Radiosensitivity of Vascular Tissue: I. Differential Radiosensitivity of Capillaries: A Quantitative in Vivo Study

The effects of single doses of X radiation ranging from 200 to 2000 rad were studied by direct morphometry in vivo of the mature, stable microvasculature in rabbit ear chambers. Reproducible observations in vivo of the mature microvasculature were obtained by photomicrography of identical$0.033\text{-}{\rm mm}^{2}$sites in each ear chamber prior to and 1 and 5 days following single doses of X radiation. Measurements were made directly on color photomicrographs at a total magnification of 2000×. The microvessels were divided into two groups according to size: vessels > 10 μm in diameter (arterioles and venules), and vessels ≤ 10 μm in diameter (capillaries). Vascular length and outer and inner surface areas were measured directly on the projected photomicrographs, and vascular volumes and diameters were calculated from these measured parameters. Measurements of capillary length per unit surface area disclosed a decrease in capillary density with increasing dose, resulting in a calculated intercapillary distance in excess of 300 μm, conceivably associated with a decrease in oxygen delivery by the microvasculature. With this method, radiosensitivity of the capillaries was found to be significantly greater than that of larger vessels. Computerized histogram analysis of vascular length, surface area, and volume as a function of increasing diameter (1-μm bins) confirmed the significant difference in reduction of these measured parameters between the capillaries and the larger vessels. The total microvascular volume profile dominated by the volume of larger vessels did not change much 5 days after irradiation, although capillary volume was markedly reduced. Furthermore, the basic profile of the microvasculature showed a shift to larger diameters following irradiation, thus confirming the calculated dilatation of surviving vessels. Qualitative morphologic observations revealed considerable extravasation from the microvessels and formation of micropetechiae at the site of disrupted capillaries with subsequent inflammatory changes.