Reconstituted Living Skin Equivalent™ (LSE™) is made up of a dermal equivalent (DE) on which keratinocytes are plated where they give rise to a multilayered differentiated epidermis. The dermal equivalent develops through interactions between fibroblasts and collagen fibrils that begin to form after the cell-matrix precursor is cast. The gel that forms as a result of collagen polymerization and fluid trapping is contracted uniformly in all dimensions. By securing it at ends and edges in the mold in which it is cast, the final dimensions, strength and morphology of the forming tissue are altered. The same phenomena are seen in casting tubular tissues for the fabrication of small caliber blood vessel equivalents. The cells of the dermal equivalent are biosynthetically active and enrich the matrix to different degrees with secretory products, depending on how the cells are stimulated and on the presence or absence of an epidermis. Collagen biosynthesis by dermal cells in the DE is sensitive to growth factors, ascorbate concentrations and amino acid pools. Both ascorbate and TGFβ1 increase total collagen biosynthesis at least two-fold by one week after tissue formation. With TGFβ1 present, the capacity of cells in the DE to synthesize collagen increases with time, over a two-week period. If ascorbate (200 μg/ml) is added just after the tissue is cast and daily thereafter, contraction lattice is blocked, and collagen biosynthesis is enhanced relative to contracted controls that had received 200 μg/ml ascorbate once. The increase was nearly an order of magnitude over that of controls and was coordinate with a comparable increase in hyaluronate and sulfated glycosaminoglycan (GAG) production as shown by TCA-precipitable glucosamine in the intercellular matrix of the DE. Both the LSE and the Living Dermal Equivalent™ (LDE™) exhibit complex responses to UV radiation and to various chemicals that are greatly different from responses given by monolayered cells. In general, threshold doses are elevated by one or more orders of magnitude for the tissues as compared with cells in monolayer, with the LSE exhibiting higher thresholds than the DE. The immunogenicity of the human LSE has been tested in vitro. Its cells are shown to be unable to stimulate a response in a mixed lymphocyte reaction (MLR) even after Class II antigens are induced by exposure to cytokines. The basis for the immunologic neutrality of the LSE can be referred to the absence of immune system (IS) cells normally present in skin and to the specific antigenic profiles of nonimmune system (NIS) cells that must be different from those of IS cells and which, even after Class II induction, are not allostimulatory. The generality of immunologic neutrality is an essential consideration in the fabrication of tissue and organ equivalents for grafting. The idea that it can be made a graft property has been formalized in the Neutral Allograft Hypothesis.

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