"Rodent-like" and "primate-like" types of astroglial architecture in the adult cerebral cortex of mammals: a comparative study

Evolution of astrocytes: From invertebrates to vertebrates

The central nervous system (CNS) shows incredible diversity across evolution at the anatomical, cellular, molecular, and functional levels. Over the past decades, neuronal cell number and heterogeneity, together with differences in the number and types of neuro-active substances, axonal conduction, velocity, and modes of synaptic transmission, have been rigorously investigated in comparative neuroscience studies. However, astrocytes, a specific type of glial cell in the CNS, play pivotal roles in regulating these features and thus are crucial for the brain's development and evolution. While special attention has been paid to mammalian astrocytes, we still do not have a clear definition of what an astrocyte is from a broader evolutionary perspective, and there are very few studies on astroglia-like structures across all vertebrates. Here, I elucidate what we know thus far about astrocytes and astrocyte-like cells across vertebrates. This information expands our understanding of how astrocytes evolved to become more complex and extremely specialized cells in mammals and how they are relevant to the structure and function of the vertebrate brain.

Light microscopic and heterogeneity analysis of astrocytes in the common marmoset brain

Astrocytes are abundant cells of the central nervous system (CNS) and are involved in processes including synapse formation/function, ion homeostasis, neurotransmitter uptake, and neurovascular coupling. Recent evidence indicates that astrocytes show diverse molecular, structural, and physiological properties within the CNS. This heterogeneity is reflected in differences in astrocyte structure, gene expression, functional properties, and responsiveness to injury/pathological conditions. Deeper investigation of astrocytic heterogeneity is needed to understand how astrocytes are configured to enable diverse roles in the CNS. While much has been learned about astrocytic heterogeneity in rodents, much less is known about astrocytic heterogeneity in the primate brain where astrocytes have greater size and complexity. The common marmoset (Callithrix jacchus) is a promising non‐human primate model because of similarities between marmosets and humans with respect to genetics, brain anatomy, and cognition/behavior. Here, we investigated the molecular and structural heterogeneity of marmoset astrocytes using an array of astrocytic markers, multi‐label confocal microscopy, and quantitative analysis. We used male and female marmosets and found that marmoset astrocytes show differences in expression of astrocytic markers in cortex, hippocampus, and cerebellum. These differences were accompanied by intra‐regional variation in expression of markers for glutamate/GABA transporters, and potassium and water channels. Differences in astrocyte structure were also found, along with complex interactions with blood vessels, microglia, and neurons. This study contributes to our knowledge of the cellular and molecular features of marmoset astrocytes and is useful for understanding the complex properties of astrocytes in the primate CNS.

EAAT2 Expression in the Hippocampus, Subiculum, Entorhinal Cortex and Superior Temporal Gyrus in Alzheimer's Disease

Alzheimer’s disease (AD) is a neuropathological disorder characterized by the presence and accumulation of amyloid-beta plaques and neurofibrillary tangles. Glutamate dysregulation and the concept of glutamatergic excitotoxicity have been frequently described in the pathogenesis of a variety of neurodegenerative disorders and are postulated to play a major role in the progression of AD. In particular, alterations in homeostatic mechanisms, such as glutamate uptake, have been implicated in AD. An association with excitatory amino acid transporter 2 (EAAT2), the main glutamate uptake transporter, dysfunction has also been described. Several animal and few human studies examined EAAT2 expression in multiple brain regions in AD but studies of the hippocampus, the most severely affected brain region, are scarce. Therefore, this study aims to assess alterations in the expression of EAAT2 qualitatively and quantitatively through DAB immunohistochemistry (IHC) and immunofluorescence within the hippocampus, subiculum, entorhinal cortex, and superior temporal gyrus (STG) regions, between human AD and control cases. Although no significant EAAT2 density changes were observed between control and AD cases, there appeared to be increased transporter expression most likely localized to fine astrocytic branches in the neuropil as seen on both DAB IHC and immunofluorescence. Therefore, individual astrocytes are not outlined by EAAT2 staining and are not easily recognizable in the CA1–3 and dentate gyrus regions of AD cases, but the altered expression patterns observed between AD and control hippocampal cases could indicate alterations in glutamate recycling and potentially disturbed glutamatergic homeostasis. In conclusion, no significant EAAT2 density changes were found between control and AD cases, but the observed spatial differences in transporter expression and their functional significance will have to be further explored.

