Cell outgrowth from the human ACL in vitro: regional variation and response to TGF-beta1

One of the new methods being developed to stimulate healing of the human anterior cruciate ligament (ACL) after rupture is the implantation of a biodegradable scaffold which the host cells invade, populate and remodel. One of the cellular behaviors critical to the success of this method is cell outgrowth from the ligament remnants onto an adjacent scaffold. As morphological differences have been previously reported in the proximal and distal human ACL, the primary aim of this study was to determine if the cells from the proximal and distal ACL had different outgrowth behaviors as well. A second aim was to determine whether TGF-beta1, reported to be a mitogen for fibroblasts, would affect the outgrowth behaviors from the proximal and distal ACL. To achieve these aims, explants from both the proximal and distal human ACL were placed into culture under various conditions and outgrowth measured for 42 days. In explants cultured with 10% fetal bovine serum (high serum group) the explants from the proximal ACL had an earlier start of outgrowth than the distal explants (p < 0.015), and outgrowth rates were similar in the two groups. In explants cultured with 2% fetal bovine serum (low serum group), the explants from the proximal ACL had an earlier start to outgrowth (p < 0.003) as well as a faster rate of outgrowth (p < 0.004) than the distal explants. The addition of TGF-beta1 to the low serum cultures significantly slowed the rate of outgrowth from both groups of ACL explants (p < 0.025). These results suggest that outgrowth behaviors are different in the proximal and distal human ACL, and that TGB-beta1 has an inhibitory effect on cell outgrowth from ACL explants. However, this study is only a first step, and additional experiments are needed to further optimize tissue engineering parameters for enhancement of the repair of the ACL.