Radial glia and astrocytes in developing and adult telencephalon of the lizard Gallotia galloti as revealed by immunohistochemistry with anti-GFAP and anti-vimentin antibodies

Modulatory Activity of the Endocannabinoid System in the Development and Proliferation of Cells in the CNS

The Endocannabinoid System (ECS, also known as Endocannabinoidome) plays a key role in the function of the Central Nervous System, though the participation of this system on the early development - specifically in neuroprotection and proliferation of nerve cells - has been poorly studied. Here, we collect and describe evidence regarding how cannabinoid receptors CB1R and CB2R regulate several cell markers related to proliferation. While CB1R participates in the modulation of neuronal and glial proliferation, CB2R is involved in the proliferation of glial cells. The endocannabinoids anandamide (AEA) and 2-arachidonoylglycerol (2-AG) exert significant effects on nerve cell proliferation. AEA generated during embryogenesis induces major effects on the differentiation of neuronal progenitor cells, whereas 2-AG participates in modulating cell migration events rather than affecting the neural proliferation rate. However, although the ECS has been demonstrated to participate in neuroprotection, more characterization on its role in neuronal and glial proliferation and differentiation is needed, especially in brain areas with recognized high neurogenesis rates. This has encouraged scientists to elucidate and propose specific mechanisms related with these cell proliferation mechanisms to better understand some neurodegenerative disorders such as Parkinson, Huntington and Alzheimer diseases, in which neuronal loss and poor neurogenesis are crucial factors for their onset and progression. In this review, we collect and present recent evidence published pointing to an active role of the ECS in the development and proliferation of nerve cells.

Molecular diversity and evolution of neuron types in the amniote brain

The existence of evolutionarily conserved regions in the vertebrate brain is well established. The rules and constraints underlying the evolution of neuron types, however, remain poorly understood. To compare neuron types across brain regions and species, we generated a cell type atlas of the brain of a bearded dragon and compared it with mouse datasets. Conserved classes of neurons could be identified from the expression of hundreds of genes, including homeodomain-type transcription factors and genes involved in connectivity. Within these classes, however, there are both conserved and divergent neuron types, precluding a simple categorization of the brain into ancestral and novel areas. In the thalamus, neuronal diversification correlates with the evolution of the cortex, suggesting that developmental origin and circuit allocation are drivers of neuronal identity and evolution.

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.

Radial Glial Cells: New Views on Old Questions

Radial glial cells (RGC) are at the center of brain development in vertebrates, acting as progenitors for neurons and macroglia (oligodendrocytes and astrocytes) and as guides for migration of neurons from the ventricular surface to their final positions in the brain. These cells originate from neuroepithelial cells (NEC) from which they inherit their epithelial features and polarized morphology, with processes extending from the ventricular to the pial surface of the embryonic cerebrum. We have learnt a great deal since the first descriptions of these cells at the end of the nineteenth century. However, there are still questions regarding how and when NEC transform into RGC or about the function of intermediate filaments such as glial fibrillary acidic protein (GFAP) in RGCs and their dynamics during neurogenesis. For example, it is not clear why RGCs in primates, including humans, express GFAP at the onset of cortical neurogenesis while in rodents it is expressed when it is essentially complete. Based on an ultrastructural analysis of GFAP expression and cell morphology of dividing progenitors in the developing neocortex of the macaque monkey, we show that RGCs become the main progenitor in the developing cerebrum by the start of neurogenesis, as all dividing cells show glial features such as GFAP expression and lack of tight junctions. Also, our data suggest that RGCs retract their apical process during mitosis. We discuss our findings in the context of the role and molecular characteristics of RGCs in the vertebrate brain, their differences with NECs and their dynamic behavior during the process of neurogenesis.

No rapid and demarcating astroglial reaction to stab wounds in Agama and Gecko lizards and the caiman Paleosuchus - it is confined to birds and mammals

The present study proves that rapid and demarcating astroglial reactions are confined to birds and mammals. To understand the function of post-lesion astroglial reaction, the phylogenetical aspects are also to be investigated. Considering the regenerative capabilities, reptiles represent an intermediate position between the brain regeneration-permissive fishes and amphibians and the almost non-permissive birds and mammals. Damage is followed by a rapid astroglial reaction in the mammalian and avian brain, which is held as an impediment of regeneration. In other vertebrates the reactions were usually observed following long survival periods together with signs of regeneration, therefore they can be regarded as concomitant phenomena of regeneration. The present study applies short post-lesion periods comparable to those seen in mammals and birds for astroglial reactions. Two species of lizards were used: gecko (leopard gecko, Eublepharis macularius, Blyth, 1854) and agama (bearded dragon, Pogona vitticeps, Ahl, 1926). The gecko brain is rich in GFAP whereas the agama brain is quite poor in this. Crocodilia, the closest extant relatives of birds were represented in this study by Cuvier's dwarf caiman (Paleosuchus palpebrosus, Cuvier, 1807). The post-lesion astroglial reactions of crocodilians have never been investigated. The injuries were stab wounds in the telencephalon. The survival periods lasted 3, 7, 10 or 14 days. Immunoperoxidase reactions were performed applying anti-GFAP, anti-vimentin and anti-nestin reagents. No rapid and demarcating astroglial reaction resembling that of mammalian or avian brains was found. Alterations of the perivascular immunoreactivities of laminin and β-dystroglycan as indicators of glio-vascular decoupling proved that the lesions were effective on astroglia. The capability of rapid and demarcating astroglial reaction seems to be confined to mammals and birds and to appear by separate, parallel evolution in them.

