Large spindle-shaped neurons in the anterior insula in lesser apes and monkeys

We report the presence of a large spindle-shaped neuron in layer 5b in the agranular insula in several different species of Old World monkeys and in two species of lesser apes. The Old World monkeys included five rhesus and five cynomolgus macaques, two baboons, two mangabeys, two langurs, one colobus, one blue monkey and one patas monkey. The lesser apes included two gibbons and one siamang. The large spindle-shaped neuron found in all these species shared 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, the large spindle-shaped neuron in the monkeys, gibbon and siamang had an elongate perikaryon that was symmetrical about its height and width; had a unique basal dendrite that was proximally as thick as its apical dendrite; was as large or larger than local pyramidal neurons and much larger than the small fusiform neurons in layer 6; was consistently mingled with fork neurons; and was located, as in humans, in a restricted portion of the agranular insula, anterior to the limen and medial to the superior limiting sulcus of the insula. Taken together with evidence from a more detailed analysis in the macaque monkey (Forro et al., this meeting), we conclude that the large-spindle shaped neuron found in the Old World monkeys and lesser apes is anatomically homologous with the hominoid VEN. A preliminary examination of the anterior agranular insula in several species of New World monkeys did not reveal the presence of the large spindle-shaped neuron, except for a few isolated neurons found in one spider monkey. The present findings suggest that the VEN emerged much earlier than previously proposed and was present already at least in a common ancestor of the cercopithecoids and hominoids. Although being much less numerous and smaller than in humans, the VEN in monkeys and lesser apes likely shares some primal functions and connections with the VEN in humans. Future comparative and quantitative analyses of the distribution of the VEN across primate species living in different ecological niches and having different behaviors and social organizations might provide valuable information regarding the evolutive mechanisms that lead to the marked development and crucial role of VEN in humans