Morphometric analysis of astrocytes in vocal production circuits of common marmoset (Callithrix jacchus)

Astrocytes, the star-shaped glial cells, are the most abundant non-neuronal cell population in the central nervous system. They play a key role in modulating activities of neural networks, including those involved in complex motor behaviors. Common marmosets (Callithrix jacchus), the most vocal non-human primate (NHP), have been used to study the physiology of vocalization and social vocal production. However, the neural circuitry involved in vocal production is not fully understood. In addition, even less is known about the involvement of astrocytes in this circuit. To understand the role, that astrocytes may play in the complex behavior of vocalization, the initial step may be to study their structural properties in the cortical and subcortical regions that are known to be involved in vocalization. Here, in the common marmoset, we identify all astrocytic subtypes seen in other primate's brains, including intralaminar astrocytes. In addition, we reveal detailed structural characteristics of astrocytes and perform morphometric analysis of astrocytes residing in the cortex and midbrain regions that are associated with vocal production. We found that cortical astrocytes in these regions illustrate a higher level of complexity when compared to those in the midbrain. We hypothesize that this complexity that is expressed in cortical astrocytes may reflect their functions to meet the metabolic/structural needs of these regions.

Sleep Disorders in Children With Autism Spectrum Disorder: Insights From Animal Models, Especially Non-human Primate Model

Autism Spectrum Disorder (ASD) is a heterogeneous neurodevelopmental disorder with deficient social skills, communication deficits and repetitive behaviors. The prevalence of ASD has increased among children in recent years. Children with ASD experience more sleep problems, and sleep appears to be essential for the survival and integrity of most living organisms, especially for typical synaptic development and brain plasticity. Many methods have been used to assess sleep problems over past decades such as sleep diaries and parent-reported questionnaires, electroencephalography, actigraphy and videosomnography. A substantial number of rodent and non-human primate models of ASD have been generated. Many of these animal models exhibited sleep disorders at an early age. The aim of this review is to examine and discuss sleep disorders in children with ASD. Toward this aim, we evaluated the prevalence, clinical characteristics, phenotypic analyses, and pathophysiological brain mechanisms of ASD. We highlight the current state of animal models for ASD and explore their implications and prospects for investigating sleep disorders associated with ASD.

Cortical Interlaminar Astrocytes Are Generated Prenatally, Mature Postnatally, and Express Unique Markers in Human and Nonhuman Primates

Interlaminar astrocytes (ILAs) are a subset of cortical astrocytes that reside in layer I, express GFAP, have a soma contacting the pia, and contain long interlaminar processes that extend through several cortical layers. We studied the prenatal and postnatal development of ILAs in three species of primates (rhesus macaque, chimpanzee, and human). We found that ILAs are generated prenatally likely from radial glial (RG) cells, that ILAs proliferate locally during gestation, and that ILAs extend interlaminar processes during postnatal stages of development. We showed that the density and morphological complexity of ILAs increase with age, and that ILAs express multiple markers that are expressed by RG cells (Pax6, Sox2, and Nestin), specific to inner and outer RG cells (Cryab and Hopx), and astrocyte markers (S100β, Aqp4, and GLAST) in prenatal stages and in adult. Finally, we demonstrated that rudimentary ILAs in mouse also express the RG markers Pax6, Sox2, and Nestin, but do not express S100β, Cryab, or Hopx, and that the density and morphological complexity of ILAs differ between primate species and mouse. Together these findings contribute new information on astrogenesis of this unique class of cells and suggest a lineal relationship between RG cells and ILAs.

Cortical interlaminar astrocytes across the therian mammal radiation

Interlaminar astrocytes (ILA) in the cerebral cortex possess a soma in layer I and extend an interlaminar process that runs perpendicular to the pia into deeper cortical layers. We examined cerebral cortex from 46 species that encompassed most orders of therian mammalians, including 22 primate species. We described two distinct cell types with interlaminar processes that have been referred to as ILA, that we termed pial ILA and supial ILA. ILA subtypes differ in somatic morphology, position in layer I, and presence across species. We further described rudimentary ILA that have short GFAP⁺ processes that do not exit layer I, and "typical" ILA with longer GFAP⁺ processes that exit layer I. Pial ILA were present in all mammalian species analyzed, with typical ILA observed in Primates, Scandentia, Chiroptera, Carnivora, Artiodactyla, Hyracoidea, and Proboscidea. Subpial ILA were absent in Marsupialia, and typical subpial ILA were only found in Primate. We focused on the properties of pial ILA by investigating the molecular properties of pial ILA and confirming their astrocytic nature. We found that while the density of pial ILA somata only varied slightly, the complexity of ILA processes varied greatly across species. Primates, specifically bonobo, chimpanzee, orangutan, and human, exhibited pial ILA with the highest complexity. We showed that interlaminar processes contact neurons, pia, and capillaries, suggesting a potential role for ILA in the blood-brain barrier and facilitating communication among cortical neurons, astrocytes, capillaries, meninges, and cerebrospinal fluid.