Distribution of GFAP in Squamata: Extended Immunonegative Areas, Astrocytes, High Diversity, and Their Bearing on Evolution

Squamata is one of the richest and most diverse extant groups. The present study investigates the glial fibrillary acidic protein (GFAP)-immunopositive elements of five lizard and three snake species; each represents a different family. The study continues our former studies on bird, turtle, and caiman brains. Although several studies have been published on lizards, they usually only investigated one species. Almost no data are available on snakes. The animals were transcardially perfused. Immunoperoxidase reactions were performed with a mouse monoclonal anti-GFAP (Novocastra). The original radial ependymoglia is enmeshed by secondary, non-radial processes almost beyond recognition in several brain areas like in other reptiles. Astrocytes occur but only as complementary elements like in caiman but unlike in turtles, where astrocytes are absent. In most species, extended areas are free of GFAP—a meaningful difference from other reptiles. The predominance of astrocytes and the presence of areas free of GFAP immunopositivity are characteristic of birds and mammals; therefore, they must be apomorphic features of Squamata, which appeared independently from the evolution of avian glia. However, these features show a high diversity; in some lizards, they are even absent. There was no principal difference between the glial structures of snakes and lizards. In conclusion, the glial structure of Squamata seems to be the most apomorphic one among reptiles. The high diversity suggests that its evolution is still intense. The comparison of identical brain areas with different GFAP contents in different species may promote understanding the role of GFAP in neuronal networks. Our findings are in accordance with the supposal based on our previous studies that the GFAP-free areas expand during evolution.

Molecular Markers in the Study of Non-model Vertebrates: Their Significant Contributions to the Current Knowledge of Tetrapod Glial Cells and Fish Olfactory Neurons

The knowledge of the morphological and functional aspects of mammalian glial cells has greatly increased in the last few decades. Glial cells represent the most diffused cell type in the central nervous system, and they play a critical role in the development and function of the brain. Glial cell dysfunction has recently been shown to contribute to various neurological disorders, such as autism, schizophrenia, pain, and neurodegeneration. For this reason, glia constitutes an interesting area of research because of its clinical, diagnostic, and pharmacological relapses. In this chapter, we present and discuss the cytoarchitecture of glial cells in tetrapods from an evolutive perspective. GFAP and vimentin are main components of the intermediate filaments of glial cells and are used as cytoskeletal molecular markers because of their high degree of conservation in the various vertebrate groups. In the anamniotic tetrapods and their progenitors, Rhipidistia (Dipnoi are the only extant rhipidistian fish), the cytoskeletal markers show a model based exclusively on radial glial cells. In the transition from primitive vertebrates to successively evolved forms, the emergence of a new model has been observed which is believed to support the most complex functional aspects of the nervous system in the vertebrates. In reptiles, radial glial cells are prevalent, but star-shaped astrocytes begin to appear in the midbrain. In endothermic amniotes (birds and mammals), star-shaped astrocytes are predominant. In glial cells, vimentin is indicative of immature cells, while GFAP indicates mature ones.Olfactory receptor neurons undergo continuous turnover, so they are an easy model for neurogenesis studies. Moreover, they are useful in neurotoxicity studies because of the exposed position of their apical pole to the external environment. Among vertebrates, fish represent a valid biological model in this field. In particular, zebrafish, already used in laboratories for embryological, neurobiological, genetic, and pathophysiological studies, is the reference organism in olfactory system research. Smell plays an important role in the reproductive behavior of fish, with direct influences also on the numerical consistency of their populations. Taking into account that a lot of species have considerable economic importance, it is necessary to verify if the model of zebrafish olfactory organ is also directly applicable to other fish. In this chapter, we focus on crypt cells, a morphological type of olfactory cells specific of fish. We describe hypothetical function (probably related with social behavior) and evolutive position of these cells (prior to the appearance of the vomeronasal organ in tetrapods). We also offer the first comparison of the molecular characteristics of these receptors between zebrafish and the guppy. Interestingly, the immunohistochemical expression patterns of known crypt cell markers are not overlapping in the two species.

Expression of regulatory genes in the embryonic brain of a lizard and implications for understanding pallial organization and evolution

The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9, and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six‐part model of pallial divisions. The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels. Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1‐expressing/Lhx9‐negative area at the front of the telencephalon, resembling the avian hyperpallium. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. These data open new venues for understanding the organization and evolution of the pallium.

Brain size and limits to adult neurogenesis

The walls of the cerebral ventricles in the developing embryo harbor the primary neural stem cells from which most neurons and glia derive. In many vertebrates, neurogenesis continues postnatally and into adulthood in this region. Adult neurogenesis at the ventricle has been most extensively studied in organisms with small brains, such as reptiles, birds, and rodents. In reptiles and birds, these progenitor cells give rise to young neurons that migrate into many regions of the forebrain. Neurogenesis in adult rodents is also relatively widespread along the lateral ventricles, but migration is largely restricted to the rostral migratory stream into the olfactory bulb. Recent work indicates that the wall of the lateral ventricle is highly regionalized, with progenitor cells giving rise to different types of neurons depending on their location. In species with larger brains, young neurons born in these spatially specified domains become dramatically separated from potential final destinations. Here we hypothesize that the increase in size and topographical complexity (e.g., intervening white matter tracts) in larger brains may severely limit the long-term contribution of new neurons born close to, or in, the ventricular wall. We compare the process of adult neuronal birth, migration, and integration across species with different brain sizes, and discuss how early regional specification of progenitor cells may interact with brain size and affect where and when new neurons are added.

Radial glia in the proliferative ventricular zone of the embryonic and adult turtle, Trachemys scripta elegans

To better understand the role of radial glial (RG) cells in the evolution of the mammalian cerebral cortex, we investigated the role of RG cells in the dorsal cortex and dorsal ventricular ridge of the turtle, Trachemys scripta elegans. Unlike mammals, the glial architecture of adult reptile consists mainly of ependymoradial glia, which share features with mammalian RG cells, and which may contribute to neurogenesis that continues throughout the lifespan of the turtle. To evaluate the morphology and proliferative capacity of ependymoradial glia (here referred to as RG cells) in the dorsal cortex of embryonic and adult turtle, we adapted the cortical electroporation technique, commonly used in rodents, to the turtle telencephalon. Here, we demonstrate the morphological and functional characteristics of RG cells in the developing turtle dorsal cortex. We show that cell division occurs both at the ventricle and away from the ventricle, that RG cells undergo division at the ventricle during neurogenic stages of development, and that mitotic Tbr2+ precursor cells, a hallmark of the mammalian SVZ, are present in the turtle cortex. In the adult turtle, we show that RG cells encompass a morphologically heterogeneous population, particularly in the subpallium where proliferation is most prevalent. One RG subtype is similar to RG cells in the developing mammalian cortex, while 2 other RG subtypes appear to be distinct from those seen in mammal. We propose that the different subtypes of RG cells in the adult turtle perform distinct functions.

