The marmoset: An emerging model to unravel the evolution and development of the primate neocortex

Throughout evolution, the neocortex has undergone a dramatic expansion providing the substrate for increasingly complex cognitive abilities, culminating with humans. The enlargement of the neocortex did not affect its' basic organization, which is remarkably conserved from rodents to primates. The mouse has thus proven an advantageous model to decipher the molecular and cellular mechanisms supporting neocortical development. However, it is of limited benefit when studying the mechanisms leading to the inclusion of higher order association areas, which form the largest fraction of the primate neocortex. In the quest for a suitable nonhuman primate model to study the developmental mechanism of neocortical patterning and expansion, researchers focussed on the Old World macaque, routinely employed in functional and behavioral studies. However, the species has many limitations making studies difficult and/or impractical. Therefore, in the past couple of decades, the New World common marmoset (Callithrix jacchus) has drawn much attention and become an accepted model. The marmoset has the advantage of a smooth neocortical sheet, enabling the direct correspondence between developing cortices and the comprehensive map established for the adult, with a significant amount of cortical maturation occurring during the postnatal period. This review presents the contributions of recent marmoset studies to our understanding of the mechanisms regulating corticogenesis in a complex species, the molecular control of neocortical patterning and the sequential maturation of visual cortical areas while commenting on the future of the species in the field. Furthermore, while these new findings are relevant to developmental biology, in order to understand how new cortical areas have emerged to expand cognitive abilities, they also represent a foundation for the better understanding of developmental cognitive disorders.