Mechanical Stretch Induced Skin Regeneration: Molecular and Cellular Mechanism in Skin Soft Tissue Expansion

Skin soft tissue expansion is one of the most basic and commonly used techniques in plastic surgery to obtain excess skin for a variety of medical uses. However, skin soft tissue expansion is faced with many problems, such as long treatment process, poor skin quality, high retraction rate, and complications. Therefore, a deeper understanding of the mechanisms of skin soft tissue expansion is needed. The key to skin soft tissue expansion lies in the mechanical stretch applied to the skin by an inflatable expander. Mechanical stimulation activates multiple signaling pathways through cellular adhesion molecules and regulates gene expression profiles in cells. Meanwhile, various types of cells contribute to skin expansion, including keratinocytes, dermal fibroblasts, and mesenchymal stem cells, which are also regulated by mechanical stretch. This article reviews the molecular and cellular mechanisms of skin regeneration induced by mechanical stretch during skin soft tissue expansion.

Metformin Promotes Mechanical Stretch-Induced Skin Regeneration by Improving the Proliferative Activity of Skin-Derived Stem Cells

Background

Skin expansion by mechanical stretch is an essential and widely used treatment for tissue defects in plastic and reconstructive surgery; however, the regenerative capacity of mechanically stretched skin limits clinical treatment results. Here, we propose a strategy to enhance the regenerative ability of mechanically stretched skin by topical application of metformin.

Methods

We established a mechanically stretched scalp model in male rats (n = 20), followed by their random division into two groups: metformin-treated (n = 10) and control (n = 10) groups. We measured skin thickness, collagen volume fraction, cell proliferation, and angiogenesis to analyze the effects of topical metformin on mechanically stretched skin, and immunofluorescence staining was performed to determine the contents of epidermal stem cells and hair follicle bulge stem cells in mechanically stretched skin. Western blot was performed to detect the protein expression of skin-derived stem cell markers.

Results

Compared with the control group, metformin treatment was beneficial to mechanical stretch-induced skin regeneration by increasing the thicknesses of epidermis (57.27 ± 10.24 vs. 31.07 ± 9.06 μm, p < 0.01) and dermis (620.2 ± 86.17 vs. 402.1 ± 22.46 μm, p < 0.01), number of blood vessels (38.30 ± 6.90 vs. 17.00 ± 3.10, p < 0.01), dermal collagen volume fraction (60.48 ± 4.47% vs. 41.28 ± 4.14%, p < 0.01), and number of PCNA+, Aurora B+, and pH3+ cells. Additionally, we observed significant elevations in the number of proliferating hair follicle bulge stem cells [cytokeratin (CK)15+/proliferating cell nuclear antigen (PCNA)+] (193.40 ± 35.31 vs. 98.25 ± 23.47, p < 0.01) and epidermal stem cells (CK14+/PCNA+) (83.00 ± 2.38 vs. 36.38 ± 8.96, p < 0.01) in the metformin-treated group, and western blot results confirmed significant increases in CK14 and CK15 expression following metformin treatment.

Conclusion

Topical application of metformin enhanced the regenerative capacity of mechanically stretched skin, with the underlying mechanism possibly attributed to improvements in the proliferative activity of skin-derived stem cells.

Novel pre-vascularized tissue-engineered dermis based on stem cell sheet technique used for dermis-defect healing

Insufficient donor dermis and the shortage of three-dimensional vascular networks are the main limitations in the tissue-engineered dermis (TED). To solve these problems, we initially constructed pre-vascularized bone marrow mesenchymal stem cell sheet (PBMCS) and pre-vascularized fibroblasts cell sheet (PFCS) by cell sheet technology, and then superimposed or folded them together to construct a pre-vascularized TED (PTED), aiming to mimic the real dermis structure. The constructed PTED was implanted in nude mice dorsal dermis-defect wound and the wound-healing effect was quantified at Days 1, 7 and 14 via the methods of histochemistry and immunohistochemistry. The results showed that PTED could rapidly promote the wound closure, especially at Day 14, and the wound-healing rate of three-layer PTED could reach 97.2% (P < 0.01), which was faster than the blank control group (89.1%), PBMCS (92.4%), PFCS (93.8%) and six-layer PTED (92.3%). In addition, the vessel density in the PTED group was higher than the other groups on the 14th day. Taken together, it is proved that the PTED, especially three-layer PTED, is more conducive to the full-thickness dermis-defect repair and the construction of the three-dimensional vascular networks, indicating its potential application in dermis-defect repair.

