Conservation of neuronal number and size in the entorhinal cortex of behaviorally characterized aged rats.

Despite abundant evidence of behavioral and electrophysiological dysfunction of the rodent hippocampal formation with aging, the structural basis of age-related cognitive decline remains unclear. Recently, unbiased stereological studies of the mammalian hippocampus have found little evidence to support the dogma that cellular loss accompanies hippocampal aging, thereby supporting an alternative hypothesis that aging is marked by widespread conservation of neuronal number. However, to date, the effects of aging have not been reported in another key component of memory systems in the rodent brain, the entorhinal cortex. In the present study, we stereologically estimated total neuronal number and size (cross-sectional area and cell volume) in the subdivisions and cellular layers of the rat entorhinal cortex, using the optical fractionator and nucleator, respectively. Comparisons were made among Fischer 344 rats that were young, aged-impaired, and aged-unimpaired (based on functional analysis in the Morris water maze). No significant differences in cell number or size were observed in any of the entorhinal subdivisions or laminae examined in each group. Thus, aging is associated with widespread conservation of neuronal number, despite varying degrees of cognitive decline, in all memory-related systems examined to date. These data suggest that mechanisms of age-related cognitive decline are to be found in parameters other than neuronal number or size in the cortex of the mammalian brain.