A Porcine Urinary Bladder Matrix Does Not Recapitulate the Spatiotemporal Macrophage Response of Muscle Regeneration after Volumetric Muscle Loss Injury.

Volumetric muscle loss (VML) results in irrecoverable loss of muscle tissue making its repair challenging. VML repair with acellular extracellular matrix (ECM) scaffolds devoid of exogenous cells has shown improved muscle function, but limited de novo muscle fiber regeneration. On the other hand, studies using minced autologous and free autologous muscle grafts have reported appreciable muscle regeneration. This raises the fundamental question whether an acellular ECM scaffold can orchestrate the spatiotemporal cellular events necessary for appreciable muscle fiber regeneration. This study compares the macrophage and angiogenic responses including the remodeling outcomes of a commercially available porcine urinary bladder matrix, MatriStem™, and autologous muscle grafts. The early heightened and protracted M1 response of the scaffold indicates that the scaffold does not recapitulate the spatiotemporal macrophage response of the autograft tissue. Additionally, the scaffold only supports limited de novo muscle fiber formation and regressing vessel density. Furthermore, scaffold remodeling is accompanied by increased presence of transforming growth factor and α-smooth muscle actin, which is consistent with remodeling of the scaffold into a fibrotic scar-like tissue. The limited muscle formation and scaffold-mediated fibrosis noted in this study corroborates the findings of recent studies that investigated acellular ECM scaffolds (devoid of myogenic cells) for VML repair. Taken together, acellular ECM scaffolds when used for VML repair will likely remodel into a fibrotic scar-like tissue and support limited de novo muscle fiber regeneration primarily in the proximity of the injured musculature. This is a work of the US Government and is not subject to copyright protection in the USA. Foreign copyrights may apply. Published by S. Karger AG, Basel.