Dendritic and Spine Heterogeneity of von Economo Neurons in the Human Cingulate Cortex

Abstract

The human cingulate cortex (CC), included in the paralimbic cortex, participates in emotion, visceral responses, attention, cognition, and social behaviors. The CC has spindle-shaped/fusiform cell body neurons in its layer V, the von Economo neurons (VENs). VENs have further developed in primates, and the characterization of human VENs can benefit from the detailed descriptions of the shape of dendrites and spines. Here, we advance this issue and studied VENs in the anterior and midcingulate cortex from four neurologically normal adult subjects. We used the thionin technique and the adapted "single-section" Golgi method for light microscopy. Three-dimensional (3D) reconstructions were carried out for the visualization of Golgi-impregnated VENs' cell body, ascending and descending dendrites, and collateral branches. We also looked for the presence, density, and shape of spines from proximal to distal dendrites. These neurons have a similar aspect for the soma, but features of spiny dendrites evidenced a morphological heterogeneity of CC VENs. Only for the description of this continuum of shapes, we labeled the most common feature as VEN 1, which has main dendritic shafts but few branches and sparse spines. VEN 2 shows an intermediate aspect, whereas VEN 3 displays the most profuse dendritic ramification and more spines with varied shapes from proximal to distal branches. Morphometric data exemplify the dendritic features of these cells. The heterogeneity of the dendritic architecture and spines suggests additional functional implications for the synaptic and information processing in VENs in integrated networks of normal and, possibly, neurological/psychiatric conditions involving the human CC.

The Subcortical-Allocortical- Neocortical continuum for the Emergence and Morphological Heterogeneity of Pyramidal Neurons in the Human Brain

“…“Inverted” pyramidal neurons in neocortical layer VI display an “apical” dendrite directed toward the white matter ( Feldman, 1984 ; Steger et al, 2013 ). Other neurons were also considered variations (or specializations) of pyramidal neurons, such as the Meynert-Cajal cells in layer IVb of the primary visual cortex ( Hof and Morrison, 1990 ), the Meynert neurons in layer Vb of the striate area ( Braak, 1980 ), the large Betz cells in layer Vb of the primary motor cortex ( Braak, 1980 ; Feldman, 1984 ), and the von Economo neurons (VENs) in the frontoinsular and anterior cingulate cortices, for example ( Butti et al, 2013 ; Banovac et al, 2019 ; for VENs particularities see also Cauda et al, 2014 ; Yang et al, 2019 ; Correa-Júnior et al, 2020 ). Then, it is conceivable that the term “pyramidal neuron” might refer to a variety of shapes ranging from “classic” pyramidal, “pyramidal-like,” and “modified pyramids” with variable size, somatic shape, dendritic branching pattern, length, and orientation in the neuropil.…”

“…According to morphological features, spines have been classified as stubby, wide, thin, mushroom-like, ramified, with a transitional aspect between these classes (as ‘protospines’ or ‘multispines,’ García-López et al, 2010 ), or “atypical” (also “multimorphic”) with a variety of different shapes, which includes double spines, spines with racemose appendages (with a lobed appearance and various bulbous enlargements and heads), and thorny excrescences (densely packed outgrowths showing fairly large spines with various round heads grouped around the stems) ( Fiala and Harris, 1999 ; Arellano et al, 2007a ; González-Burgos et al, 2012 ; González-Ramírez et al, 2014 ; Stewart et al, 2014 ; Dall’Oglio et al, 2015 ; Correa-Júnior et al, 2020 ). Small protrusions extending from the spine are spinules ( Brusco et al, 2014 ; Vásquez et al, 2018 ), which are active zone-free invaginating structures that can participate in synaptic plasticity, including long-term potentiation ( Petralia et al, 2018 ).…”

“…The morphological complexity of human pyramidal neurons varies from their emergence in the cortical (CoA) and basomedial (BM, but not in the MeA) nuclei of the amygdaloid complex toward the CA3 and CA1 hippocampal regions and the neocortical layers II–VI, with small pyramidal neurons in the upper layers II/III and large pyramidal neurons in the deep layer V. The following images were obtained with the Golgi-impregnation method adapted for the human postmortem brain ( Dall’Oglio et al, 2010 , 2013 , 2015 ; Vásquez et al, 2018 ). Afterward, we proceeded to the 3D reconstruction of pyramidal neurons aiming further visualization and detailing of the dendritic spines from proximal to distal branches ( Reberger et al, 2018 ; Vásquez et al, 2018 ; Correa-Júnior et al, 2020 ). Methodological advantages and technical constraints were outlined in previous reports (e.g., de Ruiter, 1983 ; Anderson et al, 2009 ; Dall’Oglio et al, 2010 , 2013 , Morales et al, 2014 ; Mohan et al, 2015 ; Tønnesen and Nägerl, 2016 ; Reberger et al, 2018 ).…”

“…Nevertheless, by forming part of the neural circuitry for complex social processing, the cingulate cortex is much more than a primitive stage of cortical evolution ( Allman et al, 2001 ; see further data in Vogt, 2015 ). It is a cytoarchitectonic and functional specialization of the neocortex with participation in emotion, interoceptive and visceral modulation, attention, cognition, and complex perceptions as self-awareness ( Allman et al, 2001 ; Butti et al, 2013 ; Cauda et al, 2014 ; Correa-Júnior et al, 2020 and references therein). Likewise, the lateral parietal lobe adjacent to allocortical structures represents an evolved neocortical structure with primary, associative, and multimodal distributed functions ( Nieuwenhuys et al, 1988 ; Miller and Vogt, 1995 ; DeFelipe, 2011 ; Pandya et al, 2015 ; Kolb and Whishaw, 2015 ).…”

“…Pyramidal neurons have been studied using complementary techniques, from the Nissl (or thionine) staining and the Golgi silver impregnation procedure to different approaches for intracellular microinjection of fluorescent dyes, serial sections for ultrastructural connectional and neurochemical profiles, in vitro and in vivo electrophysiological recordings, computational and in silico models ( Ramón y Cajal, 1909–1911 ; Lorente de Nó, 1934 ; Szentágothai, 1978 ; Mountcastle, 1979 ; Braak, 1980 ; Peters and Jones, 1984 ; Sims and Williams, 1990 ; Peters et al, 1991 ; McCormick et al, 1993 ; Segev et al, 1995 ; Somogyi et al, 1998 ; Valverde et al, 2002 ; Andersen et al, 2007 ; Spruston, 2008 ; Larriva-Sahd, 2010 ; Ramaswamy and Markram, 2015 ; Eyal et al, 2018 ; Soltesz and Losonczy, 2018 ; Cembrowski and Spruston, 2019 ; Oruro et al, 2019 ; Benavides-Piccione et al, 2020 ). For example, the Golgi method adapted for formalin-fixed human brain and light microscopy provides images of pyramidal dendrites and spine shapes from different cortical and subcortical regions ( Dall’Oglio et al, 2010 ; Reberger et al, 2018 ; Vásquez et al, 2018 ; Correa-Júnior et al, 2020 ). This can eventually add fundamental data to identify the brain cellular components and their connectivity toward physiology and behavior, as well as for modeling and theory approaches on neural structure and integrated functions of human brain areas (see a current discussion in Zeng, 2020 ).…”