Regenerative therapies are available for acute and chronic skin wounds, hair loss, periodontal injuries and disease, and diseases and injuries of the cornea.
A wide variety of topical agents have been tested for their efficacy in accelerating repair of acute wounds in normal skin. The growth factors TGF-P, FGF-2, EGF, GH, and IGF-1 can accelerate the repair of acute wounds in experimental animals. FGF-2 and GH have this effect in human patients. Other agents reported to accelerate the repair of skin wounds are extract of the Celosia argentea leaf, vanadate, oxandrolone, the opoid fentanyl, ketanserin, oleic fatty acids, pig enamel matrix, and the peptide HB-107. These agents increase the rate and extent of re-epithelialization, angiogenesis, and cellularity of granulation tissue. Removal of eschar from burn wounds by debriding agents such as papain/ urea improves repair. Still other topical agents act to reduce scarring by decreasing levels of TGF-P, thus mimicking a fetal wound environment more closely. Chitosan, the COX-2 inhibitor celicoxib, HGF, and anti-TGF-P antibodies all reduce TGF-P in wounds promote healing with less scarring, as do hydrogels composed of cross-linked hyaluronic acid and chon-droitin sulfate.
The inability of chronic skin wounds to heal on their own is by far the biggest clinical problem in wound repair. A wide variety of regenerative therapies have been devised to treat chronic wounds, including topically applied agents, infrared and near-infrared light, keratinocyte transplants, bioartificial skin equivalents, and acellular dermal regeneration templates.
The topically applied growth factors P-NGF, FGF-2, PDGF-B, EGF, TGF-P, IGF-1 have been reported to improve healing of chronic wounds in animals. Combinations of PDGF-B plus IGF-1, EGF plus insulin, and TGF-P plus PDGF-B were more effective than the individual growth factors alone. In human patients, PDGF-B, FGF-2, EGF, TGF-P, and rhKGF-2 all accel erated the closure of chronic wounds. Currently, only FGF-2, PDGF-B and rhKGF-2 are approved for clinical use. FGF-2 is sold in Japan as Fiblast, and PDGF-B and rhKGF-2 are sold in the USA as Regranex® and repifermin, respectively. Growth factors may accelerate chronic wound repair by decreasing inflammation and enhancing angiogenesis. Other topically applied agents that accelerate the repair of chronic wounds are angiotensin (1-7), thymosin P4, L-arginine, and pent-oxifylline. These agents, too, may exert their effect through anti-inflammatory and angiogenesis-promoting activities. Infrared laser light and near-infrared light emitted by LEDs has also been reported to accelerate the repair of wounds in both animals and human patients. If confirmed by large randomized, doubleblind clinical trials, acceleration of wound repair by LEDs could become the treatment of choice because of simplicity of treatment, minimal invasiveness, and low cost.
Sheets of allogeneic keratinocytes are used to enhance re-epithelialization of extensive wound areas. They are only slowly rejected by recipients and are replaced by host keratinocytes. The sheets are very fragile, so they are grown on polymer carriers that enable easier transfer to the wound surface. They suffer low take rates when applied to wounds in which the dermis has been badly damaged, because they depend on dermal fibro-blasts for basement membrane resynthesis and mitotic factors such as KGF. Thus, complete skin equivalents constructed of a biodegradable polymer scaffold seeded with allogeneic fibroblasts from human foreskin and covered with keratinocytes or an MSTSG have been designed to cover wounds with extensive dermal damage.
