The University of Michigan School of Dentistry
Dr. Kohn's research program focuses on two aspects of biomineralization. First, they seek to understand how the organizational hierarchy of mineralized tissues results in mechanical competence (or, conversely, increased susceptibility to damage/fracture). Second, they seek to mimic aspects of nature's biomineralization strategies as a design basis for developing well-controlled systems that may be used to study basic questions/hypotheses regarding cell and molecular function.
Regarding the first aspect of their program, they seek to establish structure-function relations in mineralized tissues and study functional adaptation of these tissues in response to perturbations in the local microenvironment (e.g. alterations in mechanical loading, interactions with biomaterials, disease, aging). There are two guiding principles to our analysis of structure-function relations: it is highly integrated - they develop correlations between mechanical properties, crystal structure/orientation, protein orientation, chemical composition and gene expression; and seek to establish these correlations at different levels of organizational hierarchy - from the whole bone-level down through the tissue and ultrastructural-levels to the protein and gene-levels.
Regarding the second aspect of their program, biomimetic strategies are used to develop model systems in which biological output (in-vitro and in-vivo) can be quantitatively related to a well-controlled engineering input. A more long-term, applied and clinically motivated goal is to ultimately utilize this information to develop approaches to replace and/or regenerate tissues. In an effort to create biomaterials that modulate biological response in a controlled manner, they synthesize, characterize and evaluate more biologically based biomaterials. Their biomimetic approach involves exploiting aspects of 3 strategies used by nature - organic template-mediated self-assembly and mineralization, functional gradients, and environmental responsiveness.
Reconstruction of orofacial defects represents a major clinical challenge. Existing treatments have limitations and lack predictability, necessitating development of new strategies to heal orofacial tissues. One new approach is to regenerate the issues(s) of interest by seeding autogenous stem cells stem cells into a biomaterial which supports and may even provide cues for the cells, allowing them to grow, differentiate and secrete new extracellular matrix. Towards this end, we have developed in-vitro culture methods in which human bone morrow stromal cells (BMSC) are expanded, and demonstrated that these cells are capable of forming new bone in-vivo. The formation of bone from progenitor cells is, however, variable, especially if human cells are used, and is controlled by a number of factors in cells microenvironment, including the supporting biomaterial. We therefore seek to establish material parameters that could enhance bone cell function in-vitro and in-vivo. The global hypothesis is that bone-like mineral/organic hybrids that are co-precipitated onto scaffolds used for BMSC transplantation can be used by the cells to restructure a mineralized extracellular matrix of increased volume and structural integrity, thereby enhancing bone formation by transplanted cells. Results from out and other laboratories support this hypothesis, which is tested by synthesizing 3 classes of organic/inorganic hybrid materials: organic templates whose surface self-assembles into a biological apatite via the co-precipitation of a mineral/poly amino acid hybrid layer; organic templates whose surface includes cell adhesion molecules co-adsorbed with mineral phase; incorporation of growth factors into the self-assembled mineral layer such that biomineral serves as a controlled delivery vehicle. For phases into scaffold pore surfaces leads to increased osteoblast invasion and osteoconductivity and that by the type and concentration of the organic phase within the mineral. The results of these studies may lead to biomaterials that modulate bone formation my progenitor cells. This approach had implications for cell transplantations, but may also have impact on the differentiation of host cells if used as an inductive approach to tissue engineering.
Skeletal fractures represent a significant medical and economic burden for our society. and are particularly pertinent for military populations. As retirees, particularly women, age, the likelihood of fractures increases. Moreover fractures are not limited to the elderly - a significant percentage of military recruits, particularly females, exhibit stress fractures. Bone ma-~s alone is not a significant predictor of skeletal fragility. Therefore, it is important to also delineate changes in tissue quality and how these changes in tissue ultra structure affect the mcchanistic response of bone to its physical environment. In support of this clinically and militarily important area of investigation, the overall goals of this project are to:
Reconstruction of skeletal defects represents a major clinical challenge with over 1 million surgical procedures performed each year. New strategies of regenerating bone are needed because of limitations with existing techniques. One new strategy is to create a composite graft in which autogenous cells are seeded onto a porous, degradable scaffold. The scaffold supports the cells, structurally and biologically, allowing them to grow and secrete new extracellular matrix. Optimally, tissue growth occurs concurrent with scaffold degradation. The degree of new bone formation is, however, material dependent and not predictable. We therefore seek to establish material chemistry parameters that could optimize bone cell function. In pursuit of this goal, we have developed: (I) in-vitro culture methods in which human bone marrow stromal cells (BMSCs) are expanded; (2) polymer processing techniques to reproducibly fabricate highly porous 3D poly(lactic-co-glycolic) scaffolds, which have been successfully used to engineer a number of tissues, including bone; (3) materials science design strategies which enable us to biomimetically modify both the internal microenvironment of a scaffold and the scaffold surface; and (4) a critical size cranial defect model in an immunocompromised mouse which has shown that the human BMSCs are capable of forming new bone in an animal model. The global hypothesis of the proposed research is that the extracellular microenvironment provided by the scaffold modulates the ability of human BMSCs to differentiate toward an osteoblast phenotype, and therefore controls biomineralization and structural integrity of regenerate( bone. Results from our and other laboratories support this hypothesis, which is tested by synthesizing a series of model biomimetic materials. First, we synthesize environmentally responsive or "smart" scaffolds that buffer the microenvironment upon scaffold degradation. Second, we synthesize scaffolds with a surface that self-mineralizes into a biological apatite. Third, we use functionally-graded scaffolds in which mineralization is spatially controlled. The rationale for each of these 3 biomimetic strategies lies in the way nature has designed the skeleton. The skeletal system is able to perform its functions using a minimum amount of mass because biology has utilize, design approaches which include the ability to adapt to environmental cues (i.e. "smartness"), a hierarchical. Organization consisting of elegant mineral synthesis, and an organization that is optimized for physiological. Function by having gradients in composition and structure. In the proposed studies, we aim to exploit aspects of each of these 3 biomimetic strategies in an effort to create biomaterials that will modulate biological response in a controlled manner.
Dr. Kohn has two offices at the University of Michigan. One of his offices is located in 2213 School of Dentistry, 1011 North University Avenue, Ann Arbor, MI 48109-1078. Dr. Kohn has a second office in the University of Michigan's Department of Biomedical Engineering which is located at 2107 Carl A. Gerstacker Bldg., 2200 Bonisteel Blvd., Ann Arbor, MI 48109-2099. He can be reached by telephone at (734) 764-2206 or by FAX at (734) 647-2110 (School of Dentistry) and at (734) 764-9588 or by FAX at (734) 936-1905 (Department of Biomedical Engineering).