Wound tissue can utilize a polymeric template to synthesize a functional extension of skin

Reconstituted skin in culture: a simple method with optimal differentiation

Human skin is a unique organ, which can be reconstituted in vitro and represents an interesting system for studying cell proliferation and differentiation. A simple technique for producing reconstituted skin with optimal epidermal differentiation is described and characterized. A 4-mm punch biopsy of normal human skin is deposited on the epidermal side of mortified de-epidermized human dermis maintained at the air-liquid interface with a metallic support. The culture medium contains insulin, epidermal growth factor (EGF), cholera toxin, hydrocortisone, penicillin/streptomycin and fungizone. A well-differentiated epidermis develops within 15 days. Morphological and ultrastructural studies show a neoepidermis resembling normal skin. Differentiation markers such as involucrin, filaggrin, and various cytokeratins detected with pancytokeratin antibody are present and confirm this resemblance. The keratin profile is comparable to that observed in other skin culture models. A basement-membrane-like structure is reconstituted with hemidesmosomes and anchoring-filament formation. Bullous pemphigoid (BP) antigen is observed at the dermo-epidermal junction after 21 days of culture. Moreover, both dermal substrates and punch biopsies can be kept frozen for long-term storage, with little or no loss of epidermal growth kinetics and morphology. This skin culture technique is rapid, simple, economical and reproducible. Characterization has here shown high-quality epidermal differentiation. Scientists interested in epidermal in vitro studies should take interest in all these advantages.

Mechanical stress and cellular metabolism in living soft tissue composites

A soft tissue substitute composed of a three-dimensional collagen lattice infiltrated with living fibroblasts was modelled as a two-phase composite material. The phases consisted of an extracellular matrix containing fibres and fluid and cellular inclusions. A mathematical model that had been previously derived for calculating the elastic stress and energy distribution around spherical inclusions and voids in a three-dimensional elastic medium was applied to a biological composite containing cells in an elastic medium. The model predicted an increase in stress inside the cells in the direction of applied load and a decrease in the direction perpendicular to the applied load. The model also predicted increased interfacial stress at the cell-matrix boundary in the direction of applied load with maximum and minimum values at different points on the cell periphery. It was hypothesized that the interfacial stress around the cell inclusions might play a function in signal transduction. To test this hypothesis, an average unidirectional stress was applied to the boundary of a 1 cm X 2 cm X 3 mm lattice containing fibroblasts. When cells were cultured for 7 d and placed under stress of 15 g, cell growth was increased 1.7 fold, protein synthesis decreased 48% and intracellular cAMP was increased 3.7 fold in 24 h. The changes were a function of externally applied stress. Cells cultured for 14 d and placed under stress displayed similar results. The results supported the concept that soft tissue extracellular matrix depicted as an elastic medium can affect cell growth and development.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of glycosaminoglycans on the collagen sponge component of a bilayer artificial skin

A bilayer artificial skin composed of a silicone membrane and a collagen sponge layer containing glycosaminoglycans (GAGs) was first developed by Yannas and Burke. They reported that GAGs contained in the collagen sponge layer contributed to the function of the artificial skin. In an attempt to assess the effect of GAGs in the collagen sponge layer, the electron microscopic structure, mechanical strength of collagen sponges, and cell proliferation were examined in vitro, using four kinds of collagen sponges containing: no GAG, chondroitin 6-sulphate (C6S), dermatan sulphate (DER), and hyaluronic acid (HYA). The results indicated that: (1) addition of GAGs scarcely affected the mechanical structure of collagen sponges; (2) addition of C6S and DER reinforced mechanical strength, while addition of HYA did not; (3) addition of C6S and DER significantly decreased cell proliferation.

