Author

David Levitz

Date

May 2010

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Biomedical Engineering

Institution

Oregon Health & Science University

Abstract

The ability of optical imaging techniques such as optical coherence tomography (OCT) to non-destructively characterize engineered tissues has generated enormous interest recently. The engineered tissue of interest here is the collagen gel, wherein smooth muscle cells (SMCs) are embedded in a 3D collagen I matrix. This thesis focuses on characterizing collagen gels quantitatively, by measuring the optical properties – the scattering coefficient s and anisotropy factor g – from OCT data by fitting the signal to a theoretical model. μ[subscript s] and g are macroscopic physical properties that provide information on the density and size of scattering particles in the sample (cells and collagen fibrils), respectively. To measure μ[subscript s] and g, OCT data from nanoliter-sized volume is averaged to produce a depth-dependent curve, which was fit to a model, yielding 2 observable parameters – attenuation μ and reflectivity ρ – which map back to the optical properties μ[subscript s] and g. Collagen gels were imaged by OCT at various time points over a 5 day period, and their optical properties were evaluated. In the first 24 hours, scattering in collagen gels is dominated by the collagen fibrils, which scattered light anisotropically, that is, mostly in the forward direction. However, over a 5 day period, the optical properties of collagen gels showed a 10-fold increase in reflectivity with no change in attenuation. Such changes imply a decrease in anisotropy g, which cause an increase in backscatter and imply an increase in the number of small scattering particles in the collagen gels. In trying to assess what caused the measured changes in optical properties, the activity of matrix metalloproteinases (MMPs) was identified as responsible for the anisotropy decrease. MMPs degrade the collagen matrix, breaking down collagen fibrils, which scatter light anisotropically, into small fibril fragments, which scatter light more isotropically. These findings are supported by follow up experiments in which MMP activity was altered in cellular and acellular gels, as well as experiments with the cells concentrated on one side of the collagen gel which enhanced the effect SMCs and MMP activity have on the scattering properties. All in all, these data show that SMCs embedded in a collagen gel express and activate MMPs, that these MMPs act locally to degrade collagen fibrils into smaller fragments, and that OCT can visualize and quantify the entire process.

Identifier

doi:10.6083/M4J38QGT

School

School of Medicine

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