Deciphering the Astrocyte Reaction in Alzheimer's Disease

Reactive astrocytes were identified as a component of senile amyloid plaques in the cortex of Alzheimer's disease (AD) patients several decades ago. However, their role in AD pathophysiology has remained elusive ever since, in part owing to the extrapolation of the literature from primary astrocyte cultures and acute brain injury models to a chronic neurodegenerative scenario. Recent accumulating evidence supports the idea that reactive astrocytes in AD acquire neurotoxic properties, likely due to both a gain of toxic function and a loss of their neurotrophic effects. However, the diversity and complexity of this glial cell is only beginning to be unveiled, anticipating that astrocyte reaction might be heterogeneous as well. Herein we review the evidence from mouse models of AD and human neuropathological studies and attempt to decipher the main conundrums that astrocytes pose to our understanding of AD development and progression. We discuss the morphological features that characterize astrocyte reaction in the AD brain, the consequences of astrocyte reaction for both astrocyte biology and AD pathological hallmarks, and the molecular pathways that have been implicated in this reaction.

Of Men and Mice: Modeling the Fragile X Syndrome

The Fragile X Syndrome (FXS) is one of the most common forms of inherited intellectual disability in all human societies. Caused by the transcriptional silencing of a single gene, the fragile x mental retardation gene FMR1, FXS is characterized by a variety of symptoms, which range from mental disabilities to autism and epilepsy. More than 20 years ago, a first animal model was described, the Fmr1 knock-out mouse. Several other models have been developed since then, including conditional knock-out mice, knock-out rats, a zebrafish and a drosophila model. Using these model systems, various targets for potential pharmaceutical treatments have been identified and many treatments have been shown to be efficient in preclinical studies. However, all attempts to turn these findings into a therapy for patients have failed thus far. In this review, I will discuss underlying difficulties and address potential alternatives for our future research.

Interlaminar Glia and Other Glial Themes Revisited: Pending Answers Following Three Decades of Glial Research

This review aims to highlight the various significant matters in glial research stemming from personal work by the author and associates at the Unit of Applied Neurobiology (UNA, CEMIC-CONICET), and some of the pending questions. A reassessment and further comments on interlaminar astrocytes—an astroglial cell type that is specific to humans and other non-human primates, and is not found in rodents, is presented. Tentative hypothesis regarding their function and future possible research lines that could contribute to further the analysis of their development and possible role(s), are suggested. The possibility that they function as a separate entity from the “territorial” astrocytes, is also considered. In addition, the potential significance of our observations on interspecies differences in in vitro glial cell dye coupling, on glial diffusible factors affecting the induction of this glial phenotype, and on their interference with the cellular toxic effects of cerebrospinal fluid obtained from l-DOPA treated patients with Parkinson´s disease, is also considered. The major differences oberved in the cerebral cortex glial layout between human and rodents—the main model for studying glial function and pathology—calls for a careful assessment of known and potential species differences in all aspects of glial cell biology. This is essential to provide a better understanding of the organization and function of human and non-human primate brain, and of the neurobiological basis of their behavior.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Of mice and monkeys: using non-human primate models to bridge mouse- and human-based investigations of autism spectrum disorders

The autism spectrum disorders (ASDs) arise from a diverse array of genetic and environmental origins that disrupt the typical developmental trajectory of neural connectivity and synaptogenesis. ASDs are marked by dysfunctional social behavior and cognition, among other deficits. Greater understanding of the biological substrates of typical social behavior in animal models will further our understanding of the etiology of ASDs. Despite the precision and tractability of molecular genetics models of ASDs in rodents, these organisms lack the complexity of human social behavior, thus limiting their impact on understanding ASDs to basic mechanisms. Non-human primates (NHPs) provide an attractive, complementary model for ASDs, due in part to the complexity and dynamics of social structures, reliance on vision for social signaling, and deep homology in brain circuitry mediating social behavior and reward. This knowledge is based on a rich literature, compiled over 50 years of observing primate behavior in the wild, which, in the case of rhesus macaques, is complemented by a large body of research characterizing neuronal activity during cognitive behavior. Several recent developments in this field are directly relevant to ASDs, including how the brain represents the perceptual features of social stimuli, how social information influences attention processes in the brain, and how the value of social interaction is computed. Because the symptoms of ASDs may represent extreme manifestations of traits that vary in intensity within the general population, we will additionally discuss ways in which nonhuman primates also show variation in social behavior and reward sensitivity. In cases where variation in species-typical behavior is analogous to similar variations in human behavior, we believe that study of the neural circuitry underlying this variation will provide important insights into the systems-level mechanisms contributing to ASD pathology.