Hair follicle bulge-derived stem cells promote tissue regeneration during skin expansion

Skin expansion is widely used in reconstructive surgery to obtain supplemental and optimal skin. Enhancing the regenerative capacity of expanded skin is therefore of great interest. Hair follicle bulge-derived stem cells (HFBSCs) located in hair follicle bulges are closely associated with skin; HFBSC transplantation could promote cutaneous wound repair. However, the effects of transplanted HFBSCs on skin regeneration during expansion remain unclear. The aim of the study was to reveal the potential effects of transplanted HFBSCs in the expanded skin and explore its mechanism. Our results showed higher skin area, tissue weight, epidermal thickness, dermal thickness, proliferating cell count, collagen content, microcirculatory blood flow, blood vessels, and lower retraction ratios were observed in HFBSC-injected rats compared to uninjected controls. Moreover, the transplanted HFBSCs directly contributed to tissue regeneration by differentiating into vascular endothelial cells, epidermal cells, and the outer root sheath cells of hair follicle. Higher expression of EGF, VEGF, bFGF, and TGF-β were observed in HFBSC-injected rats. Our research demonstrated the transplanted HFBSCs could promote skin regeneration by differentiating into various types of skin related cells and by up-regulating the expression of growth factors. Our results could form a basis for the development of novel strategies to enhance regeneration in expanded skin by using HFBSCs.

Establishment of a Novel Mouse Model for Soft Tissue Expansion

BACKGROUND

Despite its increasing use, not much is known about tissue expansion, and its complication rates are significantly high. Thus, there is an urgent need to establish a stable animal model to overcome the limitations and complications of tissue expansion. Although the mouse model has shown several advantages in the in-depth studies, an appropriate mouse expansion model has rarely been reported, likely because of its loose skin.

MATERIALS AND METHODS

A micro expander was designed and implanted under the scalp of a mouse (expanded group); sterilized saline was regularly injected into the expander. In sham-operated mice (control group), a silicone sheet was implanted under the scalp. Skin samples were collected 5 wk after surgery. Histologic changes including epidermal and dermal thickness and collagen fiber arrangement were analyzed. In addition, vascular density and cell proliferation ratio were determined. An ultrastructural analysis was also performed.

RESULTS

With the application of the expansion device, the skin became tight and showed area enlargement. The epidermal thickness of the expanded skin increased significantly (P < 0.01), whereas the thickness of the dermis decreased significantly (P < 0.05) as compared with the control skin. Masson staining demonstrated that collagen bundles were arranged more compactly in the expanded skin (P < 0.05) than in the controls. Furthermore, more proliferating cells (P < 0.05) and blood vessels (P < 0.01) were observed. Transmission electron microscopy showed that the fibers of expanded skin were stretched and broken into bundles of various diameters, with abundant active fibroblasts.

CONCLUSIONS

A reliable mouse model of scalp skin expansion was successfully established, which may be a promising tool for in-depth studies on skin soft tissue expansion.

Transplantation of Adipose-Derived Stem Cells Before Flap Expansion and After Expanded Flap Elevation Result in Different Effects

BACKGROUND

Studies of using mesenchymal stem cells to assist skin and soft tissue expansion have shown that stem cells can improve expansion efficiency through promoting tissue regeneration. However, the issue that whether the flap viability is also improved is unknown.

METHODS

Sixteen pigs were equally divided into 2 groups. A pair of 150 mL expanders was symmetrically inserted into each pig's dorsum. Group 1 received adipose-derived stem cells (ADSCs) injection before expansion, and group 2 received ADSCs grafting after flap elevation. After 4 weeks' expansion, a random flap measuring 2 cm × 16 cm was elevated along the long axis of each expanded flap on the pigs' back. Flap viability was measured at postoperative day 7. Histological analysis and cell tracking were performed. The expression of vascular endothelial growth factor was determined.

RESULTS

The flap viability of the ADSCs-grafted expanded flap (75.5 ± 6.6%) was similar to the control (69.4 ± 8.4%) in group 1 (transplantation before expansion). However, in group 2 (transplantation after flap elevation), the ADSCs-grafted expanded flap had a higher flap viability (92.6 ± 5.7%) compared with control (66.2 ± 7.4%). Moreover, the ADSCs-grafted expanded flap in group 1 showed increased skin thickness, collagen content, cells proliferation, vascularization, and vascular endothelial growth factor expression. Cell tracking showed that the positively stained cell differentiating into an endotheliocyte could be seen in group 2.

CONCLUSIONS

Transplantation of ADSCs before tissue expansion does not improve flap viability but can promote tissue regeneration. Injection of ADSCs after flap elevation can increase the surviving rate of the expanded flap.

Emulsified Fat Grafting Accelerates Tissue Expansion: An Experimental Study in a Rat Model

INTRODUCTION

Tissue expansion has been applied in tissue repair and reconstruction of large soft tissue defects. Stromal vascular fraction (SVF) transplantation is a promising treatment in raising expansion efficiency. However, the clinical utilization of SVF is limited because of its conventional collagenase-based production. The aim of this study was to evaluate the effect of emulsified fat (EF), SVF obtained by using mechanical method, on accelerating tissue expansion.

