A Comparison of the Cortical Structure of the Bowhead Whale (Balaena mysticetus), a Basal Mysticete, with Other Cetaceans

Abstract

Few studies exist of the bowhead whale brain and virtually nothing is known about its cortical cytoarchitecture or how it compares to other cetaceans. Bowhead whales are one of the least encephalized cetaceans and occupy a basal phylogenetic position among mysticetes. Therefore, the bowhead whale is an important specimen for understanding the evolutionary specializations of cetacean brains. Here, we present an overview of the structure and cytoarchitecture of the bowhead whale cerebral cortex gleaned from Nissl‐stained sections and magnetic resonance imaging (MRI) in comparison with other mysticetes and odontocetes. In general, the cytoarchitecture of cetacean cortex is consistent in displaying a thin cortex, a thick, prominent layer I, and absence of a granular layer IV. Cell density, composition, and width of layers III, V, and VI vary among cortical regions, and cetacean cortex is cell‐sparse relative to that of terrestrial mammals. Notably, all regions of the bowhead cortex possess high numbers of von Economo neurons and fork neurons, with the highest numbers observed at the apex of gyri. The bowhead whale is also distinctive in having a significantly reduced hippocampus that occupies a space below the corpus callosum within the lateral ventricle. Consistent with other balaenids, bowhead whales possess what appears to be a blunted temporal lobe, which is in contrast to the expansive temporal lobes that characterize most odontocetes. The present report demonstrates that many morphological and cytoarchitectural characteristics are conserved among cetaceans, while other features, such as a reduced temporal lobe, may characterize balaenids among mysticetes. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:745–760, 2019. © 2018 Wiley Periodicals, Inc.

Transcriptomic evidence that von Economo neurons are regionally specialized extratelencephalic-projecting excitatory neurons

“…von Economo neurons (VENs) are a morphologically-defined neuron type with a large, characteristic spindle-shaped cell body, thick bipolar dendrites with limited branching and a moderate density of spines, and often an axon initial segment that emanates from the side of the cell body 1,2,3 . VENs have been described in several large-brained mammals, such as humans, great apes, macaques, cetaceans, cows, and elephants, but not in rodents 4,5,6,7,8,9,10,1,11 . In humans, they are restricted to the anterior cingulate (ACC), frontoinsular (FI), and medial frontopolar regions of cerebral cortex 12 , while in most other species they are also found in the frontal and occipital poles 13 and may not be restricted to layer 5.…”

Higher neuron densities in the cerebral cortex and larger cerebellums may limit dive times of delphinids compared to deep-diving toothed whales

“…Layer II of the odontocete cortex is characterized by high neuron density and is potentially similar in function to the external granular layer of terrestrial mammals [28]. Clustering of neocortical neurons in layer II of the cortex is present in all odontocetes and in mysticetes, such as humpback and fin whales [26], but not in one mysticete, the bowhead whale [33]. Large cortex surface area is a feature of many cetacean brains [34–36].…”

Transcriptomic evidence that von Economo neurons are regionally specialized extratelencephalic-projecting excitatory neurons

“…They possess thick bipolar dendrites with limited branching and a moderate density of spines, and often have an axon initial segment that emanates from the side of the cell body [1][2][3] . VENs have been described in several large-brained mammals, such as humans, great apes, macaques, cetaceans, cows, and elephants, but not in rodents 1,[4][5][6][7][8][9][10][11] . In humans, they are restricted to the anterior cingulate (ACC), frontoinsular (FI), and medial frontopolar regions of cerebral cortex 12 , while in some other species they are also found in the frontal and occipital poles 13 and may not be restricted to layer 5.…”