Experimental study of a newly developed bilayer artificial skin

A bilayer artificial skin composed of an outer layer of silicone polymer and an inner sponge layer of collagen containing chondroitin 6-sulphate was developed by modifying the technique proposed by Yannas et al. The artificial skin was placed on the skin defects on the backs of rats. Histological observation indicated that fibroblasts and capillaries infiltrated into the pores and filled in lattice spaces, resulting in synthesis of the connective tissue matrix and absorption of the original network of collagen and chondroitin 6-sulphate. Epidermal cells migrated from the edge of the wound between the two layers. Post-operative contracture in the wound with the artificial skin was significantly less than in the control.

Fabrication and reorganization of dermal equivalents suitable for skin grafting after major cutaneous injury

The incorporation of fibroblasts into a hydrated collagen lattice results in lattice contraction and collagen reorganization to form a dermal equivalent. Lattices fabricated with 7.7 mg collagen and seeded with 1 X 10(5) cells were found to give the best results in terms of their mechanical properties and ability to maintain cell viability. Newly-cast lattices were found to be completely digested by 0.085 units/ml bacterial collagenase in 3 h, whereas after 30 d in culture, limited digestion took place over 24 h. Electrophoretic analysis showed that the proportion of cross-linked collagen in the 30 d lattice was increased by 2.5-fold compared to the initial collagen preparation. These results indicate that a dermal equivalent better suited for grafting may be produced after 20-30 d in culture.

Clinical evaluation of a new bilayer "artificial skin" composed of collagen sponge and silicone layer

A bilayer "artificial skin" composed of an outer layer of silicone and an inner sponge layer of collagen and chondroitin sulphate has been developed by modifying the technique proposed by Yannas et al. (1980). Following experimental successes, the "artificial skin" was applied clinically. It was placed on the skin defects of 10 patients. Three weeks after application the outer layer of silicone sheet was peeled off and thin split thickness skin was grafted onto the newly synthesised dermis-like tissue. Secondary skin grafts took perfectly in all cases and postoperative appearance was satisfactory.

Effect of exogenous collagen I on tissue repair in the rabbit ear and regeneration of the rat liver

Collagen type I from rat tail was tested for its effect on wound healing of 6 male rabbits and on the regeneration of liver after partial hepatectomy in 15 male rats. It had a negligible effect on the rate of wound healing and a statistically significant acceleratory effect on regeneration of the rat liver.

Collagen banded fibril structure and the collagen-platelet reaction

Bovine hide collagen dispersions were swollen in the pH range 1.6-7.0, treated with glutaraldehyde, and dialyzed to neutral pH. The intensity with which these collagens reacted with human platelets in plasma was studied by aggregometry and scanning electron microscopy. Collagen swollen at a pH below 4.25 +/- 0.30 and treated with glutaraldehyde exhibited greatly reduced platelet aggregating ability after restoration of neutral pH. In addition, the state of supramolecular order in these collagens was investigated by transmission electron microscopy and infrared spectroscopy. Native, insoluble collagen fibrils were found to lose their banded structure, as observed by transmission electron microscopy, reversibly when exposed to low ionic strength aqueous solutions below pH 4.25 +/- 0.30. During the disorder transition, which occurred by time dependent swelling of fibrils, but without their disaggregation, the packing order in the fibrils was largely abolished while the triple helical structure of individual collagen molecules was retained. Chemical modification of collagen by glutaraldehyde treatment was found to prevent recrystallization of collagen during dialysis to neutral pH but did not otherwise affect the collagen-platelet reaction. The results of altering collagen mass dose (concentration) demonstrated the critical importance of traces of banded fibrils which resisted disordering below pH 4.25. The data suggest that collagen preparations which are free of significant traces of banded fibrils, but which are made up of collagen molecules possessing triple helical structure do not induce platelet aggregation, irrespective of dose.

Review of keratinocyte culture techniques: problems of growing skin

Keratinocyte cell culture has been studied by a large number of investigators, resulting in an extensive body of often contradictory literature. The advantages and disadvantages of the two major techniques, explant and disaggregated culture, will be examined, followed by a critical evaluation of the major technical variables. Suggested standardized methods for both explant and disaggregated culture, and the indications for the use of each, will be proposed. Finally, further directions for basic research will be suggested in order to improve this useful tool.

