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No description available.

No description available.

No description available.

No description available.

In the United States, nearly half a million bone graft operations are performed annually to repair defects arising from birth defects, trauma, and disease, making bone the second most transplanted tissue. Autogenous bone is the current gold standard for bone grafts; however it is in limited supply and results in a second injury at the donor site. A promising alternative is a tissue engineered bone graft composed of a biomaterial scaffold, pharmaceutics, and osteoprogenitor cells. One source of osteoprogenitor cells is bone marrow stroma, which can be obtained from the patient - minimizing the risk of an immune response - directed in vitro to proliferate, and differentiate into a bone-like tissue. To date, tissue engineered bone grafts have not been clinically effective; thus, strategies must be developed to improve efficacy. I hypothesize that to facilitate tissue healing in a manner similar to autogenous bone tissue engineering bone must possess a mineralized collagen matrix to support tissue integration, and angiogenic factors to stimulate vascular infiltration, and osteogenic factors to direct normal bone remodeling. I propose that these factors can be synthesized by osteoprogenitor cells in vitro when cultured under the appropriate conditions. Previous work has demonstrated that perfusion culture of osteoprogenitor cells within 3D scaffolds stimulates phenotypic markers of osteoblastic differentiation, but those studies did not determine whether the effects were a consequence of shear stress or increased nutrient availability. Consequently, this work has involved studies in a planar geometry, where nutrient effects are negligible. Three studies that characterize the effect of fluid flow on osteoblastic differentiation of osteoprogenitor cells are presented here. The objective of the first study was to determine the effect of shear stress magnitude on cell density and osteocalcin deposition. In this study, radial flow chambers were used to generate a spatially dependent range of shear stresses (0.36 to 2.7 dynes/cm2) across single substrates, and immunofluorescent techniques were used to assay cell phenotype as a function of shear stress. The objective of the second study was to determine the effect of the duration of fluid flow on cell density and phenotypic markers of differentiation. Here, parallel plate flow chambers were used to generate a single shear stress at the cell surface, and entire cell layers were assayed for expression of osteoblastic genes. The objective of the third study was to compare continuous and intermittent fluid flow strategies. In this study, a microprocessor-controlled actuator was added to the flow loop to periodically halt flow, and markers of mechanosensation and osteoblastic differentiation were measured. These studies demonstrated that shear stresses of 0.36 to 2.7 dynes/cm2 stimulate late phenotypic markers of osteoblastic differentiation but not cell proliferation. In addition, this osteogenic effect is sensitive to duration of fluid flow but insensitive to the magnitude of shear stress. Further, intermittent fluid flow enhances cell retention, biochemical markers of mechanotransduction, and synthesis of the angiogenic factor vascular endothelial growth factor (VEGF). Thus, these studies suggest that intermittent fluid flow may be an attractive component of a biodynamic bioreactor for in vitro manufacture of clinically effective tissue engineered bone grafts. Future studies will further investigate intermittent fluid flow strategies and three-dimensional studies with scaffolds suitable for bone tissue engineering. / Ph. D.

