Elemental localization of the von Economo neuron and fork neuron in the insular cortex in humans and macaque monkeys

Abstract

The cerebral cortex of mammalian species is parcellated into discrete areas, or modules. Some of these modules share common features amongst most mammals; however, between higher and lower mammalian species, deviations in size, subdivisions, cellular features and connectivity of these modules exist. The cortex of higher mammals like primates, has expanded and has subdivided many times resulting in a complex neural system built of smaller modules acting in concert and forming functional networks. In line with this, in primates, the insular cortex has been shown to consist of smaller, unique modules, each having a specific structure and function. Furthermore, the research presented here, identified that in the anterior agranular and the dysgranular insula of macaque monkeys one of each of these architectonic area harbors two specialized neuronal morphotypes, the von Economo neurons (VEN) and the fork neurons (FN), whilst in the human anterior insula there are five distinct architectonic areas hosting these neurons. More than a century ago, Brodmann defined this kind of containment of specialized neuronal morphotypes with a specific cytoarchitectonic area of the primate cerebral cortex as “elemental localization”, because it suggests shared evolutional, developmental, and functional features between the specialized neurons and their host area. The discovery of novel elemental localizations within the primate brain 1) supports the theory that the ventral anterior insular cortex as well as the adjacent orbital prefrontal cortex are heterogeneously organized into smaller areas rather than being lumped together into a larger, homogeneous periallocortical (agranular) sector, and 2) provides an exclusive experimental advantage for the examination of the VEN areas and its two specialized neuronal morphotypes, VEN and FN, which are linked to internal bodily state representation and neuropsychiatric disorders.