Attachment of peptide growth factors to implantable collagen

Ingrowth of fibrovascular tissue from the woundbed into collagen-based dermal substitutes and survival of cultured epithelium after transplantation may be enhanced by attachment of heparin binding growth factor 2 (HBGF2) and epidermal growth factor (EGF) to collagen. Biotinylation of collagen and the growth factors allows immobilization of HBGF2 and EGF by high affinity binding of tetravalent avidin. Biotinylated HBGF2 and EGF (B-GF) were exposed to complexes of biotinylated collagen (B-COL)-avidin (A) and detected with peroxidase-labeled avidin (AP) followed by chromagen formation on nitrocellulose paper. Binding of biotinylated HBGF2 and EGF was specific (*, P less than 0.05), proportional to the concentration of biotinylated collagen, and resistant to ionic (NaCl) displacement. Data are expressed as mean percentages of maximum binding +/- SEMs: (table; see text) Growth response of cultured human epidermal keratinocytes to HBGF2 (population doubling time, PDT = 0.70 population doublings (PD)/day) confirmed the retention of mitogenic activity after biotinylation (PDT = 0.80 PD/day). Specific binding of biotinylated HBGF2, EGF, or other biologically active molecules (antibiotics, NSAIDs) to implantable collagen may provide a mechanism for positive therapeutic modulation of wound healing, including repair of full-thickness skin wounds with cultured cell-collagen composite grafts.

Synthesis and characterization of a model extracellular matrix that induces partial regeneration of adult mammalian skin

Regeneration of the dermis does not occur spontaneously in the adult mammal. The epidermis is regenerated spontaneously provided there is a dermal substrate over which it can migrate. Certain highly porous, crosslinked collagen-glycosaminoglycan copolymers have induced partial morphogenesis of skin when seeded with dermal and epidermal cells and then grafted on standard, full-thickness skin wounds in the adult guinea pig. A mature epidermis and a nearly physiological dermis, which lacked hair follicles but was demonstrably different from scar, were regenerated over areas as large as 16 cm2. These chemical analogs of extracellular matrices were morphogenetically active provided that the average pore diameter ranged between 20 and 125 microns, the resistance to degradation by collagenase exceeded a critical limit, and the density of autologous dermal and epidermal cells inoculated therein was greater than 5 x 10(4) cells per cm2 of wound area. Unseeded copolymers with physical structures that were within these limits delayed the onset of wound contraction by about 10 days but did not eventually prevent it. Seeded copolymers not only delayed contraction but eventually arrested and reversed it while new skin was being regenerated. The data identify a model extracellular matrix that acts as if it were an insoluble growth factor with narrowly specified physiochemical structure, functioning as a transient basal lamina during morphogenesis of skin.

Material properties of living soft tissue composites

Collagen sponges seeded with fibroblasts have been used as a soft tissue substitute in wound healing applications. This biomaterial is a good in vitro analog of a connective tissue. Therefore, analysis of the properties of this material may be useful for theoretically modeling soft tissues. Stress-strain curves for such cell-seeded collagen sponges were measured to determine composite stiffness and ultimate tensile strength. Theoretical modeling was done by defining a particle-reinforced matrix using the composite sphere model. A system of uniaxially oriented fibers was then introduced to this equivalent homogeneous media and material properties were determined using the composite cylinder model. Geometric averaging was performed to yield the stiffness and Poissons' ratio for a composite with randomly oriented fibers. Inputs to the model were constituent material properties, cell volume fraction, and fiber volume fraction. From theoretical results, material properties of soft tissues and their substitutes depend on fiber mechanical properties and volume fraction and not cellular mechanical properties and volume fraction. Therefore, the increase in experimentally observed composite stiffness with increased cell number was due to deposition of newly synthesized stiffer collagen fibers, and not due to the physical presence of cells themselves.

