The flexibility of the collagen compartment of muscle.

Collagen, the major connective tissue protein, is an integral constituent of muscle and, because of its occurrence and characteristics, is a factor contributing to the texture of meat. This brief review focuses on aspects of collagen biosynthesis and experimental data which indicate the collagens of muscle possess remarkable capacity for change. Over 90% of the intramuscular collagen of meat is located in the perimysium. This fibrillar collagen is comprised predominantly of two collagen phenotypes, types I and III, whose proportions vary with animal age, muscle type, gender and probably rate of collagen synthesis. Collagen in muscle can differ additionally in crosslinking profile. Collagen crosslinks are structures arising from the condensation of lysine or hydroxylysine residues and their aldehydes. Crosslinks link two or three collagen molecules or (collagen fibrils) together. Both the tensile strength of collagen and the toughening of meat due to its connective component are related to collagen crosslink type and concentration. With age and maturation there is a general and progressive shift in collagen type (toward more type I) and an increase in the concentration of mature crosslinks. Such alterations in collagen characteristics, rather than significant changes in collagen concentrations, are responsible for the toughening of meat as animals age. Data also suggest that management practices which alter growth and muscle accretion rates can have a profound affect on collagen characteristics. Increasing or decreasing plane of nutrition, compensatory growth, testosterone- and somatotropin-mediated growth, and exercise alter collagen crosslink and/or type proportionally. Such changes can sometimes be associated with increased shear force scores for cooked meat. Alterations in muscle collagen characteristics are linked directly or indirectly to perturbations in collagen synthesis and turnover rates accompanying growth and muscle accretion. The concept that collagen does not change once deposited extracellularly or that changes in its characteristics are unidirectional does not appear to be valid in muscle tissues. Rather collagen appears to be a very flexible component of the extracellular matrix. Potential for management practices to alter collagen characteristics therefore exists.