Exposure of the chicken and chick embryo to lethal levels of ionizing radiation results in a circulatory failure and early mortality (<24 hours). This injury response, previously characterized in relation to physical factors of the experiment (dose, dose rate, and time), involves a circulatory collapse that results from increased vascular permeability and degenerative changes of the vascular endothelium. These effects, leading to circulatory stasis and death, have been observed and recorded cinemicrographically in irradiated chick embryos. After 66 to 68 hours of incubation, embryos (including the entire area vasculosa) were explanted onto an agar-albumen medium and subjected to 250-kv X-ray exposures of 700 R or 1100 R, delivered in 12 minutes. These exposures resulted in 10% or 90% mortality, respectively, within 24 hours. Circulation in chick embryos exposed to 700 R was similar to that in nonirradiated controls. After exposure to 1100 R, the hemoconcentration observed within 30 to 45 minutes suggested a plasma loss. In localized areas circulating cells adhered to the endothelium, and thrombi formed. Vascular degeneration progressed until stasis became complete, frequently within 4 to 6 hours after exposure. A movie, illustrating these findings, is available upon request. Order movie by title and number: ANL-MP-730-153, Early changes in the microcirculation in the x-irradiated chick embryo. Argonne National Laboratory, Film Center, D-2-149, 9700 S. Cass Avenue, Argonne, Illinois, 60439.

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