Microstructural brain development after perinatal cerebral white matter injury assessed by diffusion tensor magnetic resonance imaging

OBJECTIVE

Brain injury in premature infants is characterized predominantly by perinatally acquired lesions in the cerebral white matter (WM). The impact of such injury on the subsequent development of cerebral WM is not clear. This study uses diffusion tensor magnetic resonance imaging (MRI) to evaluate the effects of cerebral WM injury on subsequent microstructural brain development in different WM areas of the brain.

METHODS

Twenty premature infants (gestational age: 29.1 +/- 1.9 weeks) were studied by conventional MRI within the first 3 weeks of life and again at term, with the addition at the latter time of diffusion tensor MRI. Ten of the preterm infants had cerebral WM injury identified by the early MRI and were matched with 10 premature infants of similar gestational age and neonatal course but with normal neonatal MRI scans. Diffusion tensor MRI at term was acquired in coronal and axial planes and used to determine the apparent diffusion coefficient, a measure of overall restriction to water diffusion, and the relative anisotropy (RA), a measure of preferred directionality of diffusion, in central WM, anterior frontal WM, occipital WM, temporal WM, and the posterior limb of the internal capsule. Diffusion vector maps were generated from the diffusion tensor analysis to define the microstructural architecture of the cerebral WM regions.

RESULTS

At term, the diffusion tensor MRI revealed no difference in apparent diffusion coefficient among preterm infants with or without perinatal WM lesions. By contrast, RA, the measure of preferred directionality of diffusion and thereby dependent on development of axonal fibers and oligodendroglia, was 25% lower in central WM, the principal site of the original WM injury. However, RA was unaffected in relatively uninjured WM areas, such as temporal, anterior frontal, and occipital regions. Notably, RA values in the internal capsule, which contains fibers that descend from the injured cerebral WM, were 20% lower in the infants with WM injury versus those without. Diffusion vector maps showed striking alterations in the size, orientation, and organization of fiber tracts in central WM and in those descending to the internal capsule.

CONCLUSIONS

Perinatal cerebral WM injury seems to have major deleterious effects on subsequent development of fiber tracts both in the cerebral WM and more distally. The ultimate impact of brain injury in the newborn should be considered as a function not only of tissue destruction, but also of impaired subsequent brain development.

Arl13b-regulated cilia activities are essential for polarized radial glial scaffold formation

The construction of cerebral cortex begins with the formation of radial glia. Once formed, polarized radial glial cells divide either symmetrically or asymmetrically to balance appropriate production of progenitor cells and neurons. Following birth, neurons use the processes of radial glia as scaffolding for oriented migration. Radial glia therefore provide an instructive structural matrix to coordinate the generation and placement of distinct groups of cortical neurons in the developing cerebral cortex. We found that Arl13b, a cilia-enriched small GTPase that is mutated in Joubert syndrome, was critical for the initial formation of the polarized radial progenitor scaffold. Using developmental stage-specific deletion of Arl13b in mouse cortical progenitors, we found that early neuroepithelial deletion of ciliary Arl13b led to a reversal of the apical-basal polarity of radial progenitors and aberrant neuronal placement. Arl13b modulated ciliary signaling necessary for radial glial polarity. Our findings indicate that Arl13b signaling in primary cilia is crucial for the initial formation of a polarized radial glial scaffold and suggest that disruption of this process may contribute to aberrant neurodevelopment and brain abnormalities in Joubert syndrome-related ciliopathies.

An intimate interplay between precocious, migrating pericytes and endothelial cells governs human fetal brain angiogenesis

