Abstract
The integrity of skeletal muscle is maintained by the intramuscular connective tissues (IMCTs) that are composed of extracellular matrix (ECM) molecules such as collagens, proteoglycans, and glycoproteins. The ECM plays an important role not only in providing biomechanical strength of the IMCT, but also in regulating muscle cell behavior. Some ECM molecules, such as decorin and laminin, modulate the activity of myostatin that regulates skeletal muscle mass. Furthermore, it has been shown that decorin activates Akt downstream of insulin-like growth factor-I receptor (IGF-IR) and enhances the differentiation of myogenic cells, suggesting that decorin acts as a signaling molecule to myogenic cells. With animal growth, the structural integrity of IMCT increases; collagen fibrils within the endomysium associate more closely with each other, and the collagen fibers in the perimysium become increasingly thick and their wavy pattern grows more regular. These changes increase the mechanical strength of IMCT, contributing to the toughening of meat. However, in highly marbled beef cattle like Wagyu, intramuscular fat deposits mainly in the perimysium between muscle fiber bundles during the fattening period. The development of adipose tissues appears to disorganize the structure of IMCT and contributes to the tenderness of Wagyu beef. The IMCT was considered to be rather immutable compared to myofibrils during postmortem aging of meat. However, several studies have shown that collagen networks in the IMCT are disintegrated and proteoglycan components are degraded during postmortem aging. These changes in ECM appear to reduce the mechanical strength of IMCT and contribute to the tenderness of uncooked meat or cooked meat at low temperature. Thus, the ECM plays a multifunctional role in skeletal muscle development and postmortem aging of meat.