The effects of neonatal gamma irradiation on the mitral-granule cell development during postnatal olfactory bulb neurogenesis in the rat
Fortney, Richard L.
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The morphological and cytological consequences resulting from single or multiple exposure of gamma irradiation to the head were qualitatively and quantitatively studied by light and electron microscopy in 30-day-old rats. At birth (day 0) neonatal albino male and female rats were divided into three experimental groups and subjected to either a single (200 rads) exposure, a consecutive five- day multiple exposure (800 rads) or sham irradiated. Brain and body weights, plus crown to rump length, were lower for both irradiated groups compared to the control. At the level of the light microscope, total olfactory bulb volume and volume-to- volume ratio for all layers for both irradiated groups were lower than the control. Total mitral and granule cell number for both irradiated groups were also lower. Ultrastructurally, total external plexiform layer synaptic number increased 23% and 38% within the single and multiple groups, respectively. Reciprocal synapse number for the single exposure group was comparable to the control group but decreased in the multiple exposure group. Both symmetrical (granule- to-mitral) and asymmetrical (mitral-to-granule) dendrodendritic synaptic density increased significantly in the multiple group, but not in the single group. Reduction in apposition length for the dendrodendritic gemmulofugal (granule-to-mitral) synapses and an increase in apposition length for the dendrodendritic mitrofugal (mitral-to-granule) synapses were noted. The results of this study indicate that neonatal exposure to a single sublethal dose of gamma irradiation at birth significantly alters the development of the olfactory bulb but does not interfere with normal synaptogenic organization of neuronal assemblies between mitral and granule cells, despite reduction in both cell types. Although no apparent abnormalities in synaptogenesis were noted for the multiple exposure group, gross morphological interference of normal laminar development was noted along with retarded growth of mitral and granule cells, despite no apparent abnormalities in the cells themselves. It is concluded that multiple exposure to gamma radiation during the first postnatal week severely affects not only the postnatal granule and mitral cell populations, but also the morphology and ultrastructure of the olfactory bulb which may have contributed to a compensatory development within the mitral-granule cell synaptic circuitry.