Epidermal Replacement Research Articles

812 Evaluation of Dermal and Epidermal Replacement Strategies for the Treatment of Full-thickness Wounds

IntroductionIn full-thickness (FT) wounds, autologous skin cell suspension (ASCS) is used in combination with widely meshed split-thickness skin grafts (mSTSGs) to obtain definitive closure, reducing donor skin requirements compared to traditional autografting (AG) techniques. In the treatment of these wounds, dermal matrices (DMs) are also often utilized to address various challenges including need for temporization, mitigation of contour defects, covering avascular structures, and modulation of scar formation. Varying DMs exist and are composed of different biologic and synthetic materials designed to address these clinical needs. The purpose of this study was to compare outcomes obtained in FT wounds using 3 different DMs coupled with ASCS+mSTSG in immediate or delayed AG procedures.MethodsA FT excisional porcine wound model was used. DMs evaluated included a single-layer dermal matrix composed of bovine dermal collagen and elastin (Col/E), a bilayer construct composed of a bovine collagen-glycosaminoglycan with a silicone epidermal layer (Col/GAG), and a synthetic bilayer polyurethane DM (Poly/U).DMs were applied, managed, and grafted following manufacturer’s recommendations. AG included ASCS+mSTSG (1:80 expansion, 3:1 mesh). Wounds were evaluated for inflammation, infection, DM take, AG take, re-epithelialization, and contracture over 49 days post-excision. Additionally, biopsies were evaluated to further inform tissue generation and healing outcomes.ResultsResults related to healing outcomes are reported in the table below. Inflammation at wound margins was noted during acute phase of healing for all DMs and visual cues for infection were noted in 1 wound for Col/E, 3 wounds for Col/GAG causing partial loss of DM, and 1 wound for Poly/U. All signs of infection were resolved by day 14.ConclusionsASCS+mSTSG can be used successfully over DMs composed of various materials in both immediate and delayed AG procedures. No difference was observed on percent AG take between the DMs, however the data suggest that time to definitive closure is impacted based on utilized DM and AG strategy, with potential implications on contracture.

Particulate matter 2.5 induced hyperpigmentation in reconstructed human epidermis model (MelaKutis®).

Particulate matter 2.5 induced hyperpigmentation in reconstructed human epidermis model (MelaKutis®).

Impact of dermal matrix thickness on split-thickness skin graft survival and wound contraction in a single-stage procedure.

Optimal treatment of full‐thickness skin injuries requires dermal and epidermal replacement. To spare donor dermis, dermal substitutes can be used ahead of split‐thickness skin graft (STSG) application. However, this two‐stage procedure requires an additional general anaesthetic, often prolongs hospitalisation, and increases outpatient services. Although a few case series have described successful single‐stage reconstructions, with application of both STSG and dermal substitute at the index operation, we have little understanding of how the physical characteristics of dermal substitutes affects the success of a single‐stage procedure. Here, we evaluated several dermal substitutes to optimise single‐stage skin replacement in a preclinical porcine model. A porcine full‐thickness excisional wound model was used to evaluate the following dermal substitutes: autologous dermal graft (ADG; thicknesses 0.15‐0.60 mm), Integra (0.4‐0.8 mm), Alloderm (0.9‐1.6 mm), and chitosan‐based hydrogel (0.1‐0.2 mm). After excision, each wound was treated with either a dermal substitute followed by STSG or STSG alone (control). Endpoints included graft take at postoperative days (PODs) 7 and 14, wound closure at POD 28, and wound contracture from POD 28‐120. Graft take was highest in the STSG alone and hydrogel groups at POD 14 (86.9% ± 19.5% and 81.3% ± 12.3%, respectively; P < .001). There were no differences in graft take at POD 7 or in wound closure at POD 28, though highest rates of wound closure were seen in the STSG alone and hydrogel groups (93.6% ± 9.1% and 99.8% ± 0.5%, respectively). ADG‐treated wounds demonstrated the least amount of wound contracture at each time point. Increase dermal substitute thickness was associated with worse percent graft take at PODs 14 and 28 (Spearman ρ of −0.50 and −0.45, respectively; P < .001). In this preclinical single‐stage skin reconstruction model, thinner ADG and hydrogel dermal substitutes outperformed thicker dermal substitutes. Both substitute thickness and composition affect treatment success. Further preclinical and clinical studies to optimise this treatment modality are warranted.