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.

Glial cytoarchitecture in the central nervous system of the soft-shell turtle, Trionyx sinensis, revealed by intermediate filament immunohistochemistry

The distribution of the intermediate filament molecular markers, glial fibrillary acidic protein (GFAP) and vimentin, has been studied in the central nervous system (CNS) of the soft-shell turtle (Trionyx sinensis) with immunoperoxidase histochemistry. GFAP immunohistochemistry pointed out the presence of different astroglial cell types. The brain pattern consists of ependymal radial glia whose cell bodies are located in the ependymal layer throughout the brain ventricular system. In the spinal cord, the ependyma is immunonegative, whereas positive radial astrocyte cell bodies are displaced from the ependyma into the periependymal position. Star-shaped astrocytes are observed only in the posterior intumescence of the spinal cord. The different regions of the CNS show a different intensity in GFAP immunostaining even in the same cellular type. Vimentin-immunoreactive structures are absent in the brain and spinal cord. The present study reports an heterogeneous feature of the astroglial pattern in the spinal cord compared to the brain which shows an ancestral condition.

Intermediate filament immunohistochemistry of astroglial cells in the leopard gecko, Eublepharis macularius

The distribution of intermediate filament molecular markers, glial fibrillary acidic protein (GFAP) and vimentin, has been studied in the central nervous system (CNS) of the adult leopard gecko, Eublepharis macularius. This immunohistochemical study points out the presence of different astroglial cell types. The main pattern is constituted by ependymal radial glia, which have their cell bodies located in the ependymal layer throughout the brain ventricular system. Radial glia proper or radial astrocytes show their cell bodies displaced from the ependymal layer into a periependymal zone and are observed only in the spinal cord. Star-shaped astrocytes are scarce. They are detected in the ventral and lateral regions of the diencephalon and mesencephalon, in the superficial layer of the optic tectum, in the ventral medulla oblongata, and in the ventral and lateral spinal cord. In the different regions of the CNS, the staining intensity appears not to be identical even in the same cellular type. The results reported in the present study show an heterogeneous feature of the astroglial pattern in E. macularius.

Astroglial cells in the central nervous system of the adult brown anole lizard, Anolis sagrei, revealed by intermediate filament immunohistochemistry

We analyzed the distribution of intermediate filament molecular markers, glial fibrillary acidic protein (GFAP), and vimentin in the brain and spinal cord of the adult brown anole lizard, Anolis sagrei. The GFAP immunoreactivity is strong and the positive structures are basically represented by fibers of different lengths and thicknesses which are arranged in a regular radial pattern throughout the central nervous system. In the brain regions that have a thicker neural wall, the radial orientation is not so evident as in the thinner areas. These fibers emerge from radial ependymoglia (tanycytes) whose cell bodies are generally GFAP-immunopositive. The glial fibers give rise to endfeet that are apposed to the subpial surface and to blood vessel walls. In the spinal cord, the optic tectum and the lateroventral regions of the mesencephalon and medulla oblongata, star-shaped astrocytes coexist with radial structures. Vimentin-immunoreactive structures are absent in the brain and spinal cord. In A. sagrei the immunohistochemical response of the astroglial intermediate filaments appears typical of a mature astroglial cell lineage, since they fundamentally express GFAP immunoreactivity. A Western-blot analysis reveals a GFAP-positive single band, common to the different nervous areas. This immunohistochemical study shows that the star-shaped astrocytes have a different distribution in saurians and while the glial pattern of A. sagrei is more evolved than in urodeles it remains immature as compared with crocodilians, avians, and mammals. This condition suggests that reptiles represent a fundamental step in the phylogenetic evolution of the vertebrate glial cells.

Distribution and ontogeny of glial fibrillary acidic protein in the snail Megalobulimus abbreviatus

Glial fibrillary acidic protein (GFAP) is the major component of intermediate glial filaments in the central nervous system of many vertebrates and invertebrates. In vertebrates, this protein is mainly expressed in mature astrocytes and provides structural cell stability. The highly conserved structure and glial specificity of this protein have allowed studies of ontogeny and phylogeny using antibodies. The present study investigated the ontogenetic profile and molecular weight of GFAP in the snail, Megalobulimus abbreviatus, particularly in cerebral ganglia and subesophageal mass, by immunohistochemistry and immunoblotting. Our results confirm and extend previous studies about glial intermediate filaments in snails, showing: (i) a higher GFAP content in cerebral ganglia than in subesophageal mass; (ii) a developmental increase of GFAP immunocontent in cerebral ganglia, as described in Vertebrates; and (iii) an electrophoretic band for GFAP of approximately 55 kDa.

