CONCEPT AND CRITERIA OF RADIOLOGIC AGEING 197 



The fundamental histopathological changes of agemg seem to be degenera- 

 tive and atrophic or invokitional changes, the end-result of which may be 

 described generally as fibro-atrophy. This fibro-atrophy involves a decrease 

 in number of parenchymal cells associated with a decrease in fine vasculature 

 in a process of arteriolocaj^illary fibrosis and with an increase in density and 

 amount of interstitial connective tissue, constituting an increase in the histo- 

 haematic barrier (connective tissue barrier between blood and parenchymal 

 cells). 



The hypertrophic cellular changes and hyperplastic or metaplastic tissue 

 changes seen with increasmg age are generally not among the fundamental 

 agemg manifestations and do not occur in all senescent individuals. These 

 changes seem to be either normal physiological compensatory responses of 

 less affected cells to degenerative changes in related cells or tissues or, Hke 

 many degenerative changes observed, are secondary to specific disease 

 processes. 



It is not yet clear at the histopathological level which of the three com- 

 ponents of the tissue fibro-atrophy of "normal ageing", i.e. the changes of 

 parenchymal cells, of connective tissue, and of fine vasculature, are primary 

 and secondary with respect to one another; or what are the relative contribu- 

 tions of one component to another, since they have mutual or reciprocal 

 influences; or to what extent these relationships or contributions differ among 

 tissues of different kinds. 



Loss of parenchymal cells may be followed by a j)rocess of replacement 

 fibrosis and reduction in fine vasculature secondary to reduced parenchymal 

 cellularity. Also, non-specific damage to the endothelium of fine vasculature 

 may result in interruption or impedence of circulation, increase in inter- 

 stitial colloid and in fibrillar density of connective tissue, increase in histo- 

 haematic barrier, consequent loss of parenchymal cells through hypoxia and 

 reduced nutrition, then replacement fibrosis and reduction in fine vasculature. 

 The success of regeneration of parenchymal cells depends greatly on the ade- 

 quacy of the microcirculation and on the permeability or mtegrity of the 

 histohaematic barrier. 



None of these histopathological components of the tissue fibro-atrophy of 

 ageing are due necessarily to inherent changes in the tissue components 

 involved. Even the increasing fibrillar density of interstitial connective tissue 

 may be caused by forces originating elsewhere in the body. All of these changes 

 are non-specific changes which may be brought about by a variety of agents 

 or factors, including adrenal cortical reactions and perhaps any agent 

 eliciting a response of the adrenal cortex as in stress phenomena. The non- 

 specificity of basic histopathological ageing changes is compatible with the 

 concept proposed by Jones (1956) that ageing is a result of the accumulation 

 of non-specific injuries. 



