Effects of neonatal gamma-ray irradiation on rat hippocampus--I. Postnatal maturation of hippocampal cells.

The axons of dentate granule cells, the mossy fibres, establish synaptic contacts with the thorny excrescences of the apical dendrite of CA3 pyramidal neurons. Dentate granule granule cells develop postnatally in rats, whereas the CA3 pyramidal cells are generated before birth. In the present studies, using unilateral neonatal gamma-ray irradiation to destroy the granule cells in one hemisphere, we have studied the effect of mossy fibre deprivation on the development of their targets. We show that such "degranulation" prevents the normal development of giant thorny excrescences, suggesting that the development of thorny excrescences in CA3 pyramidal neurons is under the control of mossy fibres. In contrast, irradiation of the hippocampus of the neonatal rat does not affect the development of the dendritic arborization of CA3 pyramidal cells and their non-mossy dendritic spines.