Cell therapy in treatment of skin burns

The review summarizes the results of modern research in the field of cell therapy for skin burns. The relevance of conducting these studies both in Russia and in the world is described. The methods of influence on the skin regeneration after burns in addition to cell therapy are indicated. A history reference on the development of cell therapy for burn skin lesions is given. The documents governing the conduct of cell therapy in Russia are presented. The advantages and disadvantages of cell technology using keratinocytes are described. The methods of cell therapy for skin burns using fibroblasts are analyzed. The role of three-dimensional tissue-engineered structures – scaffolds in the regeneration of the skin is described. Their classification is given by the duration of wound coverage (permanent, semi-permanent and temporary), by composition (cellular, acellular), by type of material (synthetic, biological, which are divided into allogeneic and autologous). The main representatives of each group that are used in research as therapy in the treatment of skin burns are described: Biobrane, Integra, Dermagraft, TransCyte, Hyalograft 3D, Laser skin epidermal replacement, TissueTech autograft system. The data on the experimental testing of each of the representatives are presented. The issue of improving vascularization of tissue-engineering structures using bioreactors was also raised. According to the results of the review, it was concluded that the use of three-dimensional structures in the treatment of burn skin lesions shows the greatest efficacy and safety among pronounced cell therapy options in clinical practice. At the same time, the existing drawbacks of the analyzed samples require further study and analysis.

Skin Stem Cells in Silence, Action, and Cancer.

In studying how stem cells make and maintain tissues, nearly every chapter of a cell biology textbook takes on special interest. The field even allows us to venture where no chapters have yet been written. In studying this basic problem, we are continually bombarded by nature's surprises and challenges. Stem cell biology has captured my interest for nearly my entire scientific career. Below, I focus on my laboratory's contributions to this fascinating field, to which so many friends and colleagues have made seminal discoveries equally deserving of this award.

Epidermal dynamics and patterns of intraepidermal melanoma

Dermatopathologists know that the epidermis represents a dynamic compartment and that its cells mature from the basal layer to the skin surface in approximately 45 days. What may seem intuitive - but not obvious - is that the dynamics of the epidermis can affect the patterns of melanoma cells within the epidermis. Here this conjecture is explored with an abstract, theoretical model. To control the independent effects of epidermal replacement velocity and thickness as well as rate of melanoma cell penetration of the epidermis, an abstraction of the epidermis was created and simulated via computer. Simulated plots of the epidermis show that the number and pattern of melanoma cells in the epidermis is affected by the probability of melanoma cells entering the epidermis, by the velocity of epidermal replacement and by epidermal thickness. This analysis suggests that the dynamics of the epidermis are sufficient to affect the patterns of melanoma cells within the epidermis.

The interfollicular epidermal stem cell saga: sensationalism versus reality check

Adult stem cells in rapidly renewing tissues have been classically defined as rare, relatively quiescent cells with the unique capacity to constantly self-renew and regenerate tissues during homeostasis. Although this view remains firmly embedded in the skin field, particularly in the area of hair follicle stem cell biology, it has been challenged by a number of notable publications in 2007. These papers leave an uncomfortable feeling with the reader if one believes that stem cells and transit amplifying cells are two polar opposites and 'never the twain shall meet.' Even if you do not subscribe to this extreme view, the implications appear to be far-reaching given that the majority of techniques devised for stem cell identification have used the fundamental tenet that the proliferating compartment is comprised of two distinct, mutually exclusive compartments, i.e. a minor proportion of long-lived quiescent stem cells with unlimited self-renewal and a large pool of rapidly cycling, short-lived transient amplifying cells with limited or no self-renewal capacity in normal steady-state conditions. However, these recent findings have resulted in papers that could be described as sensationalistic because they make little or no attempt to reconcile their observations with the large bulk of historical data with direct bearing on the interpretation of stem cell activity in normal steady-state conditions. Here, we offer some explanations that may help to integrate all of the data while presenting a case that both quiescent stem cells and cycling 'transit amplifying' cells contribute to epidermal replacement.