Glial fibrillary acidic protein and vimentin immunoreactivity of astroglial cells in the central nervous system of the African lungfish,Protopterus annectens (Dipnoi: Lepidosirenidae)

The distribution of glial intermediate filament molecular markers, glial fibrillary acidic protein (GFAP), and vimentin, in the brain and spinal cord of the African lungfish, Protopterus annectens, was examined by light microscopy immunoperoxidase cytochemistry. Glial fibrillary acidic protein immunoreactivity is clear and is evident in a radial glial system. It consists of fibers of different lengths and thicknesses that are arranged in a regular radial pattern throughout the central nervous system (CNS). They emerge from generally immunopositive radial ependymoglia (tanycytes), lining the ventricular surface, and are directed from the ventricular wall to the meningeal surface. These fibers give rise to endfeet that are apposed to the subpial surface and to blood vessel walls forming the glia limitans externa and the perivascular glial layer, respectively. GFAP-immunopositive star-shaped astrocytes were not found in P. annectens CNS. In the gray matter of the spinal cord, cell bodies of immunopositive radial glia are displaced from the ependymal layer. Vimentin-immunopositive structures are represented by thin fibers mostly localized in the peripheral zones of the brain and the spinal cord. While a few stained fibers appear in the gray matter, the ependymal layer shows no antivimentin immunostaining. In P. annectens the immunocytochemical response of the astroglial intermediate filaments is typical of a mature astroglia cell lineage, since they primarily express GFAP immunoreactivity. This immunocytochemical study shows that the glial pattern of the African lungfish resembles that found in tetrapods such as urodeles and reptiles. The glial pattern of lungfishes is comparable to that of urodeles and reptiles but is not as complex as that of teleosts, birds, and mammals.

Radial glial cells, proliferating periventricular cells, and microglia might contribute to successful structural repair in the cerebral cortex of the lizard Gallotia galloti

Reptiles are the only amniotic vertebrates known to be capable of spontaneous regeneration of the central nervous system (CNS). In this study, we analyzed the reactive changes of glial cells in response to a unilateral physical lesion in the cerebral cortex of the lizard Gallotia galloti, at 1, 3, 15, 30, 120, and 240 days postlesion. The glial cell markers glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), S100 protein, and tomato lectin, as well as proliferating cell nuclear antigen (PCNA) were used to evaluate glial changes occurring because of cortical lesions. A transitory and unilateral upregulation of GFAP and GS in reactive radial glial cells were observed from 15 to 120 days postlesion. In addition, reactive lectin-positive macrophage/microglia were observed from 1 to 120 days postlesion, whereas the expression of S100 protein remained unchanged throughout the examined postlesion period. The matricial zones closest to the lesion site, the sulcus lateralis (SL) and the sulcus septomedialis (SSM), showed significantly increased numbers of dividing cells at 30 days postlesion. At 240 days postlesion, the staining pattern for PCNA, GFAP, GS, and tomato lectin in the lesion site became similar to that observed in unlesioned controls. In addition, ultrastructural data of the lesioned cortex at 240 days postlesion indicated a structural repair process. We conclude that restoration of the glial framework and generation of new neurons and glial cells in the ventricular wall play a key role in the successful structural repair of the cerebral cortex of the adult lizard.

Development of vimentin and glial fibrillary acidic protein immunoreactivities in the brain of gray mullet (Chelon labrosus), an advanced teleost

Previous studies in teleosts have revealed the presence of the intermediate filaments vimentin (Vim) and glial fibrillary acidic protein (GFAP) in glial cells of the spinal cord and/or some brain regions, but there is no comprehensive study of their distribution and developmental changes in fishes. Here, the distribution of Vim and GFAP immunoreactivities was studied in the brain of larvae, juveniles, and adults of an advanced teleost, the gray mullet (Chelon labrosus). A different sequence of appearance was observed for expression of these proteins: Vim levels decreased with age, whereas GFAP increased. In general, both immunoreactivities were expressed early in perikarya and endfeet of ependymocytes (tanycytes), whereas expression in radial processes appeared later. In large larvae, the similar expression patterns of Vim and GFAP suggest that some of these glial cells contain both proteins. Subependymal radial glia cells were observed mainly in the optic tectum, exhibiting Vim and GFAP immunoreactivity. The only immunoreactive cells with astrocyte-like morphology were observed in the optic chiasm of the adult, and they were positive for both GFAP and Vim. The perivascular processes of glial cells showed a different distribution of Vim and GFAP during development and had a caudorostral sequence of appearance of immunoreactivities similar to that observed for ependymal and radial glia cells. Several circumventricular organs (the organon vasculosum hypothalami, saccus vasculosus, and area postrema) exhibited highly specialized Vim- and/or GFAP-expressing glial cells. The glial cells of the midline septa of several brain regions were also Vim and/or GFAP immunoreactive. In the adult brain, tanycytes retain Vim expression in several brain regions. As in other vertebrates, the regions with Vim-immunoreactive ventricular and midline glia may represent areas with the capability of plasticity and regeneration in adult brain.

S100 immunoreactive glial cells in the forebrain and midbrain of the lizard Gallotia galloti during ontogeny

We identified S100 immunoreactive cells in the brain of the lizard Gallotia galloti during ontogeny using immunohistochemical techniques for light and electron microscopy. In double labeling experiments with antibodies specific for S100A1 and S100B (anti-S100) and proliferative cell nuclear antigen (anti-PCNA), myelin basic protein (anti-MBP), phosphorylated neurofilaments (SMI-31), glial fibrillary acidic protein (anti-GFAP), or glutamine synthetase (anti-GS), we detected S100-like immunoreactivity in glial cells but never in neurons. Restricted areas of the ventricular zone were stained in the hypothalamus from E32 to postnatal stages, and in the telencephalon at E35, E36, and in adults. S100 immunoreactivity was observed predominantly in scattered PCNA-negative cells that increased in number from E35 to the adult stage in the myelinated tracts of the brain and had the appearance of oligodendrocytes. Quantitative analysis revealed that all of the S100-positive glial cells were GFAP-negative, whereas most of the S100-positive glial cells were GS-positive. Ultrastructurally, most of these S100-positive/GS-positive glial cells resembled oligodendrocytes of light and medium electron density. In adult lizards, a small subpopulation of astrocyte-like cells was also stained in the pretectum. We conclude that in the lizard S100 can be considered a marker of a subpopulation of oligodendrocytes rather than of astrocytes, as is the case in mammals. The S100-positive subpopulation of oligodendrocytes in the lizard could represent cells actively involved in the process of myelination during development and in the maintenance of myelin sheaths in the adult.

