Vascular regression in the kidney: changes in 3D vessel structure with time post-irradiation

Though angiogenesis has been investigated in depth, vascular regression and rarefac-tion remain poorly understood. Regression of renal vasculature accompanies many pathological states such as diabetes, hypertension, atherosclerosis, and radiotherapy. Radiation decreases microvessel density in multiple organs, though the mechanism is not known. By using a whole animal (rat) model with a single dose of partial body irradiation to the kidney, changes in the volume of renal vasculature were recorded at two time points, 60 and 90 days after exposure. Next, a novel vascular and metabolic imaging (VMI) technique was used to computationally assess 3D vessel diameter, volume, branch depth, and density over multiple levels of branching down to 70 ??m. Four groups of rats were studied, of which two groups received a single dose of 12.5 Gy X-rays. The kidneys were harvested after 60 or 90 days from one irradiated and one non-irradiated group at each time point. Measurements of the 3D vasculature showed that by day-90 post-radiation, when renal function is known to deteriorate, total vessel volume, vessel density, maximum branch depth, and the number of terminal points in the kidneys decreased by 55%, 57%, 28%, and 53%, respectively. Decreases in the same parameters were not statistically significant at 60 days post-irradiation. Smaller vessels with internal diameters of 70-450 ??m as well as large vessels of diameter 451-850 ??m, both decreased by 90 days post-radiation. Vascular regression in the lungs of the same strain of irradiated rats has been reported to occur before 60 days supporting the hypothesis that this process is regulated in an organ-specific manner and occurs by a concurrent decrease in luminal diameters of small as well as large blood vessels.