Cellular organization of cortical barrel columns is whisker-specific

Abstract

The cellular organization of the cortex is of fundamental importance for elucidating the structural principles that underlie its functions. It has been suggested that reconstructing the structure and synaptic wiring of the elementary functional building block of mammalian cortices, the cortical column, might suffice to reverse engineer and simulate the functions of entire cortices. In the vibrissal area of rodent somatosensory cortex, whisker-related "barrel" columns have been referred to as potential cytoarchitectonic equivalents of functional cortical columns. Here, we investigated the structural stereotypy of cortical barrel columns by measuring the 3D neuronal composition of the entire vibrissal area in rat somatosensory cortex and thalamus. We found that the number of neurons per cortical barrel column and thalamic "barreloid" varied substantially within individual animals, increasing by ∼2.5-fold from dorsal to ventral whiskers. As a result, the ratio between whisker-specific thalamic and cortical neurons was remarkably constant. Thus, we hypothesize that the cellular architecture of sensory cortices reflects the degree of similarity in sensory input and not columnar and/or cortical uniformity principles.

Keywords: GAD67; NeuN; VPM; barrel cortex; soma counts.

Fig. 1.

Automated detection of all excitatory and inhibitory neuron somata in the vibrissal areas of rat somatosensory cortex and thalamus. (A) Large-scale, high-resolution confocal image stacks from 50-µm-thick brain sections, cut tangentially to the cortical surface from the pia to the WM. (Left) GAD67 projection images allow delineation of anatomical reference structures (red) in each section, such as pia, L4 barrels, and WM. (Right) NeuN projection images in the same sections. (B) Confocal image stacks from 50-µm-thick sections of vibrissal thalamus, cut tangentially from the dorsal medial (d.m.) to the ventral lateral (v.l.) direction. (Left) GAD67 projection images allow delineation of anatomical reference structures (red), such as RT (Left), VPM (Right), and individual barreloids. (Right) NeuN projection images in the same sections. (C) Optical section of GAD67 image stack from A (*) superimposed with landmarks representing automatically detected inhibitory somata (red). (D) NeuN-positive somata were automatically detected within the same area. (E) Area from B (*) with automatically detected NeuN-positive neuron somata. Brightness has been adjusted in all panels for visualization purposes.

Fig. 2.

Whisker-specific laminar cellular organization of rat vibrissal cortex. (A) Tangential view of all neuron somata in the vibrissal cortex of one animal. Somata are assigned to their closest barrel column [row colors: A (red), B (pink), C (yellow), D (green), E (blue), Greek arc (gray)] or to the septum (white). *D5 barrel column excluded from analysis. (B) Semicoronal view of the somata within the dashed region in A. A 3D reconstruction of pia and WM surfaces allows for determining the position of all neuron somata with respect to cytoarchitectonic layer borders (20). (C) A 2D average projection of the 3D excitatory neuron density. L4 barrels are clearly visible as segregated spots of high neuron density (24). (D) A 2D average projection of the 3D inhibitory neuron density. Segregation between barrels and septa is not evident. L2 and upper L5 are separable as bands of high inhibitory neuron density, as reported previously (22). (E) Average distribution of excitatory and inhibitory neuron somata along the vertical column axis for columns in arc-2. Shaded regions are ±1 SD. Dashed lines represent column-specific layer borders. (F) Average distribution of excitatory/inhibitory somata across all barrel columns and septa.

Fig. 3.

Whisker-specific horizontal cellular organization of rat vibrissal cortex. (A) Average volume per barrel column based on the four vibrissal cortices analyzed, showing a significant increase from the A- to the E-row. (B) Average number of all neurons (excitatory and inhibitory) per barrel column, increasing from the A- to the E-row concomitantly with the column volume. (C) The average neuron density (excitatory and inhibitory) per barrel column is constant across the barrel field and larger than septal neuron density (box). (D) The average fraction of inhibitory neurons (IN) is similar across the barrel field and does not differ between columns and the septum (box). (E) The average distribution of excitatory/inhibitory neurons in different cortical layers, measured along arc-2 (from left to right: A2–E2), shows a clear separation into barrel columns and septa only in L4, where excitatory neurons delineate the barrels. (F) The average distribution of excitatory/inhibitory neurons in different cortical layers, measured along the C-row (from left to right: C1–C4), is not indicative of a separation between barrel columns and septa at the cellular level.

Fig. 4.

The ratio between whisker-specific cortical and thalamic neurons is constant. (A) The average volume per barreloid across VPM increases from the A- to the E-row, as in vibrissal cortex. (B) The average number of neurons per barreloid also increases from the A- to the E-row. (C) The average neuron density per barreloid increases from the E- toward the A-row. (D) The average ratio between the number of neurons per barrel and corresponding barreloid is highly preserved. (E) Relationship between the number of neurons per barreloid in VPM and the number of neurons in the corresponding barrel is linear. Error bars are ±1 SD. (F) The linear relationship between neurons per barreloid and the respective number of target neurons in cortex is more pronounced for granular L4, compared with supra- and infragranular layers.