Failure of the subcallosal sling to develop after embryonic X-irradiation is correlated with absence of the cavum septi.
During embryonic development of the rodent forebrain, a cavity normally appears at the midline just below the corpus callosum. This cavity, the cavum septi, is present in mice by gestational day 18, but is subsequently obliterated by growth of the septal nuclei and neuropil. After x-irradiation of pregnant mice with 125r on gestational day 14.5, the cavum septi did not develop. This dramatic developmental abnormality was accompanied by delayed fusion of the septum, and a reduction in the population of subventricular cells that normally migrate to form a sling of cells extending from the medial aspect of the lateral ventricles to the midline. In normal animals formation of the cavum septi involves degeneration of this subcallosal sling of SV cells. Thus absence of the cavum after x-irradiation may be due to the premature killing of subventricular cells before their migration toward the midline.