Exposure to environmental enrichment prior to a cerebral cortex stab wound attenuates the postlesional astroglia response in rats

Modulation of astroglial components involved in reactive postlesional responses in the rat cerebral cortex was analyzed following exposure to environmental enrichment (EE) condition prior to injury. For this purpose, changes in % immunoreactive (IR) area of GFAP, vimentin, EAAT1 and ezrin were evaluated in the perilesional zone after placing a cortical stab wound in the visual cerebral cortex of adult rats. GFAP-IR postlesional reactive astrocytosis in the perilesional cortex was significantly lower in the animal group exposed to EE during postnatal development. This GFAP-IR reaction seems to be associated with existing astroglia, because neither BrdU- nor endogenous Ki-67-labeled nuclei were found in the perilesional cortex analyzed. Increased ezrin-IR area in the visual cortex of rats exposed to EE condition suggests the formation of new synapses or the enhancement of astroglial involvement in the existing ones. No effects of EE were found on either EAAT1- or vimentin-IR area. Results suggest that exposure to EE conditions prior to injury attenuates the postlesional astroglia GFAP-response in the perilesional cortex of rats. Whether this attenuated postlesional astroglia GFAP-response promotes or not protective effects on the cortical neuropil remains to be explored in futures studies.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals

Interpreting the evolution of neuronal types in the cerebral cortex of mammals requires information from a diversity of species. However, there is currently a paucity of data from the Xenarthra and Afrotheria, two major phylogenetic groups that diverged close to the base of the eutherian mammal adaptive radiation. In this study, we used immunohistochemistry to examine the distribution and morphology of neocortical neurons stained for nonphosphorylated neurofilament protein, calbindin, calretinin, parvalbumin, and neuropeptide Y in three xenarthran species-the giant anteater (Myrmecophaga tridactyla), the lesser anteater (Tamandua tetradactyla), and the two-toed sloth (Choloepus didactylus)-and two afrotherian species-the rock hyrax (Procavia capensis) and the black and rufous giant elephant shrew (Rhynchocyon petersi). We also studied the distribution and morphology of astrocytes using glial fibrillary acidic protein as a marker. In all of these species, nonphosphorylated neurofilament protein-immunoreactive neurons predominated in layer V. These neurons exhibited diverse morphologies with regional variation. Specifically, high proportions of atypical neurofilament-enriched neuron classes were observed, including extraverted neurons, inverted pyramidal neurons, fusiform neurons, and other multipolar types. In addition, many projection neurons in layers II-III were found to contain calbindin. Among interneurons, parvalbumin- and calbindin-expressing cells were generally denser compared to calretinin-immunoreactive cells. We traced the evolution of certain cortical architectural traits using phylogenetic analysis. Based on our reconstruction of character evolution, we found that the living xenarthrans and afrotherians show many similarities to the stem eutherian mammal, whereas other eutherian lineages display a greater number of derived traits.

Truncated TrkB-T1 regulates the morphology of neocortical layer I astrocytes in adult rat brain slices

By altering their morphology, astrocytes, including those involved in the maintenance and plasticity of neurons and in clearance of transmitter, play important roles in synaptic transmission; however, the mechanism that regulates the morphological plasticity of astrocytes remains unclear. Recently, we reported that T1, a subtype of TrkB (a family of BDNF-specific receptors), altered astrocytic morphology through the control of Rho GTPases in primary astrocyte cultures. In this study, we extended this observation to investigate acute neocortical slices from adult rats. T1 siRNA-expression vectors were electroporated into astrocytes in neocortical layer I of living rats. In both normal slices and control vector-electroporated slices, BDNF induced the elongation of the astrocytic processes and increased the branching of processes in slices after 1 h incubation. In contrast, in T1 siRNA-electroporated slices, no such significant morphological changes were observed in the astrocytes. In addition, the number of synaptophysin+ sites in contact with GFAP+ processes increased in a BDNF-T1-dependent manner without the increase in the total synaptophysin+ sites. Therefore, the present study provides evidence of the regulation of layer I astrocytic morphology by the BDNF-T1 signal in adult rat neocortical slices.

