CONCEPT AND CRITERIA OF RADIOLOGIC AGEING 199 



to another. Secondary to damage to capillary and arteriolar endothelium, 

 there is some degree of pericapillary or interstitial oedema and an increase in 

 the amount of interstitial colloid, followed by an increase in fibrillar density 

 of the interstitial connective tissue. Some increase in subendothelial connective 

 tissue in arterioles also develops. These changes in connective tissues 

 constitute an increase in the histohaematic barrier and a temporal advance 

 in the development of arteriolocapillary fibrosis, which are progressive 

 processes in "normal" ageing. These manifestiations increase progressively 

 with passing time in the irradiated tissues as they do in non-irradiated tissues, 

 but remain temporally advanced in degree in the former as compared with 

 the latter. As these progressive changes gradually reduce the capacity of the 

 vasculo-connective tissues to support the full complement of parenchymal 

 cells, owing to the development of relative hypoxia and malnutrition, the 

 parenchymal cells gradually become decreased in number either as a result of 

 precocious ageing and death or as a result also of decrease in cell reproduction 

 in the case of renewable cells. This loss of parenchymal cells occurs earlier 

 in the irradiated tissues than in the non-irradiated tissues, according to the 

 earlier changes in histohaematic barrier and fine vasculature. With the gradual 

 decrease in number of parenchymal cells there occurs a "replacement fibrosis" 

 with increase in interstitial mesenchymal elements and further increase in 

 the histohaematic barrier and with further reduction of fine vasculature 

 secondary to the loss of parenchyma. 



The influence of the early parenchymal damage on the early changes in 

 vasculoconnective tissue and on the late effects or ageing manifestations is 

 best described for different t}^es of tissue in terms of the nature and relative 

 radiosensitivity of the parenchymal cells they contain. 



Vegetative inter-mitotic parenchymal cells, which divide regularly but 

 differentiate little or not at all, e.g. basal cells of epidermis, are generally 

 highly sensitive to the destructive action of radiation. Differentiating 

 inter-mitotic cells, which divide regularly and differentiate to some extent 

 between divisions, e.g. myelocytes, are somewhat less sensitive but still 

 relatively sensitive cells in general. Certain mesenchymal elements, including 

 endothelial cells of fine vasculature, are intermediate in sensitivity between 

 these highly sensitive cells and the relatively resistant reverting post-mitotic 

 parenchymal cells and the extremely resistant fixed post-mitotic parenchymal 

 cells. The reverting post-mitotic parenchymal cells are variably differentiated 

 cells which do not divide regularly but are capable of dividing upon apj)ro- 

 priate stimulus, e.g. hepatic cells when a partial hepatectomy is performed. 

 The fixed post-mitotic 'parenchyynal cells are highly differentiated cells which have 

 lost completely their ability to divide under any circumstances, e.g. the 

 neuron. Some of these, like the neurons, are long-lived, age and die with- 

 out replacement; others, lilvc polymorphonuclear leucocytes, are relatively 