Long‐Term Effects of Neonatal Capsaicin Treatment on Intraepidermal Nerve Fibers and Keratinocyte Proliferation in Rat Glabrous Skin

Innervation is required to preserve several aspects of skin homeostasis. Previous studies in rodents have shown that sciatic nerve transection leads to epidermal thinning and reduced keratinocyte proliferation. As the sciatic nerve is composed of sensory and motor axons, it is not clear whether skin alterations reflect motor or sensory disturbances. In this study, we used neonatal capsaicin treatment to evaluate whether sensory chemical denervation affects keratinocyte proliferation at 1, 3, and 6 months of age. Using design-based stereological methods, we estimated the total length of intraepidermal nerve fibers (IENF) that were of peptidergic type and the number of bromodeoxyuridine-labeled (BrdU(+) ) nuclei in the hind paw glabrous epidermis of control and capsaicin-treated rats. We found that the treatment decreased the total fiber length of IENF immunoreactive for both protein gene product 9.5 (PGP(+) ) and of IENF immunoreactive for calcitonin gene-related peptide (CGRP(+) ). The length of PGP(+) fibers decreased by 83%, 81%, and 77% and that of CGRP(+) fibers decreased by 48%, 58%, and 58% at 1, 3, and 6 months, respectively. Double-immunofluorescence staining for neural beta III tubulin and CGRP revealed that the majority of the remaining fibers in the epidermis after capsaicin treatment were of peptidergic type. The number of BrdU(+) nuclei was similar in both groups. Our findings suggest that IENF present after capsaicin treatment are sufficient to maintain epidermal replacement.

Use of a novel autologous cell-harvesting device to promote epithelialization and enhance appropriate pigmentation in scar reconstruction

Epidermal replacement is an important step in the management of patients with post-traumatic and iatrogenic scars. Skin-colour variation from disease or trauma causes significant changes in self-image and appearance. The aim of our study was to analyse the results obtained with a novel autologous cell-harvesting system (ReCell) for epidermal replacement in patients with post-traumatic scars that had not improved with any other surgical procedure. We recruited 30 patients with post-traumatic or iatrogenic scars admitted to our department over 2 years. The primary endpoints of the study were: (i) time for complete epithelialization (both treated area and biopsy site) and (ii) aesthetic and functional quality of the epitheliaization (colour, joint contractures). Infections, inflammations or any adverse effects of the procedure were also reported. In total, 30 patients were analysed. The aesthetic and functional outcomes were rated by both patient and surgeon. Pigmentation was rated by the Vancouver Scar Scale. Most (80%) of the patients had an excellent or good outcome, with pigmentation rated as normal in 60% of the group. The procedure is a feasible, simple and safe technique. It gives similar results to skin grafting but because it harvests from much smaller areas, can open possible future applications in the management of patients with large scars.

To Be or Not to Be a Flatworm: The Acoel Controversy

Since first described, acoels were considered members of the flatworms (Platyhelminthes). However, no clear synapomorphies among the three large flatworm taxa - the Catenulida, the Acoelomorpha and the Rhabditophora - have been characterized to date. Molecular phylogenies, on the other hand, commonly positioned acoels separate from other flatworms. Accordingly, our own multi-locus phylogenetic analysis using 43 genes and 23 animal species places the acoel flatworm Isodiametra pulchra at the base of all Bilateria, distant from other flatworms. By contrast, novel data on the distribution and proliferation of stem cells and the specific mode of epidermal replacement constitute a strong synapomorphy for the Acoela plus the major group of flatworms, the Rhabditophora. The expression of a piwi-like gene not only in gonadal, but also in adult somatic stem cells is another unique feature among bilaterians. These two independent stem-cell-related characters put the Acoela into the Platyhelminthes-Lophotrochozoa clade and account for the most parsimonious evolutionary explanation of epidermal cell renewal in the Bilateria. Most available multigene analyses produce conflicting results regarding the position of the acoels in the tree of life. Given these phylogenomic conflicts and the contradiction of developmental and morphological data with phylogenomic results, the monophyly of the phylum Platyhelminthes and the position of the Acoela remain unresolved. By these data, both the inclusion of Acoela within Platyhelminthes, and their separation from flatworms as basal bilaterians are well-supported alternatives.