In order to better understand the process of angiogenesis in the developing human brain, we have examined the spatial relationship and relative contributions of endothelial cells and pericytes, the two primary cell types involved in vessel growth, together with their relation with the vascular basement membrane. Pericytes were immunolocalized through use of the specific markers nerve/glial antigen 2 (NG2) proteoglycan, endosialin (CD248) and the platelet-derived growth factor receptor beta (PDGFR-beta), while endothelial cells were identified by the pan-endothelial marker CD31 and the blood brain barrier (BBB)-specific markers claudin-5 and glucose transporter isoform 1 (GLUT-1). The quantitative analysis demonstrates that microvessels of the fetal human telencephalon are characterized by a continuous layer of activated/angiogenic NG2 pericytes, which tightly invest endothelial cells and participate in the earliest stages of vessel growth. Immunolabelling with anti-active matrix metalloproteinase-2 (aMMP-2) and anti-collagen type IV antibodies revealed that aMMP-2 producing endothelial cells and pericytes are both associated with the vascular basement membrane during vessel sprouting. Detailed localization of the two vascular cell types during angiogenesis suggests that growing microvessels of the human telencephalon are formed by a pericyte-driven angiogenic process in which the endothelial cells are preceded and guided by migrating pericytes during organization of the growing vessel wall.

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

Defective associations between blood vessels and brain parenchyma lead to cerebral hemorrhage in mice lacking alphav integrins

Mouse embryos genetically null for the alphav integrin subunit develop intracerebral hemorrhages at midgestation and die shortly after birth. A key question is whether the hemorrhage arises from primary defects in vascular endothelial cells or pericytes or from other causes. We have previously reported normal initiation of cerebral vessels comprising branched tubes of endothelial cells. Here we show that the onset of hemorrhage is not due to defects in pericyte recruitment. Additionally, most alphav-null vessels display ultrastructurally normal endothelium-pericyte associations and normal interendothelial cell junctions. Thus, endothelial cells and pericytes appear to establish their normal relationships in cerebral microvessels. However, by both light and electron microscopy, we detected defective associations between cerebral microvessels and the surrounding brain parenchyma, composed of neuroepithelial cells, glia, and neuronal precursors. These data suggest a novel role for alphav integrins in the association between cerebral microvessels and central nervous system parenchymal cells.

GABAergic basal forebrain neurons project to the neocortex: the localization of glutamic acid decarboxylase and choline acetyltransferase in feline corticopetal neurons

Our objective was to determine whether GABAergic and cholinergic basal forebrain neurons project to the neocortex. The retrograde connectivity marker wheat germ agglutinin lectin-bound horseradish peroxidase was injected into the neocortex of adult cats. Histo- and immunohistochemical methods were combined to label sequentially connectivity and transmitter markers (glutamic acid decarboxylase; choline acetyltransferase) in forebrain neurons. The labels of each marker were identified by correlative light and electron microscopy. Two principal types of doubly labeled neurons were demonstrated. The connectivity marker was colocalized with glutamic acid decarboxylase or choline acetyltransferase. The neurons were located in the basal forebrain. Their ultrastructural, cellular, and regional organization supported 2 conclusions. (1) GABAergic basal forebrain neurons project to the neocortex. This is important new morphological evidence for the origin of inhibitory neocortical afferents from a subcortical brain site. (2) The GABAergic and cholinergic basal forebrain neurons projecting to the neocortex exhibit remarkable structural similarities. The transmitter diversity of these intertwined neocortical afferents may be significant for the pathology and treatment of human neurological disorders such as Alzheimer's disease.

An ultrastructural study of the normal synaptic organization of the optic tectum and the degenerating tectal afferents from retina, telencephalon, and contralateral tectum in a teleost, Holocentrus rufus

The ultrastructure of the optic tectum in the squirrel fish, Holocentrus rufus, has been studied and the normal synaptic organization is described. Synaptic terminals were classified into eight types (S1-S6, F1, F2) by their morphology and synaptic relations. The distribution pattern for each type of terminal was determined by counting the relative number of terminals in each layer. Most S1 terminals are localized in Stratum marginale (SM), whereas S2 terminals are most common in Stratum fibrtosum et griseum superficiale (SFGS). S3, S4, S5, F1, and F2 terminals are limited mainly to SFGS and Stratum griseum centrale (SGC). S6 terminals are most frequently seen in SGC and Stratum griseum periventriculare (SPV). In order to determine the origins of the various types of synaptic terminals in the optic tectum, the telencephalon, eye, and optic tectum were removed unilaterally and areas of resultant degeneration examined. Electron microscopic observations show that nearly all S2 terminals in SFGS of the contralateral optic tectum degenerate after eye enucleation, whereas some S4 terminals in SFGS and SGC exhibit degenerative changes after removal of the ipsilateral telencephalon. Unilateral ablation of the optic tectum was associated with degenerative changes in occasional S5 terminals within SGC of contralateral optic tectum. All experiments resulted in some increased electron density of S3 and F terminals, some of which were identified as F2 terminals. The possible origins of S1 and S6 terminals, which were not altered in the present experiments, are discussed.

