Barreloids in adult rat thalamus: Three‐dimensional architecture and relationship to somatosensory cortical barrels

Histochemical staining for cytochrome oxidase (CO) and axonal transport of horseradish peroxidase (HRP) were used to investigate thalamocortical connections in the vibrissa‐barrel system of adult rats. CO staining revealed that the medial division of the ventrobasal thalamic nucleus (VBm) consists of intensely stained rod‐like configurations, containing thalamocortical projection neurons and intervening neuropil, separated by lighter‐stained septa. CO‐dark rods span the thickness of VBm, are arranged in a pattern of rows and arcs that resembles the distribution of vibrissae on the mystacial pad, and are similar to the cytoarchitectonic structures termed “barreloids” in the mouse thalamus. Based upon the dimensions of CO‐dark structures and the numerical density of neurons in VBm we estimated that a barreloid in the rat may contain 250‐300 neurons.

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