Studies that do not support a tissue specific hypothesis

Most of the clinically utilized ECM scaffolds are used in non-homologous locations with varying degrees of success. Urinary bladder and/or small intestine derived ECM scaffolds have been utilized in lung [193], myocardial [110-119, 122] and skeletal muscle [91, 96, 97, 103-107] repair. Taken together, hese studies provide useful comparisons of the host response to the same non- tissue specific ECM scaffolds (e.g. urinary bladder ECM) implanted in several different anatomic locations, and may elucidate general remodeling trends.

There are few reports of the use of non-tissue specific lung ECM for lung applications. One example is application of urinary bladder ECM (UBM) for prevention of pulmonary

fibrosis. Particulate and enzymatically digested forms of UBM were intratracheally infused into the lung and shown to mitigate bleomycin induced pulmonary fibrosis [193]. Lung ECM was not evaluated in this model, but these results show that non-lung ECM can also provide beneficial effects to injured lung tissue in vivo.

One of the most well characterized ECM materials is small intestinal submucosa (SIS) [194], which has been used in multiple clinical skeletal muscle applications such as rotator cuff repair and ventral hernia repair, as well as multiple pre-clinical studies to treat a multitude of tissues. SIS implantation in muscle locations have shown greatly increased myogenesis and functional improvements in musculotendinous junction and abdominal wall injury locations [105, 107], though this same material in a composite VML/periosteal injury was predisposed to promote bone and connective tissue formation rather than muscle [195]. This suggests that the effect of SIS may depend on implantation location and type of injury induced. Urinary bladder derived ECM has a similar constructive remodeling response to SIS in muscle injury locations [99, 196, 197]. These scaffolds are degraded and remodeled with islands of skeletal muscle forming within the injury location. Studies have also shown that an anti-inflammatory macrophage phenotype is associated with improved remodeling and myogenesis in these models [196]. Stem cell seeded bladder ECM scaffolds markedly improved myogenesis and functional recovery in VML models, and was dependent on seeding density and preconditioning methodology prior to implantation. Acellular bladder ECM in this model improved function to a lesser extent, and with less myogenesis [198-200].

Non-cardiac specific urinary bladder and SIS ECM has also been investigated for cardiac repair applications in vivo, and have been shown to encourage remodeling and improve cardiac function [110-113, 115]. Cell seeding [122] or infusion with bioactive factors (e.g. exogenous

growth factors) prior to implantation further improves this functional recovery [116]. Like cardiac ECM, ECM degradation products prepared from small intestine ECM also encourages functional remodeling and protection from dilation and fibrosis [114, 201]. Most in vivo evaluations of cardiac ECM efficacy have been compared to synthetic materials such as Dacron, which are the clinical standard of care, rather than to other ECM devices [123, 179]. Both cardiac and non-tissue specific ECM have been more effective as cardiac patch materials compared to synthetics, though these studies do not establish tissue specific effects. Remlinger et al., however, did compare cardiac patches cut from whole heart ECM [202, 203] to patches composed of urinary bladder ECM for right ventricle outflow tract repair. Though both types of ECM promoted endothelialization and α-actinin positive cardiomyocyte formation, bladder ECM was more quickly and more effectively remodeled with new cardiomyocytes over the time course of this study than cardiac ECM [203]. However, the cardiac and urinary bladder ECM used in this study was very different in thickness and geometry prior to implantation, which may have affected remodeling.

There is also a growing body of evidence supporting the use of these non-tissue specific ECM scaffolds for clinical repair of VML and cardiac defects. Urinary bladder ECM has been implanted in the repair of VML defects in quadriceps, tibialis anterior, and biceps muscles, which have resulted in greatly increased function [204]. This functional improvement was measured not only by force production, but also task completion, such as the ability to perform a range of different motions and activities necessary for normal ambulation [204] . Multiple case studies have reported the use of SIS ECM scaffolds for repairing congenital atrial septal defects (and other cardiac repair applications) in humans [117, 119, 205, 206]. There has been a low

incidence of adverse events, and follow up studies suggest good incorporation and remodeling. Tissue specific ECM scaffolds have not yet been clinically translated for these tissues, however. In both skeletal and cardiac muscle repair applications, non-homologous scaffolds are degraded and remodeled with reported increases in contractile function. Different studies have either shown advantages, disadvantages, or no difference in comparing tissue specific and non- specific ECM. Each of these conclusions would lead to discordant interpretations. A tissue specific advantage would support the logic that the ECM of every tissue has been optimized during evolution to be the ideal scaffold for that tissue. However, the success of non-homologous ECM sources would imply that the compositional differences between different ECMs does not significantly affect tissue remodeling outcome in these applications, or that certain ECM sources universally provide necessary support to a wide array of tissues.