Neuronal volume of the hippocampal regions in ageing

The hippocampal formation (HF) has an important role in different human capacities, such as memory processing and emotional expression. Both extensive changes and limited variations of its components can cause clinically expressed dysfunctions. Although there remains no effective treatment for diseases caused by pathological changes in this brain region, detection of these changes, even minimally, could allow us to develop early interventions and establish corrective measures. This study analysed the neuronal islands of layer II of the entorhinal cortex (EC), the neuronal clumps of the external principal layer of the presubiculum (PrS) and the dentate granule cells of the dentate gyrus (DG), which represent the prominent structural regions within the HF circuit. Subjects from two age groups (younger or older than 65 years) were studied and their neuronal size assessed by the point-sampled intercepts stereological method. The quantitative ${\overline{v}}_v\left(soma\right)$ estimate was a volume of roughly 8,500 µm³ for EC layer II neurons, and DG granule neurons and presubicular neurons were five and 10 times smaller, respectively. The older age group showed a ${\overline{v}}_v\left(soma\right)$ increase of 2%, 18% and 28% with respect to the younger group in the PrS, DG and EC regions, respectively. None of these regions showed interhemispheric differences. This quantitative estimation is relevant because the observed variance in the ${\overline{v}}_v\left(soma\right)$ estimates suggests that biological variation is the main contributory factor, with intercepts and measurements having a smaller impact. Therefore, we suggest that age has a limited influence on neuronal volume variation in these HF regions, which needs to be compared with similar measurements in neurodegenerative disorders such as Alzheimer's.