The purpose of bioartificial skin is to obviate the need to take large areas of donor skin for full-thickness skin grafts. Bioartificial skin equivalents are temporary living dressings that prevent wound contraction and stimulate repair of the wound by host cells. There are a number of FDA-approved skin equivalents on the market that are similar in nature. The first two of these were Apligraf® and Dermagraft®, which are used for the treatment of the most recalcitrant chronic wounds. Apligraf® consists of a bioartificial dermis of human fibroblasts in bovine type I collagen covered with allo-geneic keratinocytes or an autogeneic MSTSG. The bioartificial dermis of Dermagraft® is constructed by growing human fibroblasts on a mesh of Vicryl. Keratinocytes or an MSTSG are added after placement of the construct on the wound bed. Apligraf® has been shown to accelerate the healing of venous ulcers and increase the percentage of ulcers that are closed, while Dermagraft® has been shown to do the same for diabetic ulcers. Skin equivalents have proven their worth in avoiding the need to harvest autogeneic full-
thickness skin grafts. They suffer several problems, however. First, lacking blood vessels, they are slow to vascularize if the underlying wound bed is severely damaged; thus, ways are being sought to seed endothe-lial cells into the construct that will form blood vessels. Second, they do no better than MSTSGs alone in restoring cosmetic appearance, sensation, or skin functions such as thermal regulation, because the repaired skin has no hair follicles, sebaceous glands, or sweat glands. Third, they are cryopreserved until use, which reduces their cellularity. Thus, it is cheaper to use an acellular dermal regeneration matrix covered with keratinocytes or MSTSGs to treat wounds with deep dermal damage. The matrix slowly degrades as it is invaded by host fibroblasts, which repair the dermis.
Dermal regeneration templates can be either natural (derived from tissue) or bioartificial. Several processed collagenous matrices have been approved for clinical use. One of these, Alloderm®, is cadaver dermal matrix. It has been evaluated clinically for burns and was reported to provide good cosmetic appearance and function when used in full thickness. Permacol® is porcine dermal matrix and is effective in hernia repair. SIS is porcine small intestine submucosa. One variant of SIS, SurgisisTM, has been shown to be superior to polypropylene mesh in hernia repair, and another, OasisTM, accelerates the healing of diabetic ulcers. Primatrix™ is fetal bovine dermal matrix that is approved for chronic wounds and acute incisional and excisional wounds, including burns. The most widely used bioartificial dermal matrix is Integra®, which consists of bovine dermal collagen and chondroitin 6-sulfate. Clinical assessments of Integra® have reported results superior to those of other constructs for exci-sional wounds, including burns. Epidermal coverings do not take well on dermal regeneration templates when the dermis is badly damaged, due to slow vascu-larization. Thus, they are often applied in a two-step procedure in which the dermal template is put on the wound first and allowed to revascularize, after which keratinocytes or an MSTSG are added.
Male pattern baldness is a genetic deficiency leading to hair loss on the temples and crown of the scalp due to sensitivity of the hair follicles in these regions to dihydrotestosterone (DHT). This hormone shortens the anagen stage and lengthens the telogen stage of hair regeneration, leaving empty and involuted hair follicles. Two drugs, minoxidil (Rogaine) and finasteride (Propecia) are available to combat hair loss. Rogaine slows hair loss by an unknown mechanism, whereas Propecia promotes hair growth by blocking the formation of DHT. Shh stimulates hair regrowth in mice suffering from cyclophosphamide-induced alopecia, suggesting that it may be useful in the treatment of hair loss. Other approaches to restoring hair involve the culture of bulge stem cells with dermal papilla cells from DHT-insensitive follicles to create new hair follicles that can be implanted into the scalp.
The current technique to restore periodontal tissue is guided tissue regeneration, which does not produce predictably good outcomes. Other experimental approaches in animals that have reported some success are irradiation with infrared laser light, and the use of stem cells isolated from the periodontal ligament and expanded in culture. When mixed with hydroxyapatite/ tricalcium phosphate carrier and transplanted sub-cutaneously in mice, the cells formed cementum/PDL-like structures. Also envisioned are the regeneration of dentin from pulp stem cells and the creation of tooth buds by combining alveolar epithelium and pulp stem cells. Porcine tooth buds have been created in this way and can grow and develop after implantation into the abdomen of host rats. Nonmammalian research models such as urodele amphibians, sharks, and crocodilians, all of which naturally replace tooth buds, will be useful in understanding the biology and chemistry of tooth development.
The cornea is unable to regenerate if limbal tissue, which provides the stem cells for regeneration, is compromised. If part of the limbus in either eye remains undamaged, pieces of this tissue can be removed and cultured on human amniotic membrane to form a larger sheet. Pieces of this epithelial sheet can then be transplanted as an autogeneic sectorial or ring graft to regenerate and maintain the damaged cornea. In the absence of autogeneic limbal epithelium, oral epithelium has been used as a source of stem cells. These cells form corneal epithelium when transplanted to the corneal stroma. These techniques have been reported to improve vision in human patients.
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