Premorbid exercising in specific cognitive tasks prevents impairment of performance in parkinsonian monkeys

Adult Cebus apella monkeys were exposed to either one, two or four series of cognitive tasks that place a demand on working memory and inhibitory control (Spatial Delayed Response and Object Retrieval Detour), before administration of the neurotoxin 1-methyl-1-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Following MPTP treatment, monkeys receiving more than one series successfully reached criteria at delays similar to those attained during the pre-MPTP stage for the Spatial Delayed Response task and avoided increased perseveration in the Object Retrieval Detour task. Results provide evidence that protection towards a neurotoxin in specific cognitive performances can be increased by repeated exposure to task-specific cognitive demands and that motor and cognitive impairment following MPTP treatment can be effectively dissociated in primates.

Neuropathological and genetic findings in autism: the significance of a putative minicolumnopathy

Autism is a condition manifested as abnormalities of relatedness, communication, range of interests, and repetitive behaviors. Despite alarming prevalence estimates and exhortations to research, little is known regarding its pathophysiology. Recent reports of a putative minicolumnopathy explain changes in brain size, gray/white matter ratios, and interareal connectivity. This article summarizes possible links between minicolumns and other topics-cortical modularity, age of onset, gliosis, and genetics-relevant to the pathophysiology of autism.

Semicentennial Tribute to the Ingenious Neurobiologist Christfried Jakob (1866–1956). 1. Works from Germany and the First Argentina Period, 1891–1913

This study, and the companion paper that follows, pays homage to the life and work of Christfried (also Christian or Christofredo) Jakob, a German-born neuropathologist who adopted Argentina as his country of vocation. Rated by von Economo and Koskinas among the three most important pre-1925 cortical neuro-anatomists, alongside Ramón y Cajal, Jakob is little known in the English literature. He has left an impressive record of publications, 30 richly illustrated monographs and 200 articles that span over a vast array of neurological themes, including cortical development and evolution, and the visceral brain. The present paper reviews works from his German years and the first visit to Argentina in 1899-1910. The companion paper covers his works (all in Spanish) during his 'second Argentina period', after 1913.

Cerebral cortex astroglia and the brain of a genius: a propos of A. Einstein's

The glial fibrillary acidic protein immunoreactive astroglial layout of the cerebral cortex from Albert Einstein and other four age-matched human cases lacking any known neurological disease was analyzed using quantification of geometrical features mathematically defined. Several parameters (parallelism, relative depth, tortuosity) describing the primate-specific interlaminar glial processes did not show individually distinctive characteristics in any of the samples analyzed. However, A. Einstein's astrocytic processes showed larger sizes and higher numbers of interlaminar terminal masses, reaching sizes of 15 microm in diameter. These bulbous endings are of unknown significance and they have been described occurring in Alzheimer's disease. These observations are placed in the context of the general discussion regarding the proposal--by other authors--that structural, postmortem characteristics of the aged brain of Albert Einstein may serve as markers of his cognitive performance, a proposal to which the authors of this paper do not subscribe, and argue against.

Immunohistochemical analysis of subcortical white matter astroglia of infant and adult primate brains, with a note on resident neurons

An immunohistochemical analysis of brain subcortical white matter astroglia from human (infant, adult) and adult monkey (Cebus apella, Macaca nemestrina) cases without any known neurological disease, is described. Expression of synaptic vesicle-associated proteins, excitatory amino acid transporters (EAAT1 and EAAT2) and GABAA Ralpha2 receptor produced coarse punctate labeling in human adult white matter astrocytes. A finer, generalized, punctate labeling was observed in human infants and adult C. apella monkeys. Labeling of neuronal somata and processes with microtubule-associated proteins (MAP2a-c) and neuron nuclear (NeuN) antibodies, was also observed in subcortical white matter of humans and monkeys. Results suggest competence of subcortical white matter astroglia of the primate brain to participate in various transmitter regulatory pathways. It is also proposed that, collectively with resident neurons, they may exert some role in affecting the transfer of information that takes place through the various associational and projecting fiber systems coursing through this brain compartment.