MATERIALS AND METHODS

The microstructure of EF fragments and the proportion of mesenchymal stem cells (MSCs; CD45-/CD34+) in EF were detected. Wistar rats were divided into the following 3 groups randomly: the 1-mL EF group, the 0.5-mL EF group, and the control group. The tissue expansion was carried out twice a week to maintain the capsule pressure at 60 mm Hg. After 4 weeks, inflation volume and histological changes, which includes collagen content, cell proliferation, and capillary density, were observed to evaluate the effect of EF on tissue expansion.

RESULTS

Mechanical emulsification effectively destroyed the mature adipocytes in adipose tissue. The proportion of MSCs population in the EF fragments was 12.40 ± 0.86%. After expansion, the inflation volume and the levels of collagen deposition, cell proliferation, and capillary density of the expanded tissue in the 1-mL EF group were significantly higher than that in the 0.5-mL EF group and the control group (P < 0.05). However, all these regenerative indicators in the 0.5-mL EF group showed no statistical difference from the control group (P > 0.05). The thickness of epidermal and dermal layers showed no significant difference among the 3 groups (P > 0.05).

CONCLUSIONS

Our findings suggested that EF grafting can be used as a new alternative to increase tissue expansion efficiency.

Macrophages are necessary for skin regeneration during tissue expansion

Background

Tissue expansion is a procedure that promotes skin regeneration by mechanical stretch. During the stress and relaxation cycle, the skin undergoes a repeated microtrauma which triggers an immune response leading to the recruitment of macrophages to repair the damaged tissue. Macrophages have been found to be necessary for tissue repair and wound healing, but their effects on skin regeneration during mechanical stretch remain unclear.

Methods

The dynamic changes of macrophages in the rat skin tissues undergoing expansion were quantitatively determined by immunohistochemistry staining. The area of the expanded skin, skin thickness, dermal collagen density, cell proliferation and tissue vascularization were examined to determine the effects of macrophages on the expanding skin. The phenotypes of macrophages and the growth factors related to skin regeneration were also examined to evaluate the underlying mechanisms for the involvement of macrophages in skin regeneration. As a comparison, the tissue samples of expanding skin in which the macrophages were depleted by topically utilizing clodronate liposomes were also evaluated.

Results

The number of skin macrophages in skin maintained in the high level during the skin expansion compared to non-expanded skin. We found that a switch from an M1- to M2-dominant response during tissue expansion. After the macrophages were depleted, the skin regeneration was inhibited, as evidenced by a smaller expansion area, thinner skin layers and decreased cell proliferation rate, collagen synthesis and, skin vascularization. The secretion of epidermal growth factor (EGF), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF) were decreased when macrophages were depleted.

Conclusions

Our findings suggest that macrophages are necessary for skin regeneration during tissue expansion. Modulating inflammation may provide a key therapeutic strategy to promote skin growth under mechanical strain.

Electronic supplementary material

The online version of this article (10.1186/s12967-019-1780-z) contains supplementary material, which is available to authorized users.

Application of Nanoscaffolds in Mesenchymal Stem Cell-Based Therapy

Regenerative medicine is an alternative solution for organ transplantation. Stem cells and nanoscaffolds are two essential components in regenerative medicine. Mesenchymal stem cells (MSCs) are considered as primary adult stem cells with high proliferation capacity, wide differentiation potential, and immunosuppression properties which make them unique for regenerative medicine and cell therapy. Scaffolds are engineered nanofibers that provide suitable microenvironment for cell signalling which has a great influence on cell proliferation, differentiation, and biology. Recently, application of scaffolds and MSCs is being utilized in obtaining more homogenous population of MSCs with higher cell proliferation rate and greater differentiation potential, which are crucial factors in regenerative medicine. In this review, the definition, biology, source, characterization, and isolation of MSCs and current report of application of nanofibers in regenerative medicine in different lesions are discussed.

Skin extracellular matrix components accelerate the regenerative potential of Lin− cells

Due to their unique properties, bone marrow-derived Lin− cells can be used to regenerate damaged tissues, including skin. The objective of our study was to determine the influence of the skin tissue-specific microenvironment on mouse Lin− cell proliferation and migration in vitro. Cells were analyzed for the expression of stem/progenitor surface markers by flow cytometry. Proliferation of MACS-purified cells in 3D cultures was investigated by WST-8 assay. Lin− cell migration was evaluated by in vitro scratch assay. The results obtained show that basement membrane matrix is more effective for Lin− cell proliferation in vitro. However, type I collagen matrix better enhances the re-epithelization process, that depends on the cell migratory properties. These studies are important for preparing cells to be used in transplantation.