A randomized trial comparing ReCell ® system of epidermal cells delivery versus classic skin grafts for the treatment of deep partial thickness burns

Background Our purpose was to directly compare results obtained with the ReCell system and the classic skin grafting for epidermal replacement in deep partial thickness burns. Materials and methods We recruited all patients with deep partial thickness burns admitted at the Burn Centre of S. Eugenio Hospital in Rome over 2 years. Enrollment was conducted with a controlled strategy – sampling chart – that allowed homogeneous groups (ReCell and skin grafting) for age, gender, type of burns and total burn surface area (TBSA). We evaluated as primary endpoints of the study the (i) time for complete epithelization (both treated area and biopsy site) and (ii) aesthetic and functional quality of the epithelization (color, joint contractures). Secondary endpoints were the assessment of infections, inflammations or any adverse effects of the ReCell procedure, particular medications assumed, postoperative pain. Results Eighty-two patients were analyzed in two homogeneous groups. All of them received adequate epidermal replacement, but skin grafting was faster than ReCell ( p < 0.05). On the contrary, ReCell biopsy areas and postoperative pain were smaller than classic grafting ( p < 0.05). The aesthetic and functional outcomes were similar between procedures. Conclusions ReCell is a feasible, simple and safe technique. It gives similar results to skin grafting but, harvesting minor areas, can open possible future applications in the management of large-burns patients.

Hyalomatrix ® – A novel dermal pre-treatment

Aims: It is well accepted that Hyaluronic Acid is crucial to wound healing. The aim of this study was to test in-vivo an esterified Hyaluronic Acid scaffold - Hyalomatrix®, to act as a dermal pre-treatment and substitute prior to the application of epidermal replacement in full thickness wounds. Hyalomatrix® is a partial benzyl ester (80% esterified) of Hyaluronic acid coupled with a thin silicone layer providing fluid loss control and a microbial barrier to function as a temporarily epidermis. Investigation of the optimal pre-treatment time was also part of the study.

Cytokeratin, filaggrin, and p63 expression in reepithelialization during human cutaneous wound healing

ABSTRACTCytokeratin (CK), filaggrin (filament aggregating protein), and p63 expression and cellular distribution during reepithelialization has not been systemically studied in the healing stage of human cutaneous wounds. We examined these proteins by immunohistochemical methods in 12 cases of skin ulcer, using seven anti‐keratin antibodies, anti‐filaggrin, and anti‐p63 antibody. At the edge of the wound in skin ulcers, CK1 and 10 expression was reduced, while CK14, 16, and 17 expression was raised. Beneath the wound bed, all layers of the epidermal tongue, deriving from sweat ducts, were positive for CK14 and 17. Both cytokeratins were also found in basal and luminal cells of the dermal duct. CK expression by epithelia continuous with hair follicles showed that, CK14, 16, and 17 were present, and CK1 and 10 were absent. Filaggrin expression was elevated in reepithelialized epithelium. Expression of p63 expression was verified in the suprabasal layer in reepithelialized epithelia. CK, filaggrin, and p63 expression in the reepithelialization stage at the wound edge and at epidermal appendages remaining in the wound bed is undifferentiated and hyperproliferative. The presence of CK14 and 17 in the remaining epidermal appendages in the pathological wound may be important in epidermal replacement.

Survey of Literature

Cytokeratin (CK), filaggrin (filament aggregating protein), and p63 expression and cellular distribution during reepithelialization has not been systemically studied in the healing stage of human cutaneous wounds. We examined these proteins by immunohistochemical methods in 12 cases of skin ulcer, using seven anti-keratin antibodies, anti-filaggrin, and anti-p63 antibody. At the edge of the wound in skin ulcers, CK1 and 10 expression was reduced, while CK14, 16, and 17 expression was raised. Beneath the wound bed, all layers of the epidermal tongue, deriving from sweat ducts, were positive for CK14 and 17. Both cytokeratins were also found in basal and luminal cells of the dermal duct. CK expression by epithelia continuous with hair follicles showed that, CK14, 16, and 17 were present, and CK1 and 10 were absent. Filaggrin expression was elevated in reepithelialized epithelium. Expression of p63 expression was verified in the suprabasal layer in reepithelialized epithelia. CK, filaggrin, and p63 expression in the reepithelialization stage at the wound edge and at epidermal appendages remaining in the wound bed is undifferentiated and hyperproliferative. The presence of CK14 and 17 in the remaining epidermal appendages in the pathological wound may be important in epidermal replacement.