Biomaterials and biomedical devices

This review discusses the factors important in the incorporation or integration of biomaterials and devices by tissue. Methods for surface modification and surface-sensitive techniques for analysis are cited. In vitro methods to evaluate the biocompatibility or efficacy of certain biomaterials and devices are presented. Present and future directions in neural prostheses, cardiovascular materials, blood or bone substitutes, controlled drug delivery, orthopedic prostheses, dental materials, artificial organs, plasma- and cytapheresis, and dialysis are discussed.

Histological study of the hydroxyapatite-collagen complex implants in periodontal osseous defects in dogs

This study was performed to determine whether the process of wound healing, following periodontal surgery, could be improved through the combined use of collagen and grafting of hydroxyapatite (HAP) particles. Twenty-four proximal defects were made in the mandibular fourth premolars and second molars of six adult mongrel dogs. Steel wires and resin were put into the defect to enhance plaque formation. At eight weeks, the wires and resin were removed. At ten weeks, HAP or HAP-collagen complex was implanted during reconstructive surgery, along the root surface treated with acid conditioning. Dogs that received no implant following a flap operation served as controls. In these three groups of animals, differences in the extent and features of healing were histopathologically examined two months later. Animals implanted with a HAP-collagen complex showed a larger amount of new cementum formation when compared with HAP-implanted or control animals. In addition, in animals from the HAP-complex group, the interdigitation between the root surface and the gingival connective tissue fibers tended to be reinforced resulting in suppressed epithelial downgrowth. However, neither bone formation nor the reformation of the periodontium was promoted in the HAP-collagen complex group. These results suggest that implantation of an HAP-collagen complex promotes cemento-genesis of the demineralized root surface and can establish a stronger interdigitation between the root surface and the gingival connective tissue fibers.

Structure of a collagen-GAG dermal skin substitute optimized for cultured human epidermal keratinocytes

Collagen and glycosaminoglycan (GAG) dermal skin substitutes (membranes) were studied as substrates for cultured human epidermal keratinocytes. Structure of dermal substitutes was optimized for pore size to promote ingrowth of fibrovascular tissue from the wound bed and for culture of human keratinocytes of the membrane's surface. Pore size of the freeze-dried material was regulated by control of the temperature of freezing between -50 degrees C and -20 degrees C and by concentration of starting materials between 0.17% and 1.62% wt/vol. A nonporous surface of collagen-GAG was laminated to the membranes to provide a planar substrate for cultured epidermal keratinocytes. Thickness of dermal substitutes was regulated by control of the volume and concentration of starting materials. Biotin was conjugated to solubilized collagen for binding with avidin of specific quantities of biologically active molecules. The optimized membranes are suitable substrates for the culture of human epidermal keratinocytes, and together with the cells yield a composite material that is histologically similar to skin.

Effects of heparin on vascularization of artificial skin grafts in rats

Artificial skin is a recent development in the clinical care of the severely burned patient. Its manufacture entails the covalent bonding of collagen and polysaccharide, followed by the coating of one surface with a thin layer of silicone rubber. Artificial skin was grafted onto rats and examined for neovascularization at 7 days. Vascular patency was shown by perfused yellow latex casts. Five percent of the patent vessels grew into the graft soaked in physiological buffered saline (PBS). When the graft was soaked in heparin, 1 mg/ml buffered saline solution, before grafting, 54% of the patent vessels in the grafted area had grown into the matrix. These experiments show that the local application of heparin promotes early ingrowth of blood vessels into the healing site. The vascularity of artificial skin can be modified by heparin, which promotes angiogenesis, and leads to earlier deposits of greater amounts of new connective tissue.

Porous collagen sponge wound dressings: in vivo and in vitro studies

Collagen-based materials can be formed into a three-dimensional sponge for use as a wound dressing and as a support for cell cultured skin components. Factors such as biocompatibility, morphological structure and addition of non-collagenous molecules to collagen are analyzed and discussed. Large pores or channels, interchannel communications and combinations of macromolecules of the connective tissue enhance wound tissue infiltration in vivo as well as cell growth in vitro into collagen sponges. The presence of such factors can be useful in patients with excised burn wounds and pressure skin ulcers.

