Neuronal ectopic masses induced by prenatal irradiation in the rat.

Ectopic neuronal masses below the subcortical white matter were seen in the brains of postnatal rats after 200 cGy irradiation at embryonic day 14. In contrast with the laminated organisation of the cortex located above the subcortical white matter, the ectopic masses were formed of confluent nodules composed of pyramidal and non-pyramidal neurons distributed at random, with no laminar organisation. Afferent and efferent fibres to/from the ectopic masses running together with fibres passing the subcortical white matter indicated that the ectopic masses were heavily connected to neighbouring structures. Examination of irradiated embryos revealed that the ectopic masses originated from ectopic periventricular rosettes, composed of germinal cells, which were formed shortly after irradiation. Neuronogenesis in these rosettes did not follow an inside-out gradient, as seen in the laminated cortex; however, early-generated neurons predominated in the external regions, whereas late-generated neurons were mainly located in the middle and internal regions of the ectopic masses.