Connective tissues--possible implications of the temporal changes for the aging process.

Two of the "old" mono-cause theories of aging have temporal changes in the connective tissues, especially those in collagen, as main events. The crosslinking of collagen is stable under physiological conditions shortly after the formation of fibrils and the additional physico-chemical stability attained later on probably does not influence the physiological functions of the tissues significantly. The temporal changes in collagen seen as increased thermal stability, decreased solubility and increased mechanical stiffness are discussed in relation to the underlying structural changes. It is concluded that the increased stability of the collagen (mainly type I) in the locomotive system and skin are not "true" aging phenomena. It is possible that the changes in the connective tissues of the lungs and kidneys contribute to the decrease of function with age. The "normal" increase of stiffness of type I collagen may contibute to the increased compliance and residual volume of the aging lung. Our present knowledge of the structure and function of collagens and ground substances in various basement membranes does not permit an evaluation of the role of connective tissues in the age changes in the alveolo--capillary complex, the glomeruli and the exchange between tissues and capillaries in general.