The effects of heparin on the physicochemical properties of reconstituted collagen

Pepsin-solubilized bovine dermal collagen was reconstituted in 0.02 M sodium phosphate (pH 7.2), concentrated to 30-40 mg/ml, and adjusted to physiological ionic strength by addition of sodium chloride. These preparations, at 4-15 degrees C, are fibrillar suspensions composed of fibrils of varying diameters and nonassociated molecules. Addition of heparin to these suspensions promoted a dose-dependent increase in average fibril diameter as measured by turbidimetry and electron microscopic analyses. These effects were relatively specific for heparin and heparin-like glycosaminoglycans. Chondroitin sulfate and hyaluronic acid had little or no effect on fibrillar diameters under these conditions, whereas dermatan sulfate had an intermediate effect on fibrillar reorganization. Differential scanning calorimetry revealed that addition of optimal concentrations of heparin generated fibrils of higher stability and that this effect was associated with the disappearance of structures of lower stability, including nonassociated molecules and thin fibrils. Light microscopic analyses of the fibrillar collagen/heparin matrix showed it to be a more open network of distinct collagen fibers than was observed with the fibrillar collagen preparation alone. Binding experiments indicated that heparin bound to fibrillar collagen in a saturable fashion with a Kd of approximately 4 X 10(-7) M. Creep experiments provided evidence that the addition of heparin to fibrillar collagen suspensions greatly reduces the gelation phenomenon that is normally observed when such suspensions are warmed to 37 degrees C. These differences in fibrillar architecture may be in part responsible for differences noted in the biological response to fibrillar collagen and fibrillar collagen/heparin implants in vivo (McPherson et al., 1988).

Diffusivity of 125I-labelled macromolecules through collagen: mechanism of diffusion and effect of adsorption

Diffusion of angiotensin II, albumin and aldolase was studied through collagen membranes with swelling ratios between 4 and 15. The diffusion coefficient was measured from the time-lag for the onset of steady-state flux through the membrane. Binding of macromolecules to collagen was evaluated from the results of sorption studies conducted as a function of macromolecular concentration. Results presented indicate that the diffusion of macromolecules through collagen membrane is slowed by electrostatic and hydrogen bonding between individual macromolecular chains and collagen. The extent of adsorption is increased as the molecular weight of the diffusant increases. Diffusion of water soluble macromolecules through collagen occurs rapidly, suggesting that diffusion occurs through water filled channels as opposed to between collagen molecules. The results of these studies are useful in understanding diffusion through connective tissues and in the design of drug delivery systems based on collagen.

The vascularization of artificial skin grafts in rats: its modification by protamine

Artificial skin is a recent development in the clinical care of the severely burned patient. Its manufacture involves the covalent bonding of collagen and polysaccharide, followed by the coating of one surface with a thin layer of silicone rubber. Neovascularization and its modification in artificial skin were studied. Experimental artificial skin was grafted onto rats and examined for vascular growth in the graft at 7 days. This was revealed by latex-perfused vascular casts which were processed for histological study. An area including the graft bed and graft matrix was viewed and examined for latex-filled vessels. Thirty-seven percent of the total vessels, identified by residual latex, had grown into the graft. When artificial skin was treated with protamine at 10 mg/ml buffered saline solution before grafting, only 6% of the total perfused blood vessels were found in the graft matrix. The remainder was found in the graft bed. Moreover, increases in the numbers of perfused blood vessels and vessel diameters were observed in the graft bed at the interface below the graft pretreated with protamine. Protamine inhibited vessel growth into the matrix, but promoted an increased number of dilated blood vessels in the surrounding graft bed. These dilated vessels were related to an altered vessel architecture.