A Clinical Investigation on the Characteristics and Outcomes of Treating Chronic Lower Extremity Wounds using the TissueTech Autograft System

The application of tissue engineering technology to wound healing has resulted in the development of a number of "living skin equivalents." These have become a viable option in the treatment of chronic, nonhealing wounds. Such wounds present a major cost burden as well as increased morbidity and mortality. Unique among skin tissue engineering technology is the TissueTech autograft system, as it incorporates an autologous dermal substitute-Hyalograft 3D-and an autologous epidermal replacement, Laserskin autograft. Each includes a matrix of a hyaluronic acid ester to promote cellular migration and graft take.

Ultrastructure of neoblasts in microturbellaria: significance for understanding stem cells in free-living Platyhelminthes

Summary Platyhelminths possess a unique stem cell system that is claimed to be totipotent. It is supposed to be competent for the renewal of all cell types, including germ cells, during postembryonic development and regeneration. A connection to stem cells in the embryo has been postulated repeatedly. This cell type is now most frequently termed “neoblast”. Light microscopy can reveal only a few neoblast characters, and ultrastructural studies have shown additional characters for discriminating possible types andlor stages. While some progress has been made in this respect for triclad turbellarians (freshwater planarians), rather little is known about the microturbellarians. We have investigated the fine structure of neoblasts of hatchlings and adults of Macrostomum hystricinum marinurn, a member of a primitive taxon in the “Turbellaria”-Rhabditophora (a paraphyletic group giving rise to the main parasitic flatworm taxa). In Macrostomum, one population of neoblasts is located in lateral bands along the main longitudinal nerve cords within the body cavity. Another population is found in the gastrodermis in a basi-epithelial position. Based on their cytoplasmic and nuclear organization, three stages in neoblast differentiation have been distinguished. The first and second stages are characterized by cytoplasm lacking organelles except free ribosomes and scattered mitochondria, a finding identical with the picture known from the “classical” planarian neoblast. In the first stage, heterochromatin is scattered over the nucleus in isolated clumps in a typical speckled (checkerboard) appearance; a nuclear lamina is weakly developed. In stage 2 the heterochromatin forms strands and clumps connected to each other. In stage 3 the nucleus is characterized by more prominent heterochromatin strands and by heterochromatin attachments to the well developed nuclear lamina. In this last stage a rough endoplasmic reticulum (rER) and Golgi complex are also present, indicating the entrance into cytoplasmic differentiation. Early epidermal replacement cells are located baso-epithelially, which show a nuclear organization similar to stage 3 neoblasts. Observations of stem cells in regenerating specimens and on isolated neoblasts are reported briefly. The data show that from the three types of differentiating cells distinguished recently in regenerative blastemas of planarians, the first stage (“undifferentiated cells”) resembles stage 2 neoblasp described here for postembryonic development. The results are compared with observations that have been published for neoblasts in other free-living platyhelminths.

A Molecular Test of Platyhelminth Phylogeny: Inferences from Partial 28S rDNA Sequences

