Pattern Formation in Regenerating Tissues
To understand the concept of pattern formation and its role in regeneration, the basic differences between repair and regeneration needs to be explained. Repair is defined as tissue restoration of a damaged tissue, without organized patterning. For exampl
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1 Tissue Repair Versus Pattern Formation in Regenerating Tissues To understand the concept of pattern formation and its role in regeneration, the basic differences between repair and regeneration need to be explained. Repair is defined as tissue restoration of a damaged tissue, without organized patterning. For example, vertebrates including humans are capable of reconstituting a functional liver following removal of up to 70 % of the original liver mass [1, 2]. Thus, liver repair includes reconstitution of the same volume but it lacks reconstruction of the same tissue pattern. Similarly, wound healing of the skin includes tissue repair by formation of a scar tissue that lacks some of the characteristic features of the original tissue. These are both examples for tissue repair that lack true regeneration or patterning. When looking at regeneration of whole body parts in salamanders, the whole limb is restored into its original form. In this case, the re-establishment of patterns is necessary because otherwise regeneration is meaningless.
2 Models of Pattern Formation in Regenerative Tissues Traditionally regenerating tissues have been favorable models in the theoretical and experimental approaches to pattern formation. Issues of the exact restoration of patterns during regeneration have been explicitly discussed by researchers such as T. H. Morgan in the nineteenth and the beginning of the twentieth century [3]. However, it was not until the mid 1960s that formulation of ideas to explain mechanisms of pattern formation were presented. Lewis Wolpert established the A. Hoffmann • P.A. Tsonis (*) Department of Biology and Center for Tissue Regeneration and Engineering, University of Dayton, Dayton, OH 45469-2320, USA e-mail: [email protected] V. Capasso et al. (eds.), Pattern Formation in Morphogenesis, Springer Proceedings in Mathematics 15, DOI 10.1007/978-3-642-20164-6_2, # Springer-Verlag Berlin Heidelberg 2013
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A. Hoffmann and P.A. Tsonis Concentration of Morphogen
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Threshold
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Distance from Source
Fig. 1 The morphogen hypothesis (Adapted and modified from Jaeger et al. [43] according to Wolpert [4, 5]). Localized production of a morphogen by the blue source cells could be distributed in direction of the red cells by some form of active transport and other means according to a source and sink model. The concentration of the substance when produced at a fixed value would thus specify the cellular position when looking at the generated linear gradient. Cellular response is depending on a certain morphogen threshold that is defined by the distance of a cell towards the source
“Morphogen Hypothesis” by assuming production of different morphogens in a tissue system (Fig. 1) [4, 5]. He suggested that positional information along a certain axis can be provided by a gradient of a factor and that the cells will know their position because they will respond to a corresponding concentration of the morphogen. For instance, a suggested morphogen gradient that defines the proximal-distal axis in the regener
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