Differential patterns of glial fibrillary acidic protein-immunolabeling in the brain of adult lizards

The present study describes by means of immunohistochemistry the comparative distribution of glial fibrillary acidic protein (GFAP)-positive cells in the forebrain and midbrain of three species of lizards: Eumeces algeriensis, Scincoidae; Agama impalearis, Agamidae; Tarentola mauritanica, Gekkonidae. In the species studied, the different types and proportions of glial cells expressing GFAP showed considerable variation. These cells include radial glia, oval cells, tanycytes, ependymocytes, glia limitans, and astrocytes. In Eumeces, astrocytes are particularly abundant and their processes form numerous perivascular end-feet; in addition well-differentiated ependymal cells and glia limitans express GFAP. These mature glial features are concordant with the relatively advanced phylogenetic level of Eumeces. In Tarentola, relatively few GFAP-expressing glial cells are observed, consisting mainly of radial glia and tanycytes. These features indicate a relatively immature state of the glial cell populations in this species. In Agama, GFAP-immunostained cells are confined to the periventricular and subpial brain areas; the ventricular lining contains numerous GFAP-immunopositive tanycytes and well-differentiated glia limitans. This pattern indicates that the glial cell profile in Agama exhibits characteristics intermediate between Eumeces and Tarentola, a feature which is discordant with the relatively primitive phylogenetic level of Agamidae compared to Gekkonidae. Together, the results of the present study provide novel data on the characterization of GFAP-expressing cell populations in different species of lizards. We suggest that the different glial patterns observed in the lizard brain correlates with developmental and functional aspects.

Distribution of GFAP immunoreactive structures in the rhombencephalon of the sterlet (Acipenser ruthenus) and its evolutionary implication

Previous studies have revealed that although the brains of cypriniform teleosts (iberian barb, Barbus comiza; carp, Cyprinus carpio; goldfish, Carassius auratus) are rich in glial fibrillary acidic protein (GFAP), they have, however, areas devoid of GFAP immunoreactivity. The largest ones of these are in the rhombencephalon, e.g., the zones of the sensory and motor neurons in the vagal lobe. Our studies in amniotes suggested that the GFAP immunonegative areas could be characteristic of the more advanced brains (avian and mammalian), whereas no similar areas were found in reptiles. A similar tendency was found in the Chondrichthyes, i.e., GFAP immunonegative areas appeared as brain complexity progressed. The question arose whether the GFAP immunonegative brain areas in the Teleostei were also the result of such a tendency. Within the radiation of ray-finned fishes (Actinopterygii), Chondrostei represent a less advanced level as compared to the Teleostei. Therefore, the distribution of GFAP immunoreactivity was investigated in the rhombencephalon of the sterlet (Acipenser ruthenus) as a representative of Chondrostei, and in the carp. Serial vibratome sections were processed according to the avidin-biotinylated horseradish peroxidase method.Several comparable GFAP immunoreactive structures were found in the two species: the dense periventricular ependymoglial plexus, the midsagittal glial septum, the small glial septa separating the nerve fiber bundles, and the wide glial endfeet lining the meningeal surface. In the vagal lobe in the zones adjacent to the meningeal and ventricular surfaces, the glial structures also proved to be similar. In contrast to the carp, however, no areas were found devoid of GFAP immunoreactivity in the sterlet.The results suggest that this trend of glial evolution, i.e., GFAP immunonegative areas appearing as brain complexity progressed, is a common feature shared by Actinopterygii, Amniota, and Chondrichthyes, despite their separate evolutionary histories. J. Exp. Zool. 293:395-406, 2002.

Radial glial cells as neuronal precursors: a new perspective on the correlation of morphology and lineage restriction in the developing cerebral cortex of mice

Radial glia is a ubiquitous cell type in the developing central nervous system (CNS) of vertebrates, characterized by radial processes extending through the wall of the neural tube which serve as guiding cables for migrating neurons. Radial glial cells were considered as glial precursor cells due to their astroglial traits and later transformation into astrocytes in the mammalian CNS. Accordingly, a hypothetical morphologically distinct type of precursor was attributed the role of neurogenesis. Recent evidence obtained in vitro and in vivo, however, revealed that a large subset of radial glia generates neurons. We further demonstrate here that the progeny of radial glial cells does not differ from the progeny of precursors labeled from the ventricular surface, implying that there is no obvious relation between precursor morphology and neuron-glia lineage decisions in the developing cerebral cortex of mice. Moreover, we show that many radial glial cells seem to maintain their process during cell division and discuss the implications of this observation for the orientation of cell division. These new data are then related to radial glial cells in other non-mammalian vertebrates persisting into adulthood and suggest that radial glia are not only neurogenic during development, but also in adulthood.

GFAP expression withdraws--a trend of glial evolution?

This study is a summary of investigations in the last decade with several collaborators on representatives of different vertebrate stocks. The results suggest that in the main vertebrate stocks (Agnathi, Chondrichthyes, Actinopterygii, Sarcopterygii-Amniotes), which had their parallel brain evolutions from the laminar brains to the elaborated ones, the astroglia also developed in parallel, and had a common trend of evolution. With growing brain complexity, free astrocytes arose and tended to predominate, and the spontaneous glial fibrillary acidic protein (GFAP)-expression regressed, in several areas. In the mammalian, avian, teleost, and batoid brains, therefore, large areas display a paucity, almost a lack of GFAP-immunoreactivity. The GFAP-expression in the GFAP-free areas seems to be inducible only in the presence of free astrocytes.

Continuity and discontinuity of the radial scaffolding in the forebrain of a lizard embryo

We have used ventricular injections of the fluorescent carbocyanine dye DiI (Molecular Probes, Eugene, OR, USA) to demonstrate the scaffolding of the radial ventricular cells in the embryonic forebrain of lizards. The results reveal changes of density, orientation, or thickness of the radial processes between adjacent regions. On the whole, they support the idea that the hypothalamus together with the telencephalic stalk and the telencephalic hemispheres make up the anterior region of the neural tube or secondary prosencephalon, as proposed by the prosomeric model. In contrast, the thalamic eminence seems part of the diencephalon.

