Neuronal and Glial Cell Number in the Hippocampus after Experimental Traumatic Brain Injury: Analysis by Stereological Estimation

Fluid percussion (FP) brain injury causes spatial memory dysfunction in rats regardless of injury location (midline vs. lateral). Standard histological analysis of the injured brains shows hippocampal neuronal loss after lateral, but not midline FP injury. We have used the optical volume fractionator (OVF) stereological procedure to quantify neuronal loss and glial proliferation within specific subregions of the hippocampus after midline or lateral FP injury. The OVF method is a design-based cell counting procedure, which combines cellular numerical density estimates (from the optical disector) with volume estimates (generated by point counting and the fractionator stereology method) to produce an estimate of the absolute cell number. Fifteen adult male Sprague-Dawley rats were randomly divided into 3 groups (n = 5/group): midline injury, lateral injury and naive. A single fluid percussion pulse was delivered to anesthetized rats in the injured groups. At 14 days post-injury, strict morphological criteria enabled the estimation of neurons, astrocytes, oligodendrocytes, and microglia in defined hippocampal subregions. The results confirm that hippocampal neurons are selectively vulnerable to brain injury, particularly observed as a significant loss in the hilus following both types of injury and in area CA3 after lateral injury. In contrast, the number of astrocytes and oligodendrocytes remains unaffected by brain injury, regardless of subregion. However, the significant increase in microglia number (bilaterally after midline and ipsilateral following lateral injury) suggests that underlying cellular processes continue weeks following injury. The implications of the observed cell population changes are discussed in relation to the reported cognitive deficits associated with both lateral and midline FP brain injury.