The fundamental components of bone tissue engineering are (a) progenitor cells which subsequently express tissue matrix, (b) scaffolds which can act as temporary frameworks to support bone growth, and (c) growth factors to induce osteoblast regeneration. A variety of growth factors are involved during the differentiation cascade and these chemical and biological signals dynamically interact with cell populations to facilitate the differentiation. Therefore, enhanced expression of endogenous growth factor genes might facilitate abundant existence of growth factors in the surrounding microenvironment, stimulate the osteogenic differentiation of progenitor cell population, and finally induce bone regeneration. This work is focused on the augmentation of osteogenic signal expressions to stimulate the downstream differentiation of transplanted bone marrow stromal cells (BMSCs) population through the optimization of a variety of properties of three dimensional (3D) biodegradable poly(propylene fumarate) (PPF) scaffold. Changes in the microenvironment of cell population would affect the responses of localized cell population and the manipulated scaffold properties might be associated with induction of endogenous osteogenic signal expressions.

First, the effect of cell-to-cell paracrine signaling distance, which can by modulated by initial cell seeding density, on the osteogenic signal expressions and osteoblastic differentiation of BMSCs on 2D PPF disks was investigated.  Next, in order to investigate the improvement of the 3D macroporous PPF scaffold by the incorporation with nanoparticle filler materials, PPF/hydroxyapatite (HA) nanocomposite scaffolds were fabricated. The effect of HA content and initial cell seeding density on the osteogenic signal expression in 3D porous system was then determined.  Finally, the incorporation of diethyl dumarate (DEF) with PPF was tested based on the photocrosslinking characteristics of PPF/DEF composite material with increased mechanical properties. The effect of two scaffold design parameters including the stiffness by modulating the DEF content as well as the pore size of porous scaffold on the signal expression and downstream osteoblastic differentiation was investigated.  In addition, the feasibility of PPPF/DEF materials for stereolithographical fabrication was also tested in this work.  

Controlling these construction parameters to optimize engineered bone substitutes could affect various cellular functions of attachment, proliferation, signal expression, and differentiation. This research provided the insight of stimulation of the expression of target endogenous genes to induce the osteogenic differentiation and bone regeneration as well as the fabrication of improved bone substitute implant materials which is clinically applicable.