Neuronal regeneration in the cerebellum of adult teleost fish, Apteronotus leptorhynchus: guidance of migrating young cells by radial glia

In contrast to mammals, adult fish exhibit an enormous potential to replace injured brain neurons by newly generated ones. In the present study, the role of radial glia, identified by immunostaining against fibrillary acidic protein (GFAP), was examined in this process of neuronal regeneration. Approximately 8 days after application of a mechanical lesion to the corpus cerebelli in the teleost fish Apteronotus leptorhynchus, the areal density of radial glial fibers increased markedly in the ipsilateral dorsal molecular layer compared to shorter survival times, or to the densities found in the intact brain or in the hemisphere contralateral to the lesion. This density remained elevated throughout the time period of up to 100 days examined. The increase in fiber density was followed approximately 2 days later by a rise in the areal density of young cells, characterized by labeling with the nuclear dye DAPI, in the ipsilateral dorsal molecular layer. Based on this remarkable spatio-temporal correlation, and the frequently observed close apposition of elongated young cells to radial glial fibers, we hypothesize that radial glia play an important role in the guidance of migrating young cells from their proliferation zones to the site of lesion where regeneration takes place.

Glial fibrillary acidic protein and vimentin immunoreactivity of astroglial cells in the central nervous system of adult Podarcis sicula (Squamata, Lacertidae)

The present immunoperoxidase cytochemical study describes the distribution of glial intermediate filament molecular markers, glial fibrillary acidic protein (GFAP) and vimentin, in the brain and spinal cord of the adult lizard, Podarcis sicula. GFAP immunoreactivity is abundant and the positive structures are mainly represented by fibres of different lengths which are arranged in a rather regular radial pattern throughout the CNS. They emerge from generally immunopositive radial ependymoglia and are directed from the ventricular wall towards the meningeal surface. The glial fibres give origin to endfeet which are apposed to the blood vessel walls and subpial surface where they form the continous perivascular and subpial glia envelopes, respectively. In the optic tectum and spinal cord, star-shaped astrocytes coexist with radial glia. In the spinal cord, cell bodies of immunopositive radial glia are displaced from the ependyma. While vimentin immunoreactive elements are almost completely absent in the brain except for a few diencephalic radial fibres, the spinal cord ependyma exhibits a clearly vimentin positivity and no GFAP staining. In the Podarcis CNS the immunocytochemical response of the astroglial intermediate filaments appears typical of mature astroglia cell lineage since it fundamentally expresses GFAP immunoreactivity. Moreover, this immunocytochemical study shows that the Podarcis fibre pattern with predominant radial glial cells is morphologically more immature than in avians and mammalians, a condition suggesting that reptiles represent a fundamental step in the phylogenetic evolution of vertebrate astroglial cells.

The functions of the preplate in development and evolution of the neocortex and hippocampus

Recently, it has been shown that the early developmental organization of the archicortical hippocampus resembles that of the neocortex. In both cortices at embryonic stages, a preplate is present, which is split by the formation of the cortical plate into a marginal zone and a subplate layer. The pioneer neurons of the preplate are believed to form a phylogenetically ancient cortical structure. Neurons in these preplate layers are the first postmitotic neurons and have important roles in the development of the cerebral cortex. Cajal-Retzius cells in the marginal zone regulate the phenotype of radial glial cells and may direct neuronal migration establishing the inside-out gradient of corticogenesis. Furthermore, pioneer neurons form the initial axonal connections with other (sub)cortical structures. A significant difference between the hippocampus and neocortex, however, is that in the hippocampus, most afferents are guided by the pioneer neurons in the prominent marginal zone, while in the neocortex most ingrowing afferent axons enter via the subplate. At later developmental periods, most pioneer neurons disappear by cell death or transform into other neuronal shapes. Here, we review the early developmental organization of the mammalian cerebral cortex (both neocortex and hippocampus) and discuss the functions and fate of pioneer neurons in cortical development, in particular that of Cajal-Retzius cells. Evaluating the developmental properties of the hippocampus and neocortex, we present the hypothesis that the distribution of the main ingrowing afferent systems in the developing neocortex, which differs from the one in the hippocampal region, may have enabled the specific evolution of the neocortex.

Heterogeneous immunoreactivity of glial cells in the mesencephalon of a lizard: a double labeling immunohistochemical study

Astrocytes and radial glia coexist in the adult mesencephalon of the lizard Gallotia galloti. Radial glia and star-shaped astrocytes express glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). The same cell markers are also expressed by round or pear-shaped cells that are therefore astrocytes with unusual morphology. Other round or pear-shaped cells, also scattered in the tegmentum and the tectum, display only GS. Electron microscopy reveals that these cells may be oligodendrocytes. In this lizard, the GS is expressed in some oligodendrocytes while this does not occur in the central nervous system of mammals in situ. These results confirm that the cellular specificity of GS is different in various species and suggest that ependymal cells are also immunoreactive for GS but they do not contain GFAP.

Distribution of radial glia in the developing telencephalon of chicks

Radial glia are known to have a sparse and uneven distribution in the telencephalon of adult birds. The present study utilizes antibodies against vimentin to reveal a more extensive, and more clearly radial, set of radial glia in the chicken telencephalon during the first half of embryogenesis. This initially extensive radial glial fiber system becomes distorted and reduced between 10 and 14 days of incubation. This reduction coincides with the cytoarchitectural differentiation of the telencephalon into its major adult subdivisions. Because developing neurons tend to migrate along radial glial fibers in both birds and mammals, a topological projection of these major subdivisions onto the embryonic ventricular zone along the radial glial fibers suggests hypotheses about lineage relationships that can be tested by subsequent experimental methods. This analysis suggests that the major components of the avian dorsal ventricular ridge, i.e., the ventral hyperstriatum, the neostriatum with its various subdivisions, part of the archistriatum, and probably also the piriform cortex, all derive from overlapping portions of the lateral pallial ventricular zone. Staining with antibodies against neurofilament suggests that this developmental parcellation of the lateral pallial complex is associated with the development of neuronal fiber systems.

