Cardiac Injury in the Aged Mouse: Comparative Ultrastructural Effects of Fission Spectrum Neutrons and γ Rays

Abstract

Late ultrastructural changes in the myocardium and myocardial microvasculature in the B6CF1 mouse were compared through 24 mo. following total-body exposure to fission spectrum neutrons or 60Co-.gamma. rays. Radiation treatment, initiated when the mice were 4 mo. old, included single doses of 788 .gamma. rad, or 80 or 240 neutron rad, and fractionated doses of 823 or 2690 total .gamma. rad or 20, 80, or 240 total neutron rad (24 fractions given in 23 wk). Early damage to myofibers and capillaries of the heart were previously described to be most severe at 30-90 days after a single exposure. Ultrastructural findings at later times after initiation of irradiation are compared here with the effects of normal aging. At 18 and 24 mo. into the experiment (mice 22 and 28 mo. of age), only minor aging changes were noted. Extracellular areas showed minimal increases in proteoglycan matrix and collagen fibers, while myocytes had increases in inclusion bodies and lipid droplets. Late (18 and 24 mo.) radiation damage seen at the ultrastructural level included myofibrillolysis, capillary degeneration, and increased accumulations of debris and lipids. The effects of 240 neutron rad, either single or fractionated, were more severe than those of a fractionated dose of 2690 .gamma. rad. Matrix (proteoglycans) accumulation and fibrosis were more prominent in neutron- than in .gamma.-treated animals. Myocardial changes generally corresponded to the degree of coronary artery damage previously reported, although there was greater individual variation. The excess fibrosis and matrix accumulation observed in neutron-treated animals agree with previous reports of a similar reaction following neutron therapy.