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.

Microvasculature of the human cerebral white matter: arteries of the deep white matter

The vascular architecture of the human cerebral deep white matter was studied using soft X-ray and diaphanized specimens, achieved by intra-arterial injection of barium and vascular stain respectively, and also by electron microscopic examination of the corrosion cast of arteries in normal adult brains. The deep white matter arteries passed through the cerebral cortex with a few branches to the cortex and ran straight through the white matter. The arteries concentrated ventriculopetally to the white matter around the lateral ventricle. Anastomoses were noted around the ventricular wall at the terminals of the deep white matter arteries. No centrifugal branches irrigating the periventricular white matter from the lenticulo-striate arteries were observed in the present study. The presence of anastomoses among the terminal branches of deep white matter arteries protects against ischemic change or infarction in this area from an occlusion of a single deep white matter artery. This may lead to development of terminal zone infarction from ischemia or vascular diseases, affecting multiple deep white matter arteries. The subcortical and deep white matter arteries had thick adventitial sheaths and large adventitial spaces in the white matter but not in the cortex. The presence or absence of the adventitial space is regarded as another characteristic difference between the arteries in the white matter and cortex. This difference may influence pathological changes in vascular lesions in these respective areas.

Disrupted myelin and axon loss in the anterior commissure of the aged rhesus monkey

This study assesses the effects of age on the composition of the anterior commissure of the rhesus monkey. The anterior commissures of nine young (5-10 years), five middle-aged (15-20 years), and eight old (25-35 years) monkeys were examined by light and electron microscopy. In all, 90-95% of the nerve fibers in the anterior commissure are myelinated. With age, the structure of the myelin sheaths of some nerve fibers is altered. Some of the axons also show signs of degeneration and this leads to a loss of nerve fibers. Thus, in young and the middle-aged monkeys the mean number of myelinated nerve fibers in the anterior commissure is 2.2 x 10(6), while in the old monkeys the mean is 1.2 x 10(6). Increasing age is correlated with a reduction in the number of myelinated nerve fibers in the anterior commissure, an increase in the frequency of structural alterations in myelin sheaths, and an increase in the frequency of occurrence of degenerating axons. However, the number of myelinated nerve fibers is the only variable that correlates with cognition: in monkeys 5-20 years of age the fewer the number of nerve fibers the poorer the cognitive performance, as measured by our Cognitive Impairment Index (CII). The most common neuroglial cells in the anterior commissure are oligodendrocytes. They account for 86% of all neuroglial cell profiles, while astrocytes account for 9%, and microglial cells for 5% of profiles. There is no apparent change with age in the total numbers of neuroglial cells, although as they age each of the neuroglial cell types acquires some inclusions in their cytoplasm. The data, together with those from previous studies, support the concept that in aging there is a ubiquitous loss of myelinated nerve fibers from the brain and that fiber loss is preceded by alterations in the structure of many of the myelin sheaths.

Differential localization and colocalization of two neuron-types of sodium-dependent inorganic phosphate cotransporters in rat forebrain

We studied by immunohistochemistry the distribution of differentiation-associated sodium-dependent inorganic phosphate (Pi) cotransporter (DNPI) in the rat forebrain, in comparison with brain-specific cotransporter (BNPI). DNPI-staining was principally seen in axonal synaptic terminals which showed a widespread but discrete pattern of distribution different from that of the BNPI-staining. In the diencephalon, marked DNPI-staining was seen in the dorsal lateral geniculate, medial geniculate, ventral posterolateral, ventral posteromedial, anterior, and reticular thalamic nuclei without the colocalization with BNPI-staining. DNPI-staining showed a strong mosaical pattern and overlapped well the BNPI-staining in the medial habenular nucleus. DNPI-staining was moderate over the hypothalamus and notably localized in neurosecretory terminals containing corticotropin-releasing hormone in the median eminence. In contrast, the BNPI-staining was region-related and strong in the ventromedial and mammillary nuclei. In the telencephalon, laminar DNPI-staining was seen over the neocortex, corresponding to the thalamocortical termination, and also found in the retrosplenial cortex and the striatum, with the highest intensity in the accumbens nucleus shell. The present results suggest that DNPI serves as a dominant Pi transport system in synaptic terminals of diencephalic neurons including thalamocortical and thalamostriatal pathways as well as the hypothalamic neuroendocrine system in the rat forebrain.