Nucleotide sequences of the region extending from the D3 to the D6 expansion segments of the 28S rDNA gene were used to reconstruct evolutionary relationships within the Platyhelminthes. Neighbor-joining and parsimony analyses of representatives of most major platyhelminth taxa revealed a basal Catenulida, a monophyletic Acoelomorpha, a sister-group relationship of Macrostomorpha and Polycladida (59% of bootstrap replications), and monophyletic Tricladida. We found no evidence of a taxon Seriata (p=.0001); however, the paraphyletic status of the Proseriata needs further investigation. Although Neodermata appeared as a monophyletic group, Monogenea was paraphyletic (p<.0001), thus not supporting the taxon Cercomeromorpha. Monopisthocotylean Monogenea was the most basal taxon among the Neodermata, and Udonella sp. consistently grouped with the Monopisthocotylea. Other relationships within the Neodermata showed Trematoda as a monophyletic taxon and, among its members, Rugogaster hydrolagi was identified as an aspidogastrean. The taxa Cestodaria (Amphilinidae and Gyrocotylidae) and Eucestoda were both supported; however, constraining these 2 clades into a monophyletic Cestoda resulted in a significantly longer tree (p=.0303). We tentatively conclude that the immediate sister group of Neodermata is Fecampiida. But because of contradictory results of the constraint analyses, we cannot exclude the possibility that the sister group to Neodermata is a taxon containing many neoophoran turbellarians (e.g., Rhabdocoela and Tricladida). Additional key words: Acoelomorpha, Neodermata, Udonellida, molecular systematics Although the Platyhelminthes, and especially the order Acoela, hold a key position in several theories of metazoan origins (Hadzi 1963; Steinbock 1963; Salvini-Plawen 1978; Ehlers 1985; Ax 1987; Nielsen 1987), their within-group relationships are poorly understood. Attempts at classifying the platyhelminths have produced a wealth of ultrastructural information, but none of these studies have resulted in a definitive phylogenetic tree (Smith et al. 1986). However, two major points regarding platyhelminth systematics have emerged from these ultrastructural studies. First, Turbellaria may be a paraphyletic group, and the term should probably be used with caution (Ax 1987). Secondly, 3 clearly defined monophyletic lineages have been identified within the phylum (Fig. 1): the Catenulida; the Acoelomorpha, consisting of the orders Acoela and Nemertodermatida; and the Rhabditophora, which comprises the remaining orders and the parasitic Neodermata (Ehlers 1985, 1986). To determine a Author for correspondence. E-mail: mkll @cisunix.unh.edu relationships among these 3 groups, however, is difficult because of a lack of convincing synapomorphies. Proposed characters such as frontal organ/frontal glands or epidermal replacement are either not homologous across the taxa (Klauser et al. 1986; Smith & Tyler 1986) or have only an extremely low probability of homology (Smith et al. 1986), respectively. Therefore, monophyly for the platyhelminths, intuitive though it may be, will be difficult to test using traditional morphological characters. In the last decade, molecular phylogenies at any taxonomic level have gained great popularity. Nucleotide sequences provide an alternative data set for the resolution of evolutionary relationships in cases where morphologists may have reached an impasse regarding a specific phylogeny, or where the number of species and structural characters needed to reliably resolve a phylogeny may be prohibitively high. Molecular systematics has attempted to resolve just such issues by providing large and novel data sets (Turbeville et al. 1991; Turbeville et al. 1992). Some of the most popThis content downloaded from 207.46.13.164 on Sun, 26 Jun 2016 07:38:12 UTC All use subject to http://about.jstor.org/terms Molecular platyhelminth phylogeny MonoI,uI pI lal Monopisthocotylea genea Neo-(incl. Udonella sp.) dermat; L Digenea [-EAspidogastrea Trematoda (incl. R. hydrolagi) Dalyellioida (incl. Fecampiida) RhabdoKalyptorhynchia coela Tricladida Seriata Proseriata

The origins of parasitism in the Platyhelminthes: a summary interpreted on the basis of recent literature

A summary is given of the 4 contributions on the origins of parasitism in the Platyhelminthes. Recent ecological literature is used to interpret some of the findings. In particular, recent findings on rugged fitness landscapes, the increasing difficulty of long-jump adaptations, complexity catastrophes, and empty phenotypic space are discussed in order to find an explanation for the large number of Neodermata and the scarcity of parasitic turbellarians; it is concluded that evolutionary stasis of symbiotic turbellarians that are “trapped” in their particular niches is responsible for the small number of symbiotic species of turbellarians, rather than competitive exclusion by the numerous neodermatans. The earliest neodermatans were probably already dependent on the production of many offspring; hence the protoneodermatan was probably a species preadapted to parasitism by high fecundity. Monogenea produce few offspring in spite of their rich and secure food supply; the protomonogenean was either a species with a complex behaviour pattern for habitat selection or, alternatively, a species with high fecundity subsequently reduced to permit the evolution of more complex behaviour patterns (switch from r- to K-strategy). The finding that embryonic replacement of the epidermis is shared by several turbellarian groups, a developmental pattern possibly used and modified in the formation of the neodermis in the Neodermata, can be interpreted as supporting the views that Neodermata and turbellarians with epidermal replacement are all monophyletic, or alternatively, that a similar developmental pattern has arisen several times due to similar selection pressures. However, the possibility of horizontal character transfer should also be considered for explaining similar characters, including similar developmental patterns, in apparently not closely related groups. Horizontal gene transfer and principles for demonstrating horizontal character transfer are discussed.