Effect of bone marrow derived mesenchymal stem cells on healing of induced full-thickness skin wounds in albino rat

BACKGROUND AND OBJECTIVES

Mesenchymal stem cells have delivered new approaches to the management of wound healing in severe skin injuries. This work was planned to evaluate the effect of bone marrow-derived mesenchymal stem cells (BMSCs) on healing of induced full thickness skin wounds in albino rats using topical & systemic injections.

METHODS AND RESULTS

Forty adult male albino rats were classified into 2 groups after induction of full thickness skin wound; untreated group and stem cell-treated group. The latter was further subdivided into topically and systemically treated ones. BMSCs were isolated & labeled by PKH26 before injection. Healing of wounds was evaluated grossly. Skin biopsies were obtained one & three weeks after wound induction. Sections were stained with Hematoxylin & Eosin, Masson's trichrome and immunohistochemichal stain for vascular endothelial growth factor (VEGF). Epidermal thicknesses and mean area percent of both collagen fibers & VEGF immunopositive cells were measured using image analyzer & results were subjected to statistical analysis. PKH26 fluorescent-labeled cells were found in the regenerated epidermis, hair follicles and dermis in BMSCs-treated groups. By the end of the third week, the wounds of BMSCs-treated groups showed full regeneration of epidermis, re-organization of collagen and decrease in VEGF immunopositive cells. Delayed wound healing was seen in 20% of systemically treated rats. Significant increase in the mean area percent of collagen fibers was detected in topically treated group.

CONCLUSIONS

Both methods of BMSCs injection were effective in healing of full thickness skin wound but topical method was more effective.

Transplantation of stromal vascular fraction as an alternative for accelerating tissue expansion

BACKGROUND

Tissue expansion has been a good option for repair and reconstruction of large soft-tissue defects. However, the long expansion process, as well as accompanied complications, still remains the major problem of this technique. In this study, a new cell therapy using the stromal vascular fraction (SVF) was applied on expanded tissue to find whether it could accelerate tissue expansion. Its role in skin regeneration during tissue expansion was also explored.

METHODS

Wistar rats were divided into two groups with eight rats in each group and received transplantation of SVF and phosphate-buffered saline (PBS), respectively. Four weeks after surgery, inflation volume was measured to evaluate the effect of SVF on tissue expansion. Angiogenesis and cell proliferation were examined to observe the histological changes. At last, the gene expression of epidermic growth factor (EGF), vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) was detected to observe the paracrine effect of SVF to explain the changes.

RESULTS

The inflation volume in the SVF group (55.5 ± 4.92 ml) was larger than in the control group (34.5 ± 2.95 ml), P < 0.05. Enhanced angiogenesis and cell proliferation were observed in the tissue treated by transplantation of SVF. SVF-conditioned expanded tissue owned higher expression of EGF, VEGF and bFGF.

CONCLUSION

These findings suggest that transplantation of SVF could shorten tissue expansion process through enhancing skin regeneration, which may be improved by growth factors such as EGF, VEGF and bFGF secreted by engrafted SVF.

Bone marrow mesenchymal stem cells increase skin regeneration efficiency in skin and soft tissue expansion

BACKGROUND

Skin and soft tissue expansion has limitations such as long hospitalization time and flap retraction after expansion. Our previous study suggested that bone marrow-derived stem cells contribute skin regeneration in skin and soft tissue expansion. In this study, the authors explored the feasibility of applying the bone marrow mesenchymal stem cells (BMMSCs) to the treatment of skin and soft tissue expansion and increasing the skin regeneration efficiency.

METHODS

Sixty silicone expanders were implanted in the backs of 15 pigs, and allogeneic BMMSCs were transplanted to skin shallow fascia layer (local transplantation, Group A) or via ear vein (systemic transplantation, Group B). Group C was the Sham operation control; and then the expanders were injected with normal saline (N.S.). Skin was obtained at different time points (days 0, 14, 21, 28, 35, and 42). The organizational structure changes of the target skin were observed in the expansion process. The distribution, differentiation, and paracrine function of labeled BMMSCs were detected.

RESULTS

Comparing with Group B (25.00 ± 1.98 cm(2)) or Group C (24.00 ± 1.10 cm(2), no transplantation), the expanded skin area of Group A (28.82 ± 1.43 cm(2)) increased, with the morphology of epidermis thickened, and dermis thinned. The BMMSCs differentiated into vascular endothelial cells and dermal fibroblasts. The quantity of newborn cells was proportional to the number of transplanted cells. The gene expression of VEGF, bFGF, EGF, and SDF in Group A was higher than those in Group B or C. The most obvious changes were on day 35.

CONCLUSIONS

The local transplanted BMMSCs could increase the skin regeneration efficiency in skin and soft tissue expansion and reduce skin shrinkage effectively after removing the expander. Growth factors, VEGF, bFGF, EGF, and SDF, are favorable to this process.