Growth and Remodeling in Engineering Soft Tissue.

Abstract

Left to their own devices, tendons and ligaments repair slowly due to low blood flow and cell content. An engineered tissue contruct (ETC) approach using multipotent bone marrow stromal cells (MSCs) without an exogenous scaffold promises a robust autologous intervention strategy. From a biomechanical perspective, two physical processes govern the development of ETCs in vitro: growth, such as protein deposition or cell proliferation, and remodeling, such as fiber reorientation or cell differentiation. This dissertation describes the development and analysis of biomechanical models of growth and remodeling critical to scaffold-less soft tissue engineering. Firstly, the thermodynamics of a class of fiber remodeling laws were investigated in detail. It was found that purely mechanical formulations of remodeling that stiffen tissue are thermodynamically inadmissible. This dissipation imbalance was quantified in a finite-element model of tendon undergoing fiber reorientation and was found to be positive under both constant displacement and constant load boundary conditions.

Next, a novel image processing algorithm was developed to quantify directionality in planar and volumetric image data for incorporation into continuum mechanical models. With a single input parameter, the method was validated in 2D against representative synthetic images of known fiber distributions and was able to distinguish in 3D between isotropic and fibroblast-aligned collagen gels imaged using confocal microscopy.

To optimize the ETC culture system for tendon and ligament, the effects of oxygen content were studied on the growth and fibroblastic differentiation of rat MSCs and tendon fibroblasts (TFbs). MSCs exhibited a significantly shorter population doubling time under hypoxic conditions (5% O2) compared to normoxia (18% O2). Collagen I mRNA and protein levels increased significantly up to 2 d in hypoxic MSC culture. Both cell types demonstrated elevated mRNAs encoding the tendon and ligament-associated transcription factor scleraxis under hypoxia.

Besides the individual contributions of these studies, the ability to model and simulate complex cell and tissue behaviors—both computationally and experimentally— portends not only patient-specific engineered tissue therapies using “computer-aided tissue engineering”, but also enables the testing of hypotheses related to important biological questions not directly approachable via conventional experiments.