Immobilization of factor XIII on collagen membranes

Factor XIII from placenta was successfully grafted onto collagen membranes by the acyl-azide procedure. The transamidase activity retained on collagen membranes was determined by measuring the increase of fluorescence resulting from dansylcadaverine incorporation into casein. We studied the effect of different factors on the grafting: concentration and composition of the factor XIII preparation in the coupling solution and influence of the preactivation of factor XIII by thrombin. Stability studies have shown that the activity of factor XIII grafted on collagen membranes was almost constant over a period of 8 months. Sterilization by gamma-irradiation of factor XIII revealed a drastic loss of activity while the use of high-energized electron bombardment caused a reduced loss of activity. The potential of such a material for biomedical use is presently under investigation.

A model for the role of hyaluronic acid and fibrin in the early events during the inflammatory response and wound healing

A model is presented outlining the molecular and cellular events that occur during the early stages of the wound healing process. The underlying theme is that there is a specific binding interaction between fibrin, the major clot protein, and hyaluronic acid (HA), a constituent of the wound extracellular matrix. This binding interaction, which could also be stabilized by other cross-linking components, provides the driving force to organize a three-dimensional HA matrix attached to and interdigitated with the initial fibrin matrix. The HA-fibrin matrix plays a major role in the subsequent tissue reconstruction processes. We suggest that HA and fibrin have both structural and regulatory functions at different times during the wound healing process. The concentration of HA in blood and in the initial clot is very low. This is consistent with the proposed interaction between HA and fibrin(ogen), which could interfere with either fibrinogen activation or fibrin assembly and cross-linking. We propose that an activator (e.g. derived from a plasma precursor, platelets or surrounding cells) is produced during the clotting reaction and then stimulates one or more blood cell types to synthesize and secrete HA into the fibrin matrix of the clot. We predict that HA controls the stability of the matrix by regulating the degradation of fibrin. The new HA-fibrin matrix increases or stabilizes the volume and porosity of the clot and then serves as a physical support, a scaffold through which cells trapped in the clot or cells infiltrating from the peripheral edge of the wound can migrate. The HA-fibrin matrix also actively stimulates or induces cell motility and activates and regulates many functions of blood cells, which are involved in the inflammatory response, including phagocytosis and chemotaxis. The secondary HA-fibrin matrix itself is then modified as cells continue to migrate into the wound, secreting hyaluronidase and plasminogen activator to degrade the HA and fibrin. At the same time these cells secrete collagen and glycosaminoglycans to make a more differentiated matrix. The degradation products derived from both fibrin and HA are, in turn, important regulatory molecules which control cellular functions involved in the inflammatory response and new blood vessel formation in the healing wound. The proposed model generates a number of testable experimental predictions.

Collagen-based wound dressing: effects of hyaluronic acid and fibronectin on wound healing

Our previous studies have shown that a collagen-based wound dressing induces the spatial deposition of wound tissue. This study was conducted to determine the effects of hyaluronic acid and fibronectin on wound healing. These macromolecules play an important role in wound healing, embryonic development and cellular migration in vitro. The effects of the addition of varying levels of fibronectin and hyaluronate to a collagen sponge were studied. Low levels of both hyaluronate and fibronectin modified the structure of the implant, and resulted in increased chemoattraction, replication and collagen deposition in an in vivo wound healing model.

Fibroblasts in isogeneic skin equivalents persist for long periods after grafting

We have fabricated skin equivalents by combining fibroblasts from female Fischer rats with collagen to form a lattice and overlaying the lattice with a suspension of epidermal cells. The epidermal cells attach and form a sheet which differentiates. These skin equivalents were then grafted to male Fischer rats in order to follow the fate of the fibroblasts after implantation. Biopsies of the skin equivalent were taken between 9 days and 13 months after grafting and examined histologically or placed in tissue culture to permit karyotyping of the resident fibroblasts. Approximately 82% of the fibroblasts from the graft biopsied at 9 days were female, with this proportion decreasing sharply to 50% at 2 weeks and 60-64% at 1 month. At 1 month, this initial sharp drop is followed by a slow, linear decline which continues through the 13th month when 42% of the fibroblasts are female. We conclude that fibroblasts of the grafted skin equivalent become permanent residents of the skin of the host rat.