Proprioception and Palisade Endings in Extraocular Eye Muscles

Palisade endings occur only in extraocular muscles, and their function is unknown. They form a cuff of nerve terminals around the tips of muscle fibers. We describe here the advantages of using antibodies to a synaptosomal-associated protein (SNAP-25) to study properties of palisade endings in man, monkey, and rat. The stain can be combined readily with other immunofluorescence procedures, and results suggest that the synapses of palisade endings do not bind alpha-bungarotoxin (i.e., are not motor), nor do they contain substance P. These double-labeling data support the hypothesis that palisade endings are non-nociceptive sensory receptors, and could serve a proprioceptive function. With SNAP-25 immunolabeling, palisade endings were identified in the rat for the first time. Thus, palisade endings appear to be present in all vertebrate extraocular muscles studied to date. Their apparent universality, which contrasts with the more variable manifestation of extraocular muscle spindles and Golgi tendon organs, would be expected if proprioceptive feedback is necessary to the function of the ocular motor system, and if palisade endings are the critical proprioceptive structure.

Development of interlaminar astroglial processes in the cerebral cortex of control and Down's syndrome human cases

Glial cytoarchitecture in human cerebral cortex is constituted by two overlapping layouts: the (general mammalian) "glial syncytium" and the (primate-specific) "interlaminar glial palisade" (IGP) composed by astroglial cells, with long, radial processes that traverse several supragranular layers. In this study, the emergence and early organization of the IGP was analyzed using immunocytochemical procedures in postmortem infantile human control and age matched, Down's syndrome (DS) cases. In control cases, first signs of a radial array of unbranched astroglial processes were apparent at the end of the period of "physiological astrocytosis" (20-40 days of postnatal life), and its general profile (except perhaps the density of cell processes) reached the adult-like configuration by the second month of life. The initial organization of the IGP was similar in control and DS cases, although a breakdown in DS became manifest by the first year of age, or earlier, albeit with individual variations. These changes tended to evolve in a "mosaic" fashion and included partial disruption of the palisade, or persistence of the "physiological astrocytosis". These observations were compared against samples from elder DS cases with an Alzheimer's type of dementia (AtD). Collectively, results suggest that DS also involves astroglial alterations during early stages of brain development, and that those changes progress with age, until an AtD ensues during adult life.

Apparent scarcity of glial fibrillary acidic protein expression in the brain of the pygmy shrew Sorex minutus as revealed by immunocytochemistry

We examined astroglial cells in the brain of the pygmy shrew Sorex minutus (Insectivora). For that purpose we labeled glial fibrillary acidic protein (GFAP) immunohistochemically in brain sections with a polyclonal antibody. Antigen retrieval experiments were performed to counteract formaldehyde fixation masking of GFAP epitopes. Our results showed remarkable paucity of GFAP-immunoreactive cells and fibers in the cerebral cortex and nuclei, as well as in the majority of the diencephalic and mesencephalic structures. In the forebrain, significant numbers of GFAP-containing astrocytes were found only in the ependyma and subventricular zones, superficial part of layer I of the cerebral cortex, and the majority of white matter structures. In the diencephalon, habenular nuclei were rich in GFAP-immunopositive astrocytes and labeled radial fibers were extended between median eminence and the third ventricle. A considerably higher density of labeled astrocytes was detected in the caudal brainstem and cerebellum. In contrast, in the mouse brain, immunoreactive astrocytes were present in large quantities in various structures. Staining of sections of the shrew brain against glutamine synthetase revealed abundance of immunofluorescent astrocytes in many areas, especially in the shrew cerebral cortex. It seems probable that in the shrew brain only a limited fraction of astroglia expresses GFAP, while other astroglial cells can be detected with different markers. It is possible that the rodent type of astroglial GFAP expression might not be common to insectivores and probably to some other mammalian orders.

Glial changes in primate cerebral cortex following long-term sensory deprivation

Significant morphological modifications in the layout of primate-specific (interlaminar) astroglia were found in somatosensory areas 3a, 3b, 1 and 2 eleven to thirteen months after transection of the posterior spinal cord in adult Macaca monkeys. These observations plus lack of evidence of a persistent reactive astrocytosis suggest that changes in the spatial arrangement of interlaminar glia may be an integral part of the long-term process of structural reorganization of the cerebral cortex following cortical deafferentation.

