Influence of maturation and aging on mechanical and biochemical properties of connective tissue in rats.

The influence of maturation and age on the physical and chemical properties of various organs of connective tissue has been studied in rats at ages of 4 weeks, 8 weeks, 4 monhs, 1 year and 2 years. The changes between young (4 weeks old) and adult (4 months to 1 year old) animals were considered as the effects of maturation, whereas the changes between adult and senescent (2 years old) rats were regarded as the effects of aging. Ultimate values, such as ultimate load, tensile strength and breaking strength, or ultimate modulus of elasticity, showed a sharp rise during maturation and a smaller but significant decrease during aging in all organs, such as skin strips, tail tendons, shaft bones, epiphyseal cartilage and aorta rings. Ultimate strain showed a similar pattern, but the maximum occurred earlier. These changes were parallel with the content of insoluble collagen. Other chemical parameters such as soluble collagen or glycosaminoglycans, showed a continuous decrease during the life span, whereas elastin rose continuously. More detailed analysis of mechanical properties in rat skin gave insight into the viscoelastic behaviour of skin. In creep experiments time until break under constant load rose continuously during the life span, whereas ultimate extension rate showed a sharp fall during maturation and a slow decrease during senescence. Stress at low extension degree and moduli of elasticity at low extension degree were decreased by maturation and increased by senescence, exactly the opposite of the changes at high extension degrees. The so-called step phenomenon was increased due to maturation and decreased due to aging. Relaxation and mechanical recovery were changed in the same direction by maturation and aging. Relative viscoelastic parameters, such as the hysteresis phenomenon and relative decrease of stress under cyclic strain, were barely influenced by the aging process. Changes of most of the mechanical parameters at high extension degrees during maturation may be explained by an increase of cross-linking of collagen. Other phenomena such as changes at low extension degrees or parameters of plasticity cannot be explained on this basis. Further studies are needed for a better understanding of the true aging process in connective tissue.