Electron microscopic study of Golgi-impregnated and gold-toned neurons and fibers in the claustrum of the cat

The claustrum is a subcortical nucleus found in the telencephalon of all placental mammals. It is a symmetrical, thin and irregular sheet of grey matter which lies between the inner surface of the insular cortex and the outer surface of the putamen. The claustrum has extensive connections with the visual, auditory, somatosensory and motor regions of the cortex, as well as with subcortical and allocortical regions. The aim of this study was to provide a detailed description of the morphology of different types of Golgi-impregnated and gold-toned neurons and fibers in the dorsal claustrum of the cat employing the combined Golgi-electron microscope Fairén method. We were able to distinguish two major types of neurons: those with dendritic spines (spiny) and those without dendritic spines (aspiny). In both groups we observed large (21–40 µm in diameter), medium-sized (16–21 µm in diameter) and small cells (10–16 µm in diameter), describing their ultrastructural organization and characteristic features, including the presence of terminal boutons. These ultrastructural findings allow us to conclude that large and medium-sized spiny claustral neurons are indeed efferent neurons, projecting to the cortex, while the small spiny and the different types of aspiny neurons are most likely inhibitory local circuit interneurons. The findings in the present study will hopefully contribute to a better understanding of the role of the claustrum.