Interlaminar astroglia of the cerebral cortex: a marker of the primate brain

Evidence for "cable-like" processes stemming from astroglial cells in the supragranular cerebral cortex has been recently presented. In addition to what could be called the "general mammalian-like" astroglial architecture (the so-called "panglial syncytium") of the cerebral cortex, composed of typical stellate astrocytes (intralaminar astrocytes), the anthropoid species, mostly catarrhines, show a manifest vertical, radial distribution of long (interlaminar) astroglial processes. It can be tentatively proposed that evolutionary pressures resulted in the progressive appearance, in primates, of a new type of glial cell. Its soma has a superficial location and unusually long cellular processes that invade, in a predominant radial fashion, the supragranular region of the cerebral cortex. Their existence has been ignored for more than a century. On the neuronal side, modular (columnar) organization of the cerebral cortex may represent an evolutionary acquisition that could optimize communication and information processing, with the least volume compromise in terms of wiring. Yet, for such columns to be functionally operative, adequate isolation from neighboring units would be required. A "mass" operation of the astroglial architecture would tend to compromise spatial definition and the degrees of freedom of such columnar modules. It is proposed that the presence of a "palisade" of interlaminar glial processes represents a relatively recent evolutionary event, instrumental for the optimization of the modular (columnar) organization of the cerebral cortex. It is interesting that the supragranular cortical region has undergone the largest growth among mammalian species during brain evolution, and has been associated with a crucial role in cortico-cortical interactions.

Disruption in the inhibitory architecture of the cell minicolumn: implications for autism

The modular arrangement of the neocortex is based on the cell minicolumn: a self-contained ecosystem of neurons and their afferent, efferent, and interneuronal connections. The authors' preliminary studies indicate that minicolumns in the brains of autistic patients are narrower, with an altered internal organization. More specifically, their minicolumns reveal less peripheral neuropil space and increased spacing among their constituent cells. The peripheral neuropil space of the minicolumn is the conduit, among other things, for inhibitory local circuit projections. A defect in these GABAergic fibers may correlate with the increased prevalence of seizures among autistic patients. This article expands on our initial findings by arguing for the specificity of GABAergic inhibition in the neocortex as being focused around its mini- and macrocolumnar organization. The authors conclude that GABAergic interneurons are vital to proper minicolumnar differentiation and signal processing (e.g., filtering capacity of the neocortex), thus providing a putative correlate to autistic symptomatology.

Chronic social stress: effects on limbic brain structures

Different types of stressors are known to activate distinct neuronal circuits in the brain. Acute physiological stimuli that are life threatening and require immediate reactions lead to a rapid stimulation of brainstem and hypothalamus to activate efferent visceral pathways. In contrast, psychological stressors activate higher-order brain structures for further interpretations of the perceived endangerment. Common to the later multimodal stressors is that they need cortical processing and, depending on previous experience or ongoing activation, the information is assembled within limbic circuits connecting, e.g., the hippocampus, amygdala and prefrontal cortex to induce neuroendocrine and behavioral responses. In view of the fact that stressful life events often contribute to the etiology of psychopathologies such as depressive episodes, several animal models have been developed to study central nervous mechanisms that are induced by stress. The present review summarizes observations made in the tree shrew chronic psychosocial stress paradigm with particular focus on neurotransmitter systems and structural changes in limbic brain regions.

Disruption of astroglial interlaminar processes in Alzheimer's disease

A palisade of long, interlaminar astroglial processes in supragranular layers of the cerebral cortex is characteristic of adult individuals of anthropoid species. In the present study, this distinctive cytoarchitectonic feature was analyzed in tissue deriving from the neocortex of cases affected by Alzheimer's disease (n=14) and age-matched control cases (n=10). Samples of different cortical areas, and in particular prefrontal, temporal and striate fields, were analyzed. Astroglia was labeled by glial fibrillary acidic protein immunoreactivity, that allowed a clear distinction between the classical, stellate intralaminar astroglia and the interlaminar glial processes. The occurrence and relative density of neuritic plaques were ascertained in the same specimens with Bielchowsky staining. In most cortical regions of cases diagnosed as severe Alzheimer's disease by the donor institutions, interlaminar astroglia was found to be markedly altered or absent, and replaced by hypertrophic intralaminar astrocytes. Cases diagnosed as milder or uncertain Alzheimer's disease showed a less consistent involvement of the interlaminar glial palisade. Alterations of the interlaminar palisade in the cortex affected by Alzheimer's disease did not strictly correlate with the density of neuritic plaques in the examined specimens. The findings indicate that loss/severe disruption of the interlaminar palisade of astroglial processes is part of the array of neuropathological changes occurring in the cerebral cortex during Alzheimer's disease. In addition, our data indicate that different types of neocortical astrocytes (namely intralaminar and interlaminar astrocytes) respond differently to the pathobiology of Alzheimer's disease in the neocortex, inasmuch as interlaminar processes tend to disappear while intralaminar processes become reactive.

