Radiation-induced Interference with Postnatal Hippocampal Cytogenesis

The long term consequences of graduated interference with the acquisition of hippocampal neurons and glia during early infancy were examined with quantitative histology and 3H-thymidine autoradiography. The head region containing the hippocampus was irradiated from day two on with either two (2X), fo~r (4X), six (6X) or eight (8X) doses of 150-200r X-rays. The animals were killed at 30,60,90 and 120 days of age. The morphology of the hippocampus was normal in all irradiated groups with the characteristic interlocking folds of the pyramidal and granular layers. While the number of pyramidal cells of Ammon's horn was unaffected, the number of granule cells of the dentate gyrus was progressively and permanently reduced from control levels by the different dosage schedules (2X, 59% reduction; 4X, 77 % ; 6X, 83 % ; 8X, 84 %). Incidental observations in control animals indicated a 20 % increase in granule ceUs between 30 and 120 days of age in agreement with earlier observations of granule cell labelling after 3H-thymidine injections in adult