Functional bone tissue engineering using human mesenchymal stem cells and polymeric scaffolds.

摘要

Functional bone tissue engineering has been necessitated by the need to treat critical size defects in bones due to birth abnormalities, trauma, and pathological conditions. Appropriate conditions for in vitro osteogenesis need to be identified to establish protocols for engineering bone tissues. The success of in vitro osteogenesis lies on the type of cell source, stimuli, and scaffold material used for engineering bone constructs. Recent investigations have established the pluripotency of mesenchymal stem cells (MSCs) and their ability to differentiate down a multitude of pathways including osteogenic. In vivo studies have shown that MSCs are primarily responsible for bone growth and regeneration and therefore have become a major candidate for bone tissue engineering. Osteogenic differentiation of MSCs via chemical stimuli has been extensively investigated using both monolayer and three-dimensional (3D) culture conditions. These investigations provided useful information on media conditions, cell seeding densities, and differentiation capabilities of MSCs. However, chemical stimulation alone might not be sufficient to accelerate osteogenesis and impart necessary mechanical strength to the final tissue construct. Mechanical strength of the final tissue construct is vital to maintain its structural integrity when exposed to physiological stresses in vivo. Stimulation of MSCs using mechanical strain might provide another method to induce MSC osteogenesis while also obtaining desired mechanical strength of the final tissue constructs. Although in vivo studies and experimental models have indicated that cyclic tensile strain could induce MSC osteogenesis, its effect on MSC osteogenesis in 3D cultures in vitro has not been investigated. The need to maintain cell viability and be able to provide chemical or mechanical cues to cells in 3D cultures requires improvements in scaffold architecture and design. While collagen provides a natural matrix for cell adhesion and growth, its contraction during culture can greatly limit culture duration and mechanical stability of the matrix. Although fibrous scaffolds can be used as an alternative to collagen scaffolds, insufficient media diffusion to the center of these 3D scaffolds could detrimentally affect uniform cell growth throughout the scaffold; hence, scaffolds with better diffusional properties need to be developed. This study investigated the use of 3D collagen matrices as a scaffold material to determine the effects of strain and chemical stimuli on osteogenic differentiation of human MSCs (hMSCs). Major attention was given to the analyses of: cell viability, matrix contraction, nuclei morphology, expression of osteogenic markers and proinflammatory cytokines, as well as changes in mechanical properties of the final tissue construct. As an approach to develop 3D fibrous scaffolds with enhanced diffusional properties, fabrication of melt spun microporous fibers using a blend of poly (lactic acid) (PLA) and sulfopolyester that could be used in 3D nonwoven scaffolds was also investigated.; The findings of this study clearly illustrated the ability of cyclic tensile strain to induce osteogenic differentiation of hMSCs when cultured in a 3D environment. Expression of proinflammatory cytokines by strained hMSCs suggested that cyclic strain might have induced modulation of bone resorption in hMSCs. The results also illustrated the effects of strain on the mechanical properties of the final tissue construct.; Microporous fibers created from melt spun composite fibers using binary blends of poly (lactic acid) and sulfopolyester could enhance diffusional properties of 3D nonwoven scaffolds fabricated using these fibers. As this body of work demonstrates, use of cyclic tensile strain combined with chemical stimulation to induce osteogenic differentiation of hMSCs could greatly assist the engineering of functional bone tissues in vitro. Microporous fibers created using polymer blends could pro