Bone is the second most transplanted tissue after blood and the need for bone graft materials continues to rise at an average annual growth rate of over 18%. An engineered bone substitute consisting of a bone-like extracellular matrix deposited on the internal pores of a resorbable biomaterial scaffold is postulated to stimulate normal bone remodeling when implanted in vivo. Part one of this engineering strategy, the deposition of bone-like extracellular matrix, can be achieved by the directed differentiation of progenitor cells such as bone marrow stromal cells (BMSCs). Part two of the engineering strategy, the biomaterial scaffold, can be fabricated with the appropriate mechanical properties using a synthetic polymer system with tunable properties like polyurethanes. Finally, BMSCs seeded within the biomaterial scaffold can be cultured in a perfusion flow bioreactor to stimulate osteoblastic differentiation and the deposition of bioactive factors. Using the three-part engineering strategy described, I hypothesize that the extracellular matrix produced by BMSCs can be modulated by two stimuli: the stiffness of the scaffold and perfusion flow. First, I propose that culturing BMSCs on polyurethane scaffolds with increasing stiffness will increase markers of osteoblastic differentiation. Secondly, I suggest that mechanically stimulating BMSCs with novel perfusion strategies will also increase markers of osteoblastic differentiation. In aim 1, a family of segmented degradable poly(esterurethane urea)s (PEUURs) were synthesized. The modulus of the PEUUR materials was systematically increased from 0.18 to 0.80 MPa by systematically increasing the molecular weight of the poly(ε-caprolactone) (PCL) soft segment from 1425 to 2700 Da. BMSCs were cultured on both rigid polymer films and on porous foam scaffolds to dissociate the effect of variation in polymer chemistry from the effect of scaffold modulus on cell phenotype. These studies demonstrated changes in osteoblastic differentiation as measured by prostaglandin E2 production, alkaline phosphatase activity (ALP) activity, and osteopontin gene expression. However, the increased levels of these phenotypic markers on the PCL 2700 material could not be attributed to scaffold chemistry or modulus. Instead, the differences may be related to polymer crystallinity or surface topography. In aim 2, novel dynamic perfusion strategies were used to investigate the influence of frequency on osteoblastic differentiation. BMSCs were seeded on porous foam scaffolds and exposed to both steady perfusion and pulsatile perfusion at 0.017, 0.050, and 0.083 Hz frequencies. The data presented here demonstrated that while some markers of osteoblastic phenotype such as ALP activity are enhanced by 0.05 Hz pulsatile flow over continuous flow, they are insensitive to frequency at low frequencies. Therefore, future studies will continue to investigate the effect of a larger range of frequencies. Additionally, fluid flow has also been shown to stimulate the deposition of bioactive factors such as BMP-2 and VEGF-A, and these growth factors are known to significantly enhance healing in bone defect models. Therefore, we plan to investigate the effect of dynamic flow strategies on the deposition of these bioactive factors. We propose that an engineered bone graft material containing a bone-like extracellular matrix and producing these growth factors will show more rapid formation of bone when implanted in vivo. / Ph. D.

Tendon injuries are a significant cause of morbidity in performance horses with superficial digital flexor tendon injury reported to represent up to 43% of overall Thoroughbred racehorse injuries. Natural repair is slow and results in inferior structural organization and biomechanical properties and, therefore, reinjury is common. The inability of tendon to regenerate after injury, or to heal with mechanical properties comparable to the original tissue, is likely attributable to low vascularity and cellularity of the tissue, low number of resident progenitor cells, and healing under weight-bearing conditions. Strategies to improve tendon healing have focused on enhancing the metabolic response of tenocytes, modulating the organization of the newly synthesized extracellular matrix, or administering progenitor cells to enhance repair. Significant research effort has been directed at the use of adult mesenchymal stem cells as a source of progenitor cells for equine tendon repair and recent clinical applications have utilized adult autologous stem cells derived either from adipose tissue or bone marrow aspirates. Isolation of a homogenous population of stem cells from bone marrow is time-consuming, and there is much variation in cell numbers, cell viability and growth rates among samples. Recently, a population of progenitor cells has been isolated from equine flexor tendons, thus providing an alternative source of progenitor cells from the target tissue for therapeutic intervention. The interaction between cells and the extracellular matrix (ECM) is an important factor in regulation of cell function. Proliferation, migration, differentiation and gene expression of many cell types are altered by adhesion to and interaction with matrix proteins and the extracellular environment. Tendon progenitor cells reside within a niche that comprises primarily parallel collagen fibers, and this niche plays an important role in regulating their function and differentiation. Culture conditions replicating this environment could be beneficial for both cell growth and matrix gene expression. The objectives of the study were to compare cell growth kinetics and biosynthetic capabilities of bone marrow mesenchymal stem cells (BMMSCs) and tendon derived progenitor cells (TPCs) cultured on commercially available bovine, highly purified bovine, porcine, and rattus collagen sources and standard tissue culture surfaces. We hypothesized that collagen type I matrix would preferentially support TPC proliferation and up regulate gene expression for collagens and organizational components of tendon and therefore provide a culture system and progenitor cell type with advantages over the current practice of BMMSC expansion on standard cell culture plastic surfaces. Cells were isolated from 6 young adult horses, expanded, and cultured on collagen-coated tissue culture plates, and no collagen control for 7 days. Samples were analyzed for cell number on days 4 and 7, and for mRNA expression of collagen type I, collagen type III, cartilage oligomeric matrix protein (COMP), and decorin on day 7. Glycosaminoglycan (GAG) synthesis was analyzed on day 7. Differences of cell number between collagen groups and cell type, and in gene expression and GAG synthesis between collagen groups and cell types, were evaluated by use of mixed-model repeated measures ANOVA. Pair-wise comparisons were made on significant differences identified with ANOVA using Tukey's post hoc test. Statistical significance was set at P<0.05. A statistical significant (P=0.05) increase in cell number for TPCs grown on rattus collagen versus control on day 4 was observed. No difference in GAG synthesis or expression of collagen type I, collagen type III, COMP or decorin mRNA was observed between collagen groups and non-collagen controls for either cell type on day 7. TPCs cultured on all collagen types yielded more cells than similarly cultured BMMSCs on day 4, but only porcine collagen was superior on day 7. TPCs synthesized more GAG than BMMSCs when cultured on control surfaces only. BMMSCs expressed more collagen type I mRNA when cultured on control, porcine and highly-purified collagen, and more collagen type III when cultured on control, porcine, highly-purified collagen, and rattus collagen, than TPCs. Tendon-progenitor cells expressed significantly more COMP when cultured on control and all collagen types, and decorin when cultured on porcine, highly purified bovine and bovine collagen when compared to BMMSCs. The results of this study revealed an advantage to culturing TPCs on randomly organized rattus collagen during the early growth phase. The beneficial effects of collagen-coated surfaces on cell proliferation is likely related to increased surface area for attachment and expansion provided by the random collagen matrix, and/or collagen-cell interactions. Tendon progenitor cells showed superior growth kinetics and expression of the matrix organizational components, COMP and decorin, than similarly cultured BMMSCs that expressed more collagen types III and I. TPCs synthesize more GAG compared to BMMSCs when cultured on plastic surfaces and there was no induction by collagen. Tendon progenitor cells should be considered as an alternative source of progenitor cells for injured equine tendons. Further in vitro studies characterizing factors that influence gene expression of both cell types is warranted. / Master of Science

