A comparison of anterior cruciate ligament fibroblast and bone marrow stromal cell potential in ligament tissue engineering

A functional tissue engineered ligament requires a viable cell source along with a mechanically robust, biodegradable scaffold. The former has included differentiated fibroblasts from ruptured anterior cruciate ligaments (ACL) along with bone marrow stromal cells (BMSC). Investigations which have employed these cell types have demonstrated that biochemical and mechanical stimulation is necessary to develop functional, ligament-like tissue in vitro . However, there is still a limited understanding of how BMSC differentiate into fibroblasts, and what is required for this to occur. In this investigation, a fibroblast-specific cytokine, Platelet Derived Growth Factor-BB (PDGF-BB) was used to model how BMSC response to extracellular cues compares to that of ACL fibroblasts. When cultured on a 2-dimensional substrate, high concentrations of PDGF-BB appeared to stimulate metabolic activity in ACL fibroblasts, however the same concentrations were inhibitory to BMSC. Similar observations were made when cells were cultured on a 3-dimensional silk scaffold; ACL fibroblast metabolic activity increased over a 21 day incubation period, while BMSC metabolic activity decreased. BMSC appeared to have limited scaffold ingrowth and collagen deposition compared to ACL fibroblasts following 50 ng/ml PDGF-BB stimulation. Collagen type I and III transcript levels followed different trends in both cell types. ACL fibroblast peak transcript expression for both collagen occurred on day 14, while BMSC peak expression was on day 2. No markers for bone or cartilage (collagen type X and II respectively) were observed. These results suggest that ACL fibroblasts and BMSC respond to PDGF differently, and indicate that to direct BMSC to the appropriate lineage will require further investigation into what is involved in fibroblast differentiation. Future studies should investigate a more appropriate molecular marker for differentiation than collagen type I:III ratios. Consequently, this study demonstrated that the innate biology of a cell will predetermine its response to extracellular cues. Differences between BMSC and fibroblasts must be better understood to ascertain development of functional ligament-like tissue in vitro.