3-Acetylpyridine-induced degeneration and regeneration in the adult lizard brain: a qualitative and quantitative analysis

The neurotoxin 3-acetylpyridine (3AP) produces highly selective neuronal damage in specific areas of the lizard brain. Following 3AP intoxication, proliferation and migration of replacement neurons born in the ventricular walls lead to regeneration of the lesioned areas. Earlier studies established the time course of 3AP-induced degeneration and subsequent regeneration in the medial cerebral cortex of adult lizards (Font, E., García-Verdugo, J.M., Alcántara, S. and Lopez-García, C., Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards, Brain Res., 551 (1991) 230-235 [13]). Complementary to our previous studies, we now provide a qualitative and quantitative account of the extent and distribution of neurotoxic damage in the brain as a whole of lizards treated with 3AP using Nissl and Golgi stains, a degeneration-sensitive reduced-silver method, and electron microscopy. Additionally, [3H]thymidine autoradiography was used to assess changes in the rate of neurogenesis caused by the 3AP treatment. Single doses of 3AP caused degenerative changes in all the cortical areas, anterior dorsal ventricular ridge, deep layers of the lateral cortex, lateral amygdaloid nucleus, and nucleus sphericus, while sparing other brain areas. The most frequent neuropathic change after 3AP treatment was clumping of the nuclear chromatin with formation of pyknotic nuclei. Occasionally, a second type of injury was observed in neurons of the cell layer of the dorsomedial cortex (DMC). 3AP also caused a conspicuous loss of dendritic spines in bipyramidal neurons of the dorsomedial and dorsal cortices possibly representing transneuronal degeneration. Numbers of [3H]thymidine-labeled cells were higher in lizards previously treated with 3AP than in controls. These results demonstrate that the neurotoxic lesion is capable of inducing an increase in the normal rate of adult neurogenesis. Whereas regeneration in the remaining areas was morphologically and histologically complete, in some animals, cell proliferation in the DMC resulted in formation of an abnormal cell plate.

Radial glia in the neocortex of adult rats: effects of neonatal brain injury

Microgyria can be induced in otherwise normal rat neocortex by a freezing injury to the cortical plate before the completion of neuronal migration. We had previously reported radial glial like-immunoreactive fibers in the area of the microgyria in 32-day-old rats. Here we demonstrate that these glial fibers, which are immunoreactive to Rat-401, vimentin, and glial fibrillary acidic protein (GFAP) antibodies, are seen in adult rats. The appearance of these fibers is hypothesized to result from the release of a trophic factor during the recovery from neonatal injury which acts to either (1) halt the transformation of radial glia to astrocytes and/or dedifferentiate already committed astrocytes, (2) create a hybrid cell, or (3) induce increased proliferation of glia.

An immunocytochemical investigation of glial morphology in the Pacific hagfish: radial and astrocyte-like glia have the same phylogenetic age

This study attempts to reconstruct the early phylogenetic history of macroglial cells among craniates. Since glia does not fossilize, such a reconstruction must be based on a cladistic comparison of glial characters in the Recent craniate taxa (hagfishes, lampreys, and gnathostomes); however, there are only few data on glial morphology and none on glial immunocytochemistry in hagfishes. Therefore, we investigated the presence and localization of various macroglia-specific epitopes in the brain and spinal cord of the Pacific hagfish, Eptatretus stouti (Myxinoidea) by means of immunocytochemistry. Antibodies directed against S100-protein and vimentin showed no cross reactivity. Antibodies directed against glial fibrillary acidic protein and glutamine synthetase labelled various glial structures. Glial fibrillary acidic protein-like immunoreactivity was observed in ependymal cells with radially oriented processes in some regions. However, throughout the entire CNS, labelling of non-ependymal cells and their processes prevailed. The processes of these cells made occasional vascular contacts and they also made contacts with neuronal perikarya. Glutamine synthetase-like immunoreactivity was also found in some processes with radial orientation and in ependymal cells; but the antibody stained mainly non-ependymal cells which gave rise to a felt-like meshwork of interdigitating fine and very fine processes penetrating the neuropil of the entire brain. Additionally, there was labelling in the walls of blood vessels and in processes enwrapping individual neurons. The occurrence of glial fibrillary acidic protein- and glutamine synthetase-like immunoreactivity in non-ependymal glial elements in the brain of hagfishes and the relative scarcity of labelling in radial glial elements necessitates a re-interpretation of the evolutionary history of glial cells. Non-ependymal macroglia with immunocytochemical and morphological characters resembling typical (mammalian) astrocytes appears to be as primitive as the various forms of radial ependymal glia.

Ependyma: phylogenetic evolution of glial fibrillary acidic protein (GFAP) and vimentin expression in vertebrate spinal cord

The phylogenetic evolution was studied of both glial fibrillary acidic protein (GFAP) and vimentin expression in the ependyma of the adult vertebrate spinal cord. Eleven species from different vertebrate groups were examined using different fixatives and fixation procedures to demonstrate any differences in immunoreactivity. GFAP expression in the ependymal cells showed a clear inverse relation with phylogenetic evolution because it was more elevated in lower than in higher vertebrates. GFAP positive cells can be ependymocytes and tanycytes, although depending on their structural characteristics and distribution, the scarce GFAP positive ependymal cells in higher vertebrates may be tanycytes. Ependymal vimentin expression showed a species-dependent pattern instead of a phylogenetic pattern of expression. Vimentin positive ependymal cells were only found in fish and rats; in fish, they were tanycytes and were quite scarce, with only one or two cells per section being immunostained. However, in the rat spinal cord, all the ependymocytes showed positive immunostaining for vimentin. The importance of the immunohistochemical procedure, the cellular nature of GFAP positive ependymal cells and the relationship between tanycytes and ependymocytes are discussed, as well as GFAP and vimentin expression.

