The neocortex of the dog. 1. A classical cytoarchitectonic map

Evolution of prefrontal cortex

Subdivisions of the prefrontal cortex (PFC) evolved at different times. Agranular parts of the PFC emerged in early mammals, and rodents, primates, and other modern mammals share them by inheritance. These are limbic areas and include the agranular orbital cortex and agranular medial frontal cortex (areas 24, 32, and 25). Rodent research provides valuable insights into the structure, functions, and development of these shared areas, but it contributes less to parts of the PFC that are specific to primates, namely, the granular, isocortical PFC that dominates the frontal lobe in humans. The first granular PFC areas evolved either in early primates or in the last common ancestor of primates and tree shrews. Additional granular PFC areas emerged in the primate stem lineage, as represented by modern strepsirrhines. Other granular PFC areas evolved in simians, the group that includes apes, humans, and monkeys. In general, PFC accreted new areas along a roughly posterior to anterior trajectory during primate evolution. A major expansion of the granular PFC occurred in humans in concert with other association areas, with modifications of corticocortical connectivity and gene expression, although current evidence does not support the addition of a large number of new, human-specific PFC areas.

The neocortex of cetaceans: cytoarchitecture and comparison with other aquatic and terrestrial species

The evolutionary process of readaptation to the aquatic environment was accompanied by extreme anatomical and physiological changes in the brain. This review discusses cortical specializations in the three major lineages of marine mammals in comparison to related terrestrial and semiaquatic species. Different groups of marine mammals adopted a wide range of strategies to cope with the challenges of aquatic living. Cetaceans and hippopotamids possess a completely agranular neocortex in contrast to phocids and sirenians; vertical modules are observed in deep layers V and VI in manatees, cetaceans, phocids, and hippopotamids, but in different cortical areas; and clustering in layer II appears in the insular cortex of hippopotamids, phocids, and cetaceans. Finally, von Economo neurons are present in cetaceans, hippopotamids, sirenians, and some phocids, with specific, yet different, cortical distributions. The interpretation of the evolutionary and functional significance of such specializations, and their relationships with the degrees of adaptation to the aquatic environment and phylogeny, remain difficult to trace, at least until comprehensive data, including representative species from all of the major mammalian families, become available.

The insular cortex: a comparative perspective

The human insular cortex is involved in a variety of viscerosensory, visceromotor, and interoceptive functions, and plays a role in complex processes such as emotions, music, and language. Across mammals, the insula has considerable morphologic variability. We review the structure and connectivity of the insula in laboratory animals (mouse, domestic cat, macaque monkey), and we present original data on the morphology and cytoarchitecture of insular cortex in less common species including a large carnivore (the Atlantic walrus, Odobenus rosmarus), two artiodactyls (the pigmy hippopotamus, Hexaprotodon liberiensis, and the Western bongo, Tragelaphus eurycerus), two cetaceans (the beluga whale, Delphinapterus leucas, and the minke whale, Balaenoptera acutorostrata), and a sirenian (the Florida manatee, Trichechus manatus latirostris). The insula shows substantial variability in shape, extent, and gyral and sulcal patterns, as well as differences in laminar organization, cellular specialization, and structural association with the claustrum. Our observations reveal that the insular cortex is extremely variable among mammals. These differences could be related to the role exerted by specific and selective pressures on cortical structure during evolution. We conclude that it is not possible to identify a general model of organization for the mammalian insular cortex.

A cytoarchitectonic and myeloarchitectonic study of the insular cortex of the bull, Bos taurus

The mammalian Insula is characterised by considerable morphologic variability, however, it shows a cytoarchitectonic homogeneity within the species so far studied. Three cytoarchitectonic areas are generally recognisable in the Insula: an "agranular", a "dysgranular" and a "granular" area. The numerous functions attributed to the Insula (visceral sensory, visceral motor, somatosensorial, supplementary motor area, area speech and/or language related, etc.) have recently attracted renewed interest. Considering the involvement of the Insula in the control of gastroenteric motility and the structural and functional complexity of this region in ruminants, it seemed interesting to analyse the cyto- and myelo-architectonic arrangement of the Insula of the bull, Bos taurus. Unlike that of the other species described, all the insular Neocortex of the bull, is of the "agranular" type. The latter includes at least four variants that differ according to the organisation of the cortical layer pattern. Considering the available evidence in laboratory animals and primates of a role played by the agranular insular cortex in gastroenteric motility control, the trademark presence of a very much extended insular agranular cortex in the Insula of the bull may be related to the necessity of motor activity control of a gastroenteric complex that is extensively more developed than in the monogastric species.