Astroglial interlaminar processes in human cerebral cortex: variations in cytoskeletal profiles

Among mammalian species, astroglial interlaminar processes are unique features of the primate cerebral cortex. The morphological diversity in the immunocytochemical expression of their cytoskeleton was analyzed. For this purpose, samples from normal human cerebral cortex from autopsy cases were used. While Fractal dimension failed to represent the actual complexity of interlaminar processes, Compression analysis allowed classification of these profiles according to their relative tortuosity. Conversion of Compression values into estimates of membrane surface suggested that profile changes could not only affect the directionality of dynamic events, but also the amount of glial cell membrane exposed to the local neuropil. Terminal segments of interlaminar processes were usually more tightly twisted than the cytoskeleton stalk, and enlarged in aged individuals. If not aberrant structures, these so-called 'terminal masses' may provide an additional means to increase local membrane availability. Based on Compression analysis, categories of the geometric variability of the cytoskeleton of cerebral cortex interlaminar glial processes are presented.

Tissue printing of astroglial interlaminar processes from human and non-human primate cerebral cortex

Astroglial interlaminar processes are unique features of the cerebral cortex of adult primates, including man. The functional role of these processes in the primate cerebral cortex is largely unknown. The development and standardization of procedures that could maximize the utilization of primate brain samples is required for the experimental analysis of the individual and collective dynamic properties of interlaminar glial processes. With this aim and in order to assess the relative stability of these glial processes in ex vivo conditions, "tissue printing" procedures were applied. "Tissue printing" allows for the acute transfer of cellular elements from fresh tissue onto an artificial substrate. Human, monkey (Cebus apella), and rat brain samples were subjected to "tissue printing" procedures followed by cell culture and immunohistochemistry. For the purpose of comparing the efficiency of this procedure on the transfer of other long glial processes, "tissue prints" of radial glial processes from neonatal rats and of Bergmann glia from cerebellar samples of adult rats were included. Nitrocellulose (with and without added fibronectin or laminin) produced the best attachment results. Interlaminar processes were not modified following 24-h incubation in a cell culture medium, with the addition of agents known to modify astroglial morphotypes in vitro (cyclic adenosine monophosphate, 40 mM K(+), or fetal calf serum). It is concluded that glia with interlaminar processes can be detached from fresh tissue using "tissue printing" procedures, can be maintained for at least 24 h in standard culture conditions, and showed a stable morphological phenotype.

Freshly isolated astrocyte (FIA) preparations: a useful single cell system for studying astrocyte properties

Astrocytes are cell constituents of the mammalian CNS whose intricate relationships with neurons, blood vessels and meninges in situ are well documented. These relationships and their complex morphologies imply numerous functions. Over the past quarter century or so, however, the main experimental basis for determining which roles are likely have been derived from studies on primary astrocyte cultures, usually prepared from neonatal rodent brains. We list a number of examples where these cultures have shown quantitative and qualitative differences from the properties exhibited by astrocytes in situ. The absence of an adequate reliable database makes proposals of likely hypotheses of astrocyte function difficult to formulate. In this article we describe representative studies from our laboratory showing that freshly isolated astrocytes (FIAs), can be used to determine the properties of astrocytes that seem more in concordance with the properties exhibited in situ. Although the cells are most easily isolated from < or =15 day old rat hippocampi they can be isolated from up to 30 day old rats. The examples we describe are that several different types of K(+) currents can be determined by patch clamp electrophysiology, of all the mGluRs only mGluR3 and 5 were detected by single cell RT-PCR, and that single cell Ca(2+) imaging shows that the mGluR5 receptor is functional. It was found that the frequency of cells expressing mGluR5 declines with the age of the animal with the mGluR5b type splice variant replacing the mGluR5a type, as occurs in the intact brain. It is concluded that FIAs can be used to determine the individual characteristics of astrocytes and their properties without the problems of indirect effects inherent in a heterogeneous system such as the slice, and without the problem of cultures unpredictably reflecting the in situ state. The FIAs obviously cannot be used to study interactions of astrocytes with the other CNS components but we propose that they will provide a good database on which hypotheses regarding such interactions can be tested in slices. FIAs can also be isolated from brain slices or intact brain after various pharmacological or electrophysiological perturbations to determine the changes in astrocyte properties that correlate with the perturbations.