Ultrastructural evidence of amygdalofugal axons terminating on cholinergic cells of the rostral forebrain

In the present study a double-label ultrastructural procedure was used to study amygdalofugal fibers contacting cholinergic cells of the rostral forebrain. Following horseradish peroxidase (HRP) injections into the basolateral amygdala, anterogradely transported HRP was detected in axon terminals contacting the dendrites of choline acetyltransferase-containing cells in the ventral pallidum.

Specific characteristic of radial glia in the human fetal telencephalon

Phenotypic characteristics of cells in the developing human telencephalic wall were analyzed using electron microscopy and immunocytochemistry with various glial and neuronal cell markers. The results suggest that multiple defined cell types emerge in the neocortical proliferative zones and are differentially regulated during embryonic development. At 5-6 weeks gestation, three major cell types are observed. Most proliferating ventricular zone (VZ) cells are labeled with radial glial (RG) markers such as vimentin, glial fibrillary acidic protein (GFAP), and glutamate astrocyte-specific transporter (GLAST) antibodies. A subpopulation of these RG cells also express the neuronal markers beta III-tubulin, MAP-2, and phosphorylated neurofilament SMI-31, in addition to the stem cell marker nestin, indicating their multipotential capacity. In addition, the presence of VZ cells that immunoreact only with neuronal markers indicates the emergence of restricted neuronal progenitors. The number of multipotential progenitors in the VZ gradually decreases, whereas the number of more restricted progenitors increases systematically during the 3-month course of human corticogenesis. These results suggest that multipotential progenitors coexist with restricted neuronal progenitors and RG cells during initial corticogenesis in the human telencephalon. Since the multipotential VZ cells disappear during the major wave of neocortical neurogenesis, the RG and restricted neuronal progenitors appear to serve as the main sources of cortical neurons. Thus, the diversification of cells in human VZ and overlying subventricular zone (SVZ) begins earlier and is more pronounced than in rodents.

Chloroquine-induced neuronal cell death is p53 and Bcl-2 family-dependent but caspase-independent

Chloroquine is a lysosomotropic agent that causes marked changes in intracellular protein processing and trafficking and extensive autophagic vacuole formation. Chloroquine may be cytotoxic and has been used as a model of lysosomal-dependent cell death. Recent studies indicate that autophagic cell death may involve Bcl-2 family members and share some features with caspase-dependent apoptotic death. To determine the molecular pathway of chloroquine-induced neuronal cell death, we examined the effects of chloroquine on primary telencephalic neuronal cultures derived from mice with targeted gene disruptions in p53, and various caspase and bcl-2 family members. In wild-type neurons, chloroquine produced concentration- and time-dependent accumulation of autophagosomes, caspase-3 activation, and cell death. Cell death was inhibited by 3-methyladenine, an inhibitor of autophagic vacuole formation, but not by Boc-Asp-FMK (BAF), a broad caspase inhibitor. Targeted gene disruptions of p53 and bax inhibited and bcl-x potentiated chloroquine-induced neuron death. Caspase-9- and caspase-3-deficient neurons were not protected from chloroquine cytotoxicity. These studies indicate that chloroquine activates a regulated cell death pathway that partially overlaps with the apoptotic cascade.

