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Characterization and Modeling of Scaffold and Growth Factors Induced Regeneration in Critical Size Limb Bone Segmental Defects

Iwona Jasiuk Department of Mechanical Science and Engineering and Institute for Genomic Biology

Authors

Iwona Jasiuk

Abstract

We focus on a medically important problem of large bone wound healing by regeneration. More specifically, we developed a small animal in vivo load bearing model to study the effect of an artificial biocompatible polymer scaffold on regeneration of long bone segmental defects of critical size in adult Xenopus laevis frog hind limbs. One bone of the dual skeletal element tarsus bone was extirpated, and a scaffold of comparable size was implanted in situ. The skin adjacent to the intramuscular incision and scaffold was sutured and the limb was permitted to heal. The scaffolds were made of 1,6 hexanediol diacrylate (HDDA) using an innovative 3D microfabrication technology, Projection Micro Sterelithography (PµSL). The process is able to precisely and locally introduce the porosity into the scaffolds. The limbs were harvested at several time points and processed for histological observation. Macrophages and lymphocytes invaded the scaffolds and no cartilage or bone formation was observed. The area previously occupied by the extirpated tarsus was filled with connective scar tissue. In contrast, a second set of scaffolds was treated with VEGF (Vascular Endothelial Growth Factor) and BMP4 (Bone Morphogenetic Protein 4) prior to implanting into the extirpated tarsus bone site. The area previously occupied by scaffold was completely filled with cartilage. Additionally, blood vessels had invaded the cartilage and bone was forming adjacent to the blood vessels. These experimental findings serve as inputs for modeling of cellular processes. In particular we simulate cellular activity at the onset of regeneration using the CompuCell3D (CC3D) software. In our simulations, the Generalized Cells include mesenchymal stem cells (MSCs), osteoblasts, chondrocytes, and other cell types. The Field can be defined as the defect space into which chemical diffusants (growth factors) are being released in order to provide for a chemotactic gradient through which cells can migrate. We set the problem geometry by representing the bone region by frozen quiescent cells which are lined at the two ends with MSCs. This is the initial condition for the model. As time evolves, the MSCs divide and differentiate into chondrocytes. These chondrocytes then bridge the critical size defect over time and ossify. We are also interested in the role that scaffold plays in the early stage of regeneration. In our CompuCell 3D simulations we also account for the presence of scaffold with a given structure and material characteristics within the gap region and study the response of cells to such a scaffold in a given biological environment.