Postnatal changes in the number of astrocytes, oligodendrocytes, and microglia in the visual cortex (area 17) of the macaque monkey: a stereological analysis in normal and monocularly deprived animals

Temporal Alterations in White Matter in An App Knock-In Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia and results in neurodegeneration and cognitive impairment. White matter (WM) is affected in AD and has implications for neural circuitry and cognitive function. The trajectory of these changes across age, however, is still not well understood, especially at earlier stages in life. To address this, we used the AppNL-G-F/NL-G-F knock-in (APPKI) mouse model that harbors a single copy knock-in of the human amyloid precursor protein (APP) gene with three familial AD mutations. We performed in vivo diffusion tensor imaging (DTI) to study how the structural properties of the brain change across age in the context of AD. In late age APPKI mice, we observed reduced fractional anisotropy (FA), a proxy of WM integrity, in multiple brain regions, including the hippocampus, anterior commissure (AC), neocortex, and hypothalamus. At the cellular level, we observed greater numbers of oligodendrocytes in middle age (prior to observations in DTI) in both the AC, a major interhemispheric WM tract, and the hippocampus, which is involved in memory and heavily affected in AD, prior to observations in DTI. Proteomics analysis of the hippocampus also revealed altered expression of oligodendrocyte-related proteins with age and in APPKI mice. Together, these results help to improve our understanding of the development of AD pathology with age, and imply that middle age may be an important temporal window for potential therapeutic intervention.

Abnormal Trajectory of Intracortical Myelination in Schizophrenia Implicates White Matter in Disease Pathophysiology and the Therapeutic Mechanism of Action of Antipsychotics

BACKGROUND

Postmortem and imaging studies provide converging evidence that the frontal lobe myelination trajectory is dysregulated in schizophrenia (SZ) and suggest that early in treatment, antipsychotic medications increase intracortical myelin (ICM). We used magnetic resonance imaging to examine whether the ICM trajectory in SZ is dysregulated and altered by antipsychotic treatment.

METHODS

We examined 93 subjects with SZ (64 men and 29 women) taking second-generation oral antipsychotics with medication exposures of 0-333 months in conjunction with 80 healthy control subjects (52 men and 28 women). Frontal lobe ICM volume was estimated using a novel dual contrast magnetic resonance imaging method that combines two images that track different tissue components.

RESULTS

When plotted against oral antipsychotic exposure duration, ICM of subjects with SZ was higher as a function of medication exposure during the first year of treatment but declined thereafter. In the age range examined, ICM of subjects with SZ was lower with increased age, while ICM of healthy control subjects was not.

CONCLUSIONS

In adults with SZ, the relationship between length of exposure to oral second-generation antipsychotics and ICM was positive during the first year of treatment but was negative after this initial period, consistent with suboptimal later adherence after initial adherence. This ICM trajectory resembles clinically observed antipsychotic response trajectory with high rates of remission in the first year followed by progressively lower response rates. The results support postmortem evidence that SZ pathophysiology involves ICM deficits and suggest that correcting these deficits may be an important mechanism of action for antipsychotics.

Real-time optical diagnosis of the rat brain exposed to a laser-induced shock wave: observation of spreading depolarization, vasoconstriction and hypoxemia-oligemia

Despite many efforts, the pathophysiology and mechanism of blast-induced traumatic brain injury (bTBI) have not yet been elucidated, partially due to the difficulty of real-time diagnosis and extremely complex factors determining the outcome. In this study, we topically applied a laser-induced shock wave (LISW) to the rat brain through the skull, for which real-time measurements of optical diffuse reflectance and electroencephalogram (EEG) were performed. Even under conditions showing no clear changes in systemic physiological parameters, the brain showed a drastic light scattering change accompanied by EEG suppression, which indicated the occurrence of spreading depression, long-lasting hypoxemia and signal change indicating mitochondrial energy impairment. Under the standard LISW conditions examined, hemorrhage and contusion were not apparent in the cortex. To investigate events associated with spreading depression, measurement of direct current (DC) potential, light scattering imaging and stereomicroscopic observation of blood vessels were also conducted for the brain. After LISW application, we observed a distinct negative shift in the DC potential, which temporally coincided with the transit of a light scattering wave, showing the occurrence of spreading depolarization and concomitant change in light scattering. Blood vessels in the brain surface initially showed vasodilatation for 3-4 min, which was followed by long-lasting vasoconstriction, corresponding to hypoxemia. Computer simulation based on the inverse Monte Carlo method showed that hemoglobin oxygen saturation declined to as low as ∼35% in the long-term hypoxemic phase. Overall, we found that topical application of a shock wave to the brain caused spreading depolarization/depression and prolonged severe hypoxemia-oligemia, which might lead to pathological conditions in the brain. Although further study is needed, our findings suggest that spreading depolarization/depression is one of the key events determining the outcome in bTBI. Furthermore, a rat exposed to an LISW(s) can be a reliable laboratory animal model for blast injury research.

Multimodal magnetic resonance imaging assessment of white matter aging trajectories over the lifespan of healthy individuals

BACKGROUND

Postmortem and volumetric imaging data suggest that brain myelination is a dynamic lifelong process that, in vulnerable late-myelinating regions, peaks in middle age. We examined whether known regional differences in axon size and age at myelination influence the timing and rates of development and degeneration/repair trajectories of white matter (WM) microstructure biomarkers.

METHODS

Healthy subjects (n = 171) 14-93 years of age were examined with transverse relaxation rate (R(2)) and four diffusion tensor imaging measures (fractional anisotropy [FA] and radial, axial, and mean diffusivity [RD, AxD, MD, respectively]) of frontal lobe, genu, and splenium of the corpus callosum WM (FWM, GWM, and SWM, respectively).

RESULTS

Only R(2) reflected known levels of myelin content with high values in late-myelinating FWM and GWM regions and low ones in early-myelinating SWM. In FWM and GWM, all metrics except FA had significant quadratic components that peaked at different ages (R(2) < RD < MD < AxD), with FWM peaking later than GWM. Factor analysis revealed that, although they defined different factors, R(2) and RD were the metrics most closely associated with each other and differed from AxD, which entered into a third factor.

CONCLUSIONS

The R(2) and RD trajectories were most dynamic in late-myelinating regions and reflect age-related differences in myelination, whereas AxD reflects axonal size and extra-axonal space. The FA and MD had limited specificity. The data suggest that the healthy adult brain undergoes continual change driven by development and repair processes devoted to creating and maintaining synchronous function among neural networks on which optimal cognition and behavior depend.

Postnatal development of the amygdala: A stereological study in macaque monkeys

Abnormal development of the amygdala has been linked to several neurodevelopmental disorders, including schizophrenia and autism. However, the postnatal development of the amygdala is not easily explored at the cellular level in humans. Here we performed a stereological analysis of the macaque monkey amygdala in order to characterize the cellular changes underlying its normal structural development in primates. The lateral, basal, and accessory basal nuclei exhibited the same developmental pattern, with a large increase in volume between birth and 3 months of age, followed by slower growth continuing beyond 1 year of age. In contrast, the medial nucleus was near adult size at birth. At birth, the volume of the central nucleus was half of the adult value; this nucleus exhibited significant growth even after 1 year of age. Neither neuronal soma size, nor neuron or astrocyte numbers changed during postnatal development. In contrast, oligodendrocyte numbers increased substantially, in parallel with an increase in