Hemispheric asymmetry of synapses in chick medial hyperstriatum ventrale following passive avoidance training: a stereological investigation

A stereological analysis was made of synapses in the left and right hemispheres of chick medial hyperstriatum ventrale (MHV) 24h after passive avoidance training (PAL) and in water trained controls (W-control). The synaptic parameters examined were (D), the mean length of the postsynaptic thickening; (NV.syn), the number of synapses per unit volume of neuropil; (VV.syn), the volume density of the pre-synaptic bouton; (NV.ves), the number of synaptic vesicles per unit volume of neuropil and (ves.syn), the mean number of synaptic vesicles per pre-synaptic bouton. No significant differences exist in NV.syn between the left or right hemispheres of W-control and trained chicks, nor is NV.syn influenced by training. However, in W-control chicks D in the right MHV is significantly greater (12%) than in the left MHV and this difference disappears on training. There are no differences in VV.syn between left and right hemispheres of W-control chicks but following training VV.syn is 22.7% greater in the left MHV than in the right MHV. Training als influences the number of synaptic vesicles; in W-control chicks NV.ves in the right MHV is 12.25% greater than in the left MHV but following training these differences are reversed. When the data are expressed as numbers of vesicles per synapse (ves.syn), values for the left hemisphere of trained chicks exceed those in the right hemisphere by a staggering 61.38%. These results are discussed in the context of biochemical and electrophysiological studies which suggest that there is lateralization of the memory trace.

Avian visual behavior and the organization of the telencephalon

Birds have excellent visual abilities that are comparable or superior to those of primates, but how the bird brain solves complex visual problems is poorly understood. More specifically, we lack knowledge about how such superb abilities are used in nature and how the brain, especially the telencephalon, is organized to process visual information. Here we review the results of several studies that examine the organization of the avian telencephalon and the relevance of visual abilities to avian social and reproductive behavior. Video playback and photographic stimuli show that birds can detect and evaluate subtle differences in local facial features of potential mates in a fashion similar to that of primates. These techniques have also revealed that birds do not attend well to global configural changes in the face, suggesting a fundamental difference between birds and primates in face perception. The telencephalon plays a major role in the visual and visuo-cognitive abilities of birds and primates, and anatomical data suggest that these animals may share similar organizational characteristics in the visual telencephalon. As is true in the primate cerebral cortex, different visual features are processed separately in the avian telencephalon where separate channels are organized in the anterior-posterior axis roughly parallel to the major laminae. Furthermore, the efferent projections from the primary visual telencephalon form an extensive column-like continuum involving the dorsolateral pallium and the lateral basal ganglia. Such a column-like organization may exist not only for vision, but for other sensory modalities and even for a continuum that links sensory and limbic areas of the avian brain. Behavioral and neural studies must be integrated in order to understand how birds have developed their amazing visual systems through 150 million years of evolution.

Synthesis and in vitro pharmacology of substituted quinoline-2,4-dicarboxylic acids as inhibitors of vesicular glutamate transport

The vesicular glutamate transport (VGLUT) system selectively mediates the uptake of L-glutamate into synaptic vesicles. Uptake is linked to an H+-ATPase that provides coupling among ATP hydrolysis, an electrochemical proton gradient, and glutamate transport. Substituted quinoline-2,4-dicarboxylic acids (QDCs), prepared by condensation of dimethyl ketoglutaconate (DKG) with substituted anilines and subsequent hydrolysis, were investigated as potential VGLUT inhibitors in synaptic vesicles. A brief panel of substituted QDCs was previously reported (Carrigan et al. Bioorg. Med. Chem. Lett. 1999, 9, 2607-2612)(1) and showed that certain substituents led to more potent competitive inhibitors of VGLUT. Using these compounds as leads, an expanded series of QDC analogues were prepared either by condensation of DKG with novel anilines or via aryl-coupling (Suzuki or Heck) to dimethyl 6-bromoquinolinedicarboxylate. From the panel of almost 50 substituted QDCs tested as inhibitors of the VGLUT system, the 6-PhCH=CH-QDC (K(i) = 167 microM), 6-PhCH2CH2-QDC (K(i) = 143 microM), 6-(4'-phenylstyryl)-QDC (K(i) = 64 microM), and 6-biphenyl-4-yl-QDC (K(i) = 41 microM) were found to be the most potent blockers. A preliminary assessment of the key elements needed for binding to the VGLUT protein based on the structure-activity relationships for the panel of substituted QDCs is discussed herein. The substituted QDCs represent the first synthetically derived VGLUT inhibitors and are promising templates for the development of selective transporter inhibitors.

