Cytoarchitecture, areas, and neuron numbers of the Etruscan Shrew cortex

Abstract

The Etruscan shrew, Suncus etruscus, is one of the smallest mammals. Etruscan shrews can recognize prey shape with amazing speed and accuracy, based on whisker-mediated tactile cues. Because of its small size, quantitative analysis of the Etruscan shrew cortex is more tractable than in other animals. To quantitatively assess the anatomy of the Etruscan shrew's brain, we sectioned brains and applied Nissl staining and NeuN (neuronal nuclei) antibody staining. On the basis of these stains, we estimated the number of neurons of 10 cortical hemispheres by using Stereoinvestigator and Neurolucida (MBF Bioscience) software. On average, the neuron number per hemisphere was found to be ~1 million. We also measured cortical surface area and found an average of 11.1 mm² (n = 7) and an average volume of 5.3 mm³ (n = 10) per hemisphere. We identified 13 cortical regions by cytoarchitectonic boundaries in coronal, sagittal, and tangential sections processed for Nissl substance, myelin, cytochrome oxidase, ionic zinc, neurofilaments, and vesicular glutamate transporter 2 (VGluT2). The Etruscan shrew is a highly tactile animal with a large somatosensory cortex, which contains a barrel field, but the barrels are much less clearly defined than in rodents. The anatomically derived cortical partitioning scheme roughly corresponds to physiologically derived maps of neocortical sensory areas.

Response to Comment on “Universality in the Evolution of Orientation Columns in the Visual Cortex“

“…Meng et al suggest greater variation in pinwheel density across species; however, those data were derived from studies with small sample sizes using first-generation optical imaging methods that are subject to various systematic errors described in detail in [SOM of (1), figures S2 and S3 and pp. [3][4][5][6][7][8][9][10][11][12]. Our analysis unambiguously indicates that pinwheel density is a genuine invariant feature of orientation column layout over a wide range of V1 sizes and that it is insensitive to hypercolumn size (Fig.…”

“…Siegrid Löwel, 7,8 David M. Coppola, 9 Leonard E. White, 10 Fred Wolf 1,2,7 * Meng et al conjecture that pinwheel density scales with body and brain size. Our data, spanning a 40-fold range of body sizes in Laurasiatheria and Euarchonta, do not support this conclusion.…”

“…They conjecture a pinwheel density scaling law that can be approximated by r = 3/2log 10 , and 450 mm 2 V1 size, respectively (3,4), pinwheel density in cats (galagos) should be 53% (28%) larger than in ferrets/tupaias. Our methods to accurately estimate pinwheel density are well suited to test this prediction.…”

“…For species in which V1 size approaches this range, it becomes hard to conceive of V1 as being organized by a system of orientation columns. A size constraint prohibiting a conventional system of orientation columns should, for instance, apply to the V1 of etruscan shrews (Suncus etruscus), which is confined to a mere 0.2 mm 2 of visual cortex (10). It also appears plausible for the last common ancestor of carnivores and primates, called the boreoeutherian ancestor [SOM of (1), pp.…”

The evolution of brains from early mammals to humans

“…By comparing brain and especially cortical organization across current members of the six major branches, and seeing what features are common, we can infer that these features were likely retained from a common early mammal ancestor, and when some specializations were lost and others acquired [2,26]. Common features are perhaps easiest to discover in small-brained mammals, since there is less brain and cortex to explore, but small-brained present-day mammals could also have simpler brains than their ancestors as features were lost when smaller brains sometimes evolved from ancestors with larger brains [27] [28]. In addition, early mammals needed to have some parts of their nervous systems develop very early since they hatched from eggs, as in present-day monotremes, or were born early in development, as in present-day marsupials, so that they could grasp maternal hair and nurse [29].…”