Distribution of glial fibrillary acidic protein-immunopositive structures in the brain of the red-eared freshwater turtle (Pseudemys scripta elegans)

The distribution of glial fibrillary acidic protein (GFAP)-immunoreactivity is described in serial Vibratome sections of the turtle brain. The results are discussed in relation to our previous studies of rat and chicken brains. In the turtle brain, the distribution of GFAP-positive elements is rather evenly abundant as compared to that observed in the chicken and rat. The GFAP-positive structures are fibers of different length and orientation, but the stellate cells are not GFAP-positive. The basic systems is the radial ependymoglia, directed from the ventricles toward the outer surface of the brain. This system also contains some transverse and randomly oriented fibers. The cell bodies are not usually GFAP-positive. The large brain tracts could be recognized by their weak immunostaining, but gray matter nuclei could not be identified on the basis of immunostaining against GFAP. The layers of the optic tectum could be distinguished, as well as the gray and white matter of brain stem and spinal cord and the molecular and granular layers of the cerebellum. In the cerebellum, a fiber system resembling the Bergmann-fibers, a strong midline raphe and coarse transverse fibers could be observed. These latter fibers have no equivalent in other cerebella. Their perikarya proved also to be GFAP-positive, and seemed to be dividing in the adult turtle brain. We conclude that the appearance of GFAP-positive stellate cells had a great importance in the evolution of avian and mammalian brains strengthening the thicker brain walls and assisting in the formation of local differences of GFAP-immunoreactivity in different brain areas.

The glial fascicle: an ontogenic and phylogenic unit guiding, supplying and distributing mammalian cortical neurons

Neurons destined for the mammalian neocortex migrate along radial glial cells (RGCs). This paper describes a comparative study of RGCs in the mouse, rat, hamster, cat and human fetus, selected as representative species of mammalian evolution. The glial fascicles display constant features throughout these species: they consist of 4-10 RGCs filled with glycogen. The glial fascicle guides and probably supplies metabolites to the migrating neurons and organizes the vertical lamination of the developing neocortical plate. The neuronal-glial unit which consists of the RGCs and their affiliated migrating neurons is conserved throughout the species studied, suggesting a designation as a phylogenic unit.

Development of radial glia and astrocytes in the spinal cord of the North American opossum (Didelphis virginiana): an immunohistochemical study using anti-vimentin and anti-glial fibrillary acidic protein

We have shown previously that rubrospinal axons grow around a lesion of their pathway in developing opossums and that a critical period exists for that plasticity. As a first step toward addressing the possibility that glial maturation and/or the development of an astrocytic response to lesioning contribute to loss of rubrospinal plasticity, we have studied the normal development of radial glia and astrocytes in the spinal cord of the opossum by immunostaining for vimentin (Vim) and glial fibrillary acidic protein (GFAP). Vim-like immunoreactivity (Vim-LI) was present in radial glia throughout the spinal cord at birth (12 days after conception), whereas GFAP-like immunoreactivity (GFAP-LI) was limited to cells of comparable morphology in the ventral part of the cervical cord. The subsequent appearance of GFAP-LI followed ventral to dorsal and rostral to caudal gradients and by postnatal day (PD) 15, it was found in radial glia throughout the cord. At the same age, processes immunostained by either antibody had lost their radial orientation in the ventral horn of the cervical cord. The subsequent transformation from radial glia to astrocytes also followed ventral to dorsal and rostral to caudal gradients. By PD30, mature appearing astrocytes were immunostained by both antibodies at thoracic levels of the spinal cord, the levels lesioned in the plasticity experiments referred to above, and by PD41, they were found at all levels of the cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of glial fibrillary acidic protein-immunopositive structures in the brain of the domestic chicken (Gallus domesticus)

The present study is the first comprehensive mapping of glial fibrillary acidic protein (GFAP)-immunopositive structures in the avian brain. Two main types of GFAP-immunopositive elements were observed: (1) nonbranching fibers, occasionally twisted or varicose, and (2) star-shaped cells. Long immunostained fibers originate from the lateral ventricle to form three bundles. Fibers of the dorsal group, emanating from the dorsal/lateral corner of the ventricle, course in lateral, anterior, and ventral directions forming a semidome, which separates the outer pallial (lateral cortical) regions from the underlying striatal mass. The middle group of fibers is directed anteriorly and laterally corresponding to the laminae frontales superior and suprema. The ventral fiber bundle is conical and traverses the lobus parolfactorius, crossing also the lamina medullaris dorsalis (the latter consisting mainly of star-shaped cells). The hippocampus, septum, and hypothalamus also contain straight radial fibers. In some areas, given their variable orientation, the fibers cannot be regarded as merely persisting radial glia. In the telencephalon, the nuclei basalis, accumbens, ectostriatum, paleostriatum primitivum, and the ventral paleostriatum are particularly rich in GFAP-positive cells, whereas the neostriatum, hyperstriatum, and paleostriatum augmentatum are almost devoid of GFAP labelling. Certain nuclei of the thalamus and the lower brainstem are conspicuous by their low levels of GFAP immunoreactivity. The Bergmann glia were GFAP-immunonegative.

Carnosine in the brain and olfactory system of amphibia and reptilia: a comparative study using immunocytochemical and biochemical methods

The pattern of distribution of carnosine-like immunoreactivity and its relation to glial fibrillary acidic protein immunoreactivity have been studied in two lizards (Gallotia galloti and Tarentola delalandii) and in two anuran amphibians (Rana esculenta and Xenopus laevis) using immunocytochemical techniques. Biochemical data obtained by paper electrophoresis show that the dipeptides carnosine and homocarnosine are both present in the brain of all the species examined. In the central nervous system of both anurans and reptilians, carnosine immunoreactivity is localized in glial cells. An important species difference is, however, seen in the olfactory system since primary olfactory neurons and their processes extending to the olfactory bulb are carnosine positive in reptiles, whereas they are not immunostained in anurans. Thus, the cellular distribution of carnosine immunoreactivity in reptilians is very similar to that observed in birds and mammals and is distinct from that seen in amphibia.