Eomes::GFP-a tool for live imaging cells of the trophoblast, primitive streak, and telencephalon in the mouse embryo

Expression of T-box family member Eomesodermin (Tbr2) is spatiotemporally restricted in the mouse embryo; initially expressed in extraembryonic lineages in the sequential progression from the trophectoderm of the blastocyst, its derivatives the extraembryonic ectoderm, and thereafter the chorion, in addition to the visceral endoderm and primitive streak at gastrula stages, and the telencephalon at later stages. We describe the spatiotemporal expression of GFP in embryos of a Tg(Eomes::GFP) BAC transgenic strain, and have compared it with the localization of endogenous Eomes transcripts and protein. Our analysis reveals the following: (1) robust easily visualized reporter expression in live hemizygous transgenic embryos, (2) increased levels of expression in live homozygous transgenic embryos that are compatible with embryo viability, and (3) a close correlation between endogenous Eomes and GFP reporter expression in BAC transgenic embryos. These features establish the Tg(Eomes::GFP) BAC transgenic strain as a novel reagent for both live imaging and the isolation of Eomes expressing cells from specific locations within the embryo.

Cytoarchitecture and ultrastructure of nucleus prethalamicus, with special reference to degenerating afferents from optic tectum and telencephalon, in a teleost (Holocentrus ascensionis)

Histological structure and neuronal geometry of the nucleus prethalamicus of holocentrid teleosts, which is homologous to the nucleus rotundus of reptiles and birds and to the nucleus lateralis posterior-pulvinar complex of mammals, were studied by means of the Bodian, Nissl, toluidine blue, and Golgi methods. Synaptic terminals were classified electron microscopically, and terminal types originating from the telencephalon and the optic tectum were determined by electron microscopy in degeneration experiments. The nucleus prethalamicus is composed of four layers, in the following order from medial to lateral: a small-cell layer, a plexiform layer, a large-cell layer, and a marginal layer. Six types of terminals (U, L, Sp, Sd, F, and P) were distinguished in the nucleus, and the distribution pattern for each type of terminal was determined by counting its relative number in each layer. Sp terminals make synaptic contacts with small-cell dendrites or somata in the small-cell layer, and degenerate after telencephalic ablations. Sd terminals synapse exclusively with spines of large-cell dendrites in both marginal and large-cell layers, and degenerate after tectal ablations. Because only large neurons have been labeled after HRP injections into the telencephalon (Ito et al., '80, '82; Ebbesson, '80; Murakami et al., '83), it is considered that these neurons relay visual information from the optic tectum onto the telencephalon. It is hypothesized that the small neurons in the nucleus, which receive telencephalic input, might modulate the large neurons' relay function.

Subcellular compartmentalization of aromatase is sexually dimorphic in the adult zebra finch brain

The vertebrate brain is a source of estrogen (E) via the expression of aromatase (E-synthase). In the zebra finch (Taeniopygia guttata), despite documented dimorphisms in E-action, no differences are detectable in circulating E, or the neural levels of aromatase transcription, activity, or somal protein expression. Studies of aromatase expression at the light- and electron-microscope levels reveal greater numbers of fibers and presynaptic boutons in adult males relative to females. We assayed aromatase activity and content in synaptosomes and microsomes from the anterior [containing lMAN and Area X (males)] and posterior telencephalon (containing HVC and RA) of adult birds. In contrast to non-song birds and mammals, both cell fractions contain abundant aromatase measurable in terms of activity (enzyme assays) and content (Western blots) with minimal enrichment in microsomes. From brain homogenates of identical concentration, aromatase activity was higher in the synaptosomal relative to the microsomal fraction, in males relative to females, and in the posterior compared to anterior telencephalon. These effects were driven by high levels of synaptosomal aromatase in the male posterior telencephalon. These data suggest that males possess more aromatase per presynaptic bouton, or a greater number of aromatase-containing presynaptic boutons than females in the posterior telencephalon. Further, the present report reveals synaptic aromatization as a considerable source of E in the zebra finch brain, and supports the idea that telencephalic synapses in and around the adult male song production nuclei may be exposed to higher levels of E compared to the female brain.

