Effects of X-Irradiation on Embryos at Critical Stages of Heart Development ‡

There is considerable evidence that differentiating cells are the most sensitive of all cells of the living organism. When embryos are x-irradiated at sublethal levels and subsequently studied histologically it is found that the regions showing the greatest radiation damage, as indicated by pycnotic nuclei, do not necessarily correspond with embryonic areas known to have a high mitotic index at the time of irradiation but rather with sites of active differentiation. The fact that the central nervous system has a protracted period of differentiation has been given as one of the reasons why it exhibits the most frequent and extensive damage following x-irradiation of embryos.' Previous studies on embryonic and larval amphibians have indicated that cardiac muscle is relatively radioresistant. 9' However, these earlier studies did not deal with the effects of x-irradiation at specific stages of cardiac development. Therefore, the present study was undertaken to determine the effects of x-irradiating embryos at successive stages of cardiac differentiation.