Distribution of a large spindle-shaped neuron in the anterior agranular insula in the macaque monkey

We report the presence of a large spindle-shaped neuron in layer 5b in the anterior agranular insula in the rhesus and cynomolgus macaque monkeys. This neuron shares numerous characteristics with the von Economo neuron (VEN), a large spindle-shaped neuron that is present in layer 5b in the anterior insula in humans and great apes and that appears to have a crucial role in self-awareness and social cognition in humans. Thus, like the VEN, the large spindle-shaped neuron in the macaque has an elongate perikaryon that is symmetrical about its height and width; has a unique basal dendrite that is proximally as thick as its apical dendrite; is larger or as large as the local pyramidal neurons and much larger than the fusiform neuron in layer 6; and expresses the neurofilament protein SMI-32 and the serotonin receptor 2b. Golgi material reveals that its dendrites have all the characteristics of a projecting neuron; and it is retrogradely labeled with neuronal tracer injected at a distant site. It is consistently mingled with fork neurons. Finally, the portion of the insula that contains this spindle-shaped neuron in the macaque is located, like in humans and great apes, in a portion of the agranular insula that is anterior to the limen and medial to the superior sulcus of the insula. We conclude that the large spindle-shaped neuron in the macaque insula is anatomically homologous with the hominoid VEN, and that the anterior agranular insula in the macaque is at least in part anatomically homologous with the human anterior insula (or frontoinsula). Further evidence from this laboratory indicates that the VEN is also present in a similar portion of the anterior agranular insula in other species of monkeys and in lesser apes (Evrard et al., this meeting). The VEN is smaller and much less frequent in the macaque than in human; it is therefore unlikely that its role in the macaque is as evolved as it appears to be in human. Nevertheless, the present demonstration offers a unique opportunity to examine in the laboratory the hodology and primal function of a brain region that appears to be central to human self-awareness and in which malformation, lesion or degeneration have a dramatic impact on human bodily and emotional feelings as well as social interaction