The ultrastructure of lung parenchyma of CBA/J mice 1 year after 14 Gy X irradiation was studied by transmission electron microscopy (TEM) and compared with the ultrastructure of age-matched control mice. The irradiated mice had received an injection of colloidal carbon just prior to death to identify patent capillaries. Regions containing carbon were compared by TEM with apparently nonperfused areas lacking carbon. The interalveolar interstitial tissue and the capillary endothelium were not different ultrastructurally in perfused and nonperfused regions. There were no appreciable differences between irradiated and unirradiated lung, except for small, infrequent focal areas with a slight increase in the number of collagen fibers. The ultrastructural examination did not reveal any lesion which might have been missed in the preliminary light microscope studies, which confirms that the lack of perfusion in CBA/J mice which have an appreciable respiratory functional deficit in the late phase is not due to the loss of patent capillaries or to fibrosis of the alveolar wall. It was concluded that the lesion must be at the "pre-capillary" level, probably damage to or loss of small blood vessels.

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