Cellular correlates of stages of memory formation in the chick following passive avoidance training

The process of memory formation has been investigated using the model of one-trial passive avoidance training in the one-day old domestic chick. We have unraveled a biochemically coherent cascade of processes which, beginning with transient ion and neurotransmitter flux, and by way of a sequence of interacting pre- and post-synaptic intracellular signalling steps, results in gene activation and the synthesis of cell adhesion molecules which appear to be the effective agents in the structural processes involved in remodelling of synaptic and neuronal circuits. Further, in a related series of experiments we have shown that these biochemical and morphological changes are accompanied by significant changes in the neurophysiological status of the neurons on the IMHV and LPO, in particular in terms of their engagement in bouts of high-frequency firing. However, much remains to be clarified, particularly the meaning of the time-dependent shifts in the location of the trace, and the ways in which these molecular and cellular events translate into changes in behavior in the animal.

The island of Calleja complex of rat basal forebrain. I. Light and electron microscopic observations

An analysis of the cells and their processes within the island of Calleja complexes (ICC) was made in light and electron microscopic preparations to determine synaptic relationships within this part of the basal forebrain. The light microscopic preparations showed that the ICC contained two cell types, granule cells and large cells. In electron microscopic preparations, the somata of granule cells were grouped together and were directly apposed to other somata of granule cells. Specialized junctions (4-6 nm wide) that occurred at sites of somal apposition suggested ephaptic coupling of granule cells. The granule cell somata had nuclei that contained clumps of heterochromatin adjacent to smooth nuclear envelopes. The perikaryal cytoplasm of these cells consisted of a relatively thin rim containing few organelles. Spinous dendrites of small diameter were occasionally found in continuity with these cells. Axon terminals rarely formed synapses with the somata of granule cells, but were more frequently found to synapse on their dendrites and dendritic spines. These features for granule cells are similar to those for medium-sized spiny neurons in the neostriatum. The somata of the large cells were found either within the core or along the dorsal margin of the ICC. The large cells had infolded nuclei and an abundant perikaryal cytoplasm that contained many organelles. Large diameter dendrites that tapered down to smaller diameters emanated in many directions from these somata. Axon terminals covered nearly the entire surface of these somata and dendrites where they commonly formed symmetric synaptic junctions. These characteristics of large cells indicate a resemblance to the large cells in the globus pallidus and ventral pallidum. Therefore, the ICC have ultrastructural features found in both the neostriatum and globus pallidus.

The effect of hyperthermia on the neuroepithelium of the 21-day guinea-pig foetus: histologic and ultrastructural study

Hyperthermia was induced in guinea-pigs on day 21 of gestation by placing them in an incubator set at 42-5 degrees-43-0 degrees C for 1 hr. At intervals thereafter foetuses were removed from the uterus and sections of the telencephalon were prepared for light and electron microscopy. The histologic and ultrastructural appearance of the telencephalon of the normal 21-day guinea-pig foetus was described for comparative purposes. Damage to cells in mitosis characterised by clumping of chromosomes, and dispersal of polysomes in interphase cells were observed immediately after hyperthermia. Breakdown of the network of junctional complexes was apparent at 4 hr and cellular proliferation was inhibited for 6-8 hr. Degenerative changes and cell deaths were observed deep in the venticular zone after 8 hr; the extent of cell death was related to the post-stressing temperature. Proliferation was resumed at 8 hr and damaged and dead cells moved outward toward the intermediate zone. Phagocytosis of debris by large mononuclear cells was a common finding. Cytoplasmic inclusions, some of which were Feulgen-positive, were present in otherwise normal ventricular cells. Occasional dead cells and empty spaces were present in the ventricular zone at 24 hr and by 48 hr the ventricular zone was normal in appearance. It was concluded that previously observed micrencephaly in the offspring of guine-pig mothers which were heat stressed on day 21 of gestation resulted from a temporary cessation of proliferation and partial depopulation of the proliferating neuroepithelium.