In this study, we demonstrated a fibrin hydrogel scaffold consisting of fibrinogen and thrombin, which had a porous, three-dimensional mesh structure and could establish perfectly fitting shapes in irregular wounds.
Full-thickness skin wounds caused by acute trauma, chronic ulcers, and deep burns have always been an intractable medical problem, which cause many physiological and functional problems [4]. Currently, the main therapeutic methods used in clinical practice are functionally limited. For instance, traditional cotton medical gauze has some inevitable disadvantages, such as limited use, difficulty in absorption, and frequent replacement, which may cause secondary trauma. Biological dressings, including pig skin and amniotic membrane, carry the risk of spreading bacteria and viruses and immunological rejection [20–23]. It has been confirmed that MSCs play important roles in all stages of wound healing [5–9], but ensuring the survival and effective functioning of MSCs at the wound site is still a difficult problem.
Our previous work demonstrated that the fibrin hydrogel scaffold possesses sustained drug release in vitro and in vivo [33]. This scaffold not only overcomes the limitations of traditional medical dressings but also provides a suitable microenvironment for hUC-MSC growth and transplantation for the treatment of skin wounds.
The advantages of hUC-MSC transplantation in skin wound healing have become popular. HUC-MSCs secrete important growth factors necessary for re-epithelialization and angiogenesis through the paracrine effect [14–19]. Herein, we found that the fibrin hydrogel scaffold enhanced the biological functions of hUC-MSCs. It facilitated the relative gene expression of growth factors (EGF, VEGF, and VEGFA) and migration-related genes (TGF-β1), which was beneficial for re-epithelialization, angiogenesis, and extracellular matrix secretion. EGF can stimulate the migration and proliferation of epidermal cells during re-epithelialization [35]. VEGF and VEGFA are critical for angiogenesis during the formation of the granulation tissue [36]. Newly formed blood vessels can provide regenerating tissues with enough oxygen and nutrition, which are essential to complete wound healing [37]. TGF-β1 belongs to the superfamily of transforming growth factor β, which regulates cell growth and differentiation. It can stimulate fibroblasts to synthesize large amounts of collagen, providing a temporary extracellular matrix for neovascularization, as well as proliferation and migration of basal cells. In addition, TGF-β1 can promote fibroblast transformation into myofibroblasts, achieving wound closure [35]. In the present study, the expression of these cytokines was higher in the hydrogel-hUC-MSC combination therapy group. Meanwhile, the fluorescence expression of K10 and K14 was high in the combination therapy group. Keratin intermediate filaments are major protein constituents in epithelial cells that provide mechanical support and fulfill a variety of additional functions [38]. We speculated that the formation of fibrin hydrogel simulates hemostasis, providing a microenvironment similar to the extracellular matrix for hUC-MSC growth, thus, inducing the paracrine effect of hUC-MSCs and promoting cytokine secretion. With the slow degradation of the hydrogel, cytokines are released to promote extracellular matrix production and increase secretion of cytokines, improving wound healing. This is a positive feedback regulation.
Interestingly, the ratio of wound re-epithelialization length and K10 and K14 immunofluorescence results in the hydrogel-hUC-MSC combination group were substantially better than those in the hUC-MSC-alone group, but the expression levels of EGF in the hUC-MSC-alone group and hydrogel-hUC-MSC combination group had no significant difference on day 3 after surgery. We speculated that hUC-MSCs could not function well without the microenvironment provided by the hydrogel, and thus, the increased expression of EGF in the hUC-MSC-alone group was not enough to recruit sufficient epidermal cells at the wound site. From this point of view, the positive effects of hydrogel on cell adhesion, proliferation, and migration were also supported. In addition, the expression of VEGFA in the hUC-MSC-alone group was lower than that in the hydrogel-hUC-MSC combination group, while the expression of VEGF in the hUC-MSC-alone group was similar to that in the hydrogel-hUC-MSC combination group on day 14 after surgery. We hypothesized that this might be because VEGFA is only one of the members of the VEGF family, and at this point, wound repair had already been in the late remodeling stage.
These findings strongly support the clinical therapeutic potential of the fibrin hydrogel scaffold loaded with hUC-MSCs. However, this study is not without limitations; the underlying mechanism of how fibrin hydrogel-hUC-MSC combination therapy promotes wound healing has not been discussed in our work, which needs further investigation.