Endotoxemia is a major cause of morbidity and mortality in horses. The presence of large amounts of circulating endotoxin inititates a number of cell signaling pathways leading to a systemic inflammatory response. Activation of these pathways causes the release of a number of pro- and anti-inflammatory mediators. An overwhelming release of these mediators leads to the development of clinical signs associated with endotoxemia. Treatment options are limited mostly to supportive care at this time. Mesenchymal stem cells (MSCs) have been shown to have anti-inflamamtory and immune modulatory effects that may have some benefit for the treatment of horses with endotoxemia. To evaluate the effect of equine MSCs on the response to endotoxin challenge, the study was performed on two different stem cell lines with peripheral blood mononuclear cells (PBMCs) used as controls. After stimulation with endotoxin, secretion of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), interleukin-10 (IL-10), and interferon gamma (IFN-γ) were determined by ELISA. The immunogenic properties of MSCs were assessed with a one-way mixed lymphocyte reaction. In addition, the ability of MSCs to alter production of cytokines from stimulated PBMCs was assessed. TNF-α was not produced by MSCs when compared to PBMCs (p = < 0.001). There was no significant difference between MSCs and PBMCs in the production of IL-6. IL-10 production was significantly different (p = <0.001) at 6 and 12 hours with MSCs producing more than PBMCs in one stem cell line only. MSCs did not stimulate proliferation of PBMCs. Co-incubation of MSCs with PBMCs decreased the production of TNF-α in both stem cell lines although it was not statistically significant (p = 0.4 and 0.9) at either time point. IL-6 secretion was suppressed at twelve hours with co-incubation. IL-10 production was increased with co-incubation in one stem cell line. MSCs secrete soluble factors that can alter PBMC cytokine production and they do not appear to be immunostimulatory. These findings have potential implication for treatment of equine inflammatory conditions. / Master of Science

Adult CBA mice at 0, 50, 73, and 90 percent levels of inbreeding were used to study variation's in chromosome number from bone marrow cells. Chromosome counts were made from 20 animals at each level of inbreeding except at the 73 percent level where only 13 animals were available. The chromosome number was determined for 24 countable cells per animal at each level, and used to determine if there were significant differences in the variation of chromosome number among the various levels of inbreeding. A modal class of 40N chromosomes was noted for the mice in all groups. However, there was considerable variation within each group. The average percentages of abnormal counts were 23.5 for the controls, 37.9 for the 50 percent inbred level, 28.3 for the 73 percent inbred level and 27.7 for the 90 percent inbred level. An analysis of variance indicated a significant difference between the various levels of inbreeding for the percentage of normal cells. Duncan's Multiple Range Test indicated that the major coutribution to the significant F-ratio came from the 50 percent level of inbreeding. Consequently, the relationship is not linear, but the higher level of abnormal cells occurred at or below the 50 percent level of inbreeding with a decreasing number of abnormal cells at the higher levels. / Master of Science