cells form osteoblasts and adipocytes. A major obstacle to this use of MSC is the initial loss of cells postimplantation. This cell death in part is due to ubiquitous nonspecific inflammatory cytokines such as FasL generated in the implant site. Our group previously found that soluble epidermal growth factor (sEGF) promotes MSC expansion. Furthermore, tethering EGF (tEGF) onto a two-dimensional surface altered MSC responses, by restricting epidermal growth factor receptor (EGFR) to the cell surface, causing sustained activation of EGFR, and promoting survival from FasL-induced death. sEGF by causing internalization of EGFR does not support MSC survival. However, for tEGF to be useful in bone regeneration, it needs to allow for MSC differentiation into osteoblasts while also protecting emerging osteoblasts from apoptosis. tEGF did not block induced differentiation of MSCs into osteoblasts, or adipocytes, a common default MSC-differentiation pathway. MSC-derived preosteoblasts showed increased Fas levels and became more susceptible to FasL-induced death, which tEGF prevented. Differentiating adipocytes underwent a reduction in Fas expression and became resistant to FasL-induced death, with tEGF having no further survival effect. tEGF protected undifferentiated MSC from combined insults of FasL, serum deprivation, and physiologic hypoxia. Additionally, tEGF was dominant in the face of sEGF to protect MSC from FasL-induced death. Our results suggest that MSCs and differentiating osteoblasts need protective signals to survive in the inflammatory wound milieu and that tEGF can serve this function.

Stem cell recruitment after injury: lessons for regenerative medicine

Tissue repair and regeneration are thought to involve resident cell proliferation as well as the selective recruitment of circulating stem and progenitor cell populations through complex signaling cascades. Many of these recruited cells originate from the bone marrow, and specific subpopulations of bone marrow cells have been isolated and used to augment adult tissue regeneration in preclinical models. Clinical studies of cell-based therapies have reported mixed results, however, and a variety of approaches to enhance the regenerative capacity of stem cell therapies are being developed based on emerging insights into the mechanisms of progenitor cell biology and recruitment following injury. This article discusses the function and mechanisms of recruitment of important bone marrow-derived stem and progenitor cell populations following injury, as well as the emerging therapeutic applications targeting these cells.

A Mechanomodulatory Device to Minimize Incisional Scar Formation

OBJECTIVE

To mechanically control the wound environment and prevent cutaneous scar formation.

APPROACH

We subjected various material substrates to biomechanical testing to investigate their ability to modulate skin behavior. Combinations of elastomeric materials, adhesives, and strain applicators were evaluated to develop topical stress-shielding devices. Noninvasive imaging modalities were utilized to characterize anatomic site-specific differences in skin biomechanical properties in humans. The devices were tested in a validated large animal model of hypertrophic scarring. Phase I within-patient controlled clinical trials were conducted to confirm their safety and efficacy in scar reduction in patients undergoing abdominoplasty surgery.

RESULTS

Among the tested materials and device applicators, a polymer device was developed that effectively off-loaded high tension wounds and blocked pro-fibrotic pathways and excess scar formation in red Duroc swine. In humans, different anatomic sites exhibit unique biomechanical properties that may correlate with the propensity to form scars. In the clinical trial, utilization of this device significantly reduced incisional scar formation and improved scar appearance for up to 12 months compared with control incisions that underwent routine postoperative care.

INNOVATION

This is the first device that is able to precisely control the mechanical environment of incisional wounds and has been demonstrated in multiple clinical trials to significantly reduce scar formation after surgery.

CONCLUSION

Mechanomodulatory strategies to control the incisional wound environment can significantly reduce pathologic scarring and fibrosis after surgery.

Enabling stem cell therapies for tissue repair: current and future challenges

Stem cells embody the tremendous potential of the human body to develop, grow, and repair throughout life. Understanding the biologic mechanisms that underlie stem cell-mediated tissue regeneration is key to harnessing this potential. Recent advances in molecular biology, genetic engineering, and material science have broadened our understanding of stem cells and helped bring them closer to widespread clinical application. Specifically, innovative approaches to optimize how stem cells are identified, isolated, grown, and utilized will help translate these advances into effective clinical therapies. Although there is growing interest in stem cells worldwide, this enthusiasm must be tempered by the fact that these treatments remain for the most part clinically unproven. Future challenges include refining the therapeutic manipulation of stem cells, validating these technologies in randomized clinical trials, and regulating the global expansion of regenerative stem cell therapies.

Therapeutic potential of bone marrow-derived mesenchymal stem cells for cutaneous wound healing

Despite advances in wound care, many wounds never heal and become chronic problems that result in significant morbidity and mortality to the patient. Cellular therapy for cutaneous wounds has recently come under investigation as a potential treatment modality for impaired wound healing. Bone marrow-derived mesenchymal stem cells (MSCs) are a promising source of adult progenitor cells for cytotherapy as they are easy to isolate and expand and have been shown to differentiate into various cell lineages. Early studies have demonstrated that MSCs may enhance epithelialization, granulation tissue formation, and neovascularization resulting in accelerated wound closure. It is currently unclear if these effects are mediated through cellular differentiation or by secretion of cytokines and growth factors. This review discusses the proposed biological contributions of MSCs to cutaneous repair and their clinical potential in cell-based therapies.

Stem cell niches for skin regeneration

Stem cell-based therapies offer tremendous potential for skin regeneration following injury and disease. Functional stem cell units have been described throughout all layers of human skin and the collective physical and chemical microenvironmental cues that enable this regenerative potential are known as the stem cell niche. Stem cells in the hair follicle bulge, interfollicular epidermis, dermal papillae, and perivascular space have been closely investigated as model systems for niche-driven regeneration. These studies suggest that stem cell strategies for skin engineering must consider the intricate molecular and biologic features of these niches. Innovative biomaterial systems that successfully recapitulate these microenvironments will facilitate progenitor cell-mediated skin repair and regeneration.