Skin provides a mechanical barrier to the external environment and acts to prevent the ingress of infectious agents. Once injured, the tissues beneath are exposed to infection; therefore, rapid and effective healing is of crucial significance to reconstruct a barrier function. Skin wound healing is a complex process, and includes many interacting processes initiated by haemostasis and the release of platelet-derived factors. The following stages are inflammation, granulation tissue formation, reepithelization and remodeling. HA is likely to play a multifaceted role in mediation of these cellular and matrix events. The proposed roles of HA in this sequence of skin wound healing events are detailed below.
Hyaluronic acid has also been used in the synthesis of biological scaffolds for wound-healing applications. These scaffolds typically have proteins such as fibronectinattached to the hyaluronan to facilitate cell migration into the wound. This is particularly important for individuals with diabetes suffering from chronic wounds.
In the early inflammatory phase of wound repair, wounded tissue is abundant in HA, probably a reflection of increased synthesis. HA acts as a promoter of early inflammation, which is crucial in the whole skin wound-healing process. In a murine air pouch model of carrageenan/IL-1-induced inflammation, HA was observed to enhance cellular infiltration. showed a dose-dependent increase of the proinflammatory cytokines TNF-α and IL-8 production by human uterine fibroblasts at HA concentrations of 10 μg/mL to 1 mg/mL via a CD44-mediated mechanism. Endothelial cells, in response to inflammatory cytokines such as TNF-α, and bacteriallipopolysaccharide, also synthesize HA, which has been shown to facilitate primary adhesion of cytokine-activated lymphocytes expressing the HA-binding variants of CD44 under laminar and static flow conditions. It is interesting to note that HA has contradictory dual functions in the inflammatory process. It not only can promote the inflammation, as stated above, but also can moderate the inflammatory response, which may contribute to the stabilization of granulation tissue matrix, as described in the following part.
Although inflammation is an integral part of granulation tissue formation, for normal tissue repair to proceed, inflammation needs to be moderated. The initial granulation tissue formed is highly inflammatory with a high rate of tissue turnover mediated by matrix degrading enzymes and reactive oxygen metabolites that are products of inflammatory cells. Stabilization of granulation tissue matrix can be achieved by moderating inflammation. HA functions as an important moderator in this moderation process, which contradicts its role in inflammatory stimulation, as described above. HA can protect against free-radical damage to cells. This may attribute to its free-radical scavenging property, a physicochemical characteristic shared by large polyionic polymers. In a rat model of free-radical scavenging property, HA has been shown to reduce damage to the granulation tissue.
In addition to the free-radical scavenging role, HA may also function in the negative feedback loop of inflammatory activation through its specific biological interactions with the biological constituents of inflammation. TNF-α, an important cytokine generated in inflammation, stimulates the expression of TSG-6 (TNF-stimulated gene 6) in fibroblasts and inflammatory cells. TSG-6, a HA-binding protein, also forms a stable complex with the serum proteinase inhibitor IαI (Inter-α-inhibitor) with a synergistic effect on the latter’s plasmin-inhibitory activity. Plasmin is involved in activation of the proteolytic cascade of matrix metalloproteinases and other proteinasesleading to inflammatory tissue damage. Therefore, the action of TSG-6/ IαI complex, which may be additionally organized by binding to HA in the extracellular matrix, may serve as a potent negative feedback loop to moderate inflammation and stabilize the granulation tissue as healing progresses. In the murine air pouch model of carragenan/IL-1 (Interleukin-1β)-induced inflammation, where HA has been shown to have a proinflammatory property, reduction of inflammation can be achieved by administrating TSG-6, and the result is comparable with systemic dexamethasone treatment.
HA plays an important role in the normal epidermis. HA also has crucial functions in the reepithelization process due to several of its properties. It serves as an integral part of the extracellular matrix of basal keratinocytes, which are major constituents of the epidermis; its free-radical scavenging function and its role in keratinocyte proliferation and migration. In normal skin, HA is found in relative high concentrations in the basal layer of the epidermis where proliferating keratinocytes are found. CD44 is collocated with HA in the basal layer of epidermis where additionally it has been shown to be preferentially expressed on plasma membrane facing the HA-rich matrix pouches. Maintaining the extracellular space and providing an open, as well as hydrated, structure for the passage of nutrients are the main functions of HA in epidermis. A report found HA content increases at the presence of retinoic acid (vitamin A). The proposed effects of retinoic acid against skin photo-damage and aging may be correlated, at least in part, with an increase of skin HA content, giving rise to increase of tissue hydration. It has been suggested the free-radical scavenging property of HA contributes to protection against solar radiation, supporting the role of CD44 acting as a HA receptor in the epidermis.
Epidermal HA also functions as a manipulator in the process of keratinocyte proliferation, which is essential in normal epidermal function, as well as during reepithelization in tissue repair. In the wound healing process, HA is expressed in the wound margin, in the connective tissue matrix, and collocating with CD44 expression in migrating keratinocytes. Kaya et al. found suppression of CD44 expression by an epidermis-specific antisense t
ransgene resulted in animals with defective HA accumulation in the superficial dermis, accompanied by distinct morphologic alterations of basal keratinocytes and defective keratinocyte proliferation in response to mitogen and growth factors. Decrease in skin elasticity, impaired local inflammatory response, and impaired tissue repair were also observed. Their observations are strongly supportive of the important roles HA and CD44 have in skin physiology and tissue repair.
Fetal wound healing and scarring
Lack of fibrous scarring is the primary feature of fetal wound healing. Even for longer periods, HA content in fetal wounds is still higher than that in adult wounds, which suggests that HA may, at least in part, reduce collagen deposition and therefore lead to reduced scarring. This suggestion is in agreement with the research of West et al., who showed in adult and late gestation fetal wound healing, removal of HA results in fibrotic scarring.
Hyaluronic acid is a common ingredient in skin-care products. Until recently, hyaluronic acid fillers were injected using a classic sharp hypodermic needle, cutting through nerves and vessels, causing pain and bruising.
In certain cases, the hyaluronic acid fillers results in a granulomatous foreign body reaction.