Origin of the epidermis in parasitic platyhelminths

The epidermis of members of the major parasitic taxon Neodermata is distinctive among flatworms, being a syncytial, insunk, non-ciliated epidermis that develops through a wholesale replacement of larval epidermis at metamorphosis when the larva attacks a host. How it arose in evolution from what must have been a turbellarian-like ancestor is not immediately evident. While many turbellarian flatworms have also adopted a symbiotic way of life, the literature on ultrastructure of epdermis in these symbionts shows quite a variety of morphologies, many not so different from that of their free-living relatives. Various turbellarians do have syncytial or insunk epidermises or reduction of epidermal ciliation as is characteristic of the Neodermata, but co-occurrence in a single turbellarian of all features common to neodermatans has not been reported. Urastoma cyprinae, for example, which is ectosymbiotic on bivalves, has a ciliated cellular epidermis that is little different from what is known of epidermises of its free-living relatives. The endoparasitic Anoplodium hymanae, from the coelom of sea cucumbers, also bears a ciliated cellular epidermis, as is typical of many other rhabdocoels, but it shows marked phagocytic activity as well as incorporation of endosymbiotic bacteria. The closest similarity to neodermatan epidermis is that of the turbellarian Genostoma kozloffi, an ectosymbiont of the crustacean Nebalia: covering the bulk of the body is a non-ciliated syncytium with multiple branching connections to insunk nucleated portions, much as in epidermis of adult neodermatans and, on its ventral surface, is a field of ciliated cellular insunk epidermis resembling the epidermis of some larval neodermatans. Developmental clues to the origin of the neodermatan epidermis can be seen in turbellarian embryos. Before hatching, embryos of proseriate and triclad embryos go through 3 generations of epidermis, each replacing the next; 2 generations of epidermis are reported in the literature on rhabdocoel embryos. This process of replacement parallels the epidermal replacement that larval neodermatans undergo at metamorphosis. Ultrastructural study of developing acoel, polyclad and macrostomid embryos shows that they, too, have epidermal replacement and growth through immigration of deeper-lying cells, comparable to the processes seen in higher flatworms. Succession of distinct generations of epidermis in such animals as the proseriates, triclads and rhabdocoels is probably an adaptation to development of ectolecithal eggs, providing the means for the embryo to use yolk that resides in vitellocytes, outside its blastomeres. We propose that the Neodermata has taken advantage of this developmental mechanism, producing successive generations of epidermal cells even in its larval stages, to counter the defenses of hosts.

Observations on the development and clinical use of artificial skin--an attempt to employ regeneration rather than scar formation in wound healing.

Artificial skin, a bilaminar membrane, is grafted on an excised wound immediately following injury. This bilayer membrane, made of a dermal and epidermal portion, is populated in place on the wound bed by the patient's own fibroblasts and epidermal cells producing a permanent skin replacement with an anatomically functioning dermis and epidermis. The dermal portion is a porous collagen-chondroitin 6-sulfate fibrous matrix arranged in a three dimensional pattern closely resembling the fiber pattern of normal dermis. A thin silastic covering serves as a temporary epidermis immediately after grafting until the patient's epidermal cells, seeded on the "neodermis", grow into a confluent epidermal replacement. The autogenous "neodermis" is produced as fibroblasts and vessels migrate from the wound bed into the artificial dermal template and, using the artificial fibers as a scaffolding, synthesize new connective tissue in the collagen fiber pattern of normal dermis rather than the pattern of scar while slowly biodegrading the artificial fibers. This replacement dermis functions as normal dermis and not as scar tissue. The patient's epidermal cells seeded on the "neodermis" grow into a confluent normal appearing epidermis and with the neodermis produce a permanent skin composed of normal functioning dermal and epidermal components produced in situ by the patient's own cells. Artificial skin has been successfully used to permanently replace skin destroyed by burn injuries ranging from 10 to over 95% BSA. The long-term functional results in these patients have been excellent and the long term cosmetic results in preliminary studies tend to be superior to autograft. Artificial skin appears to provide a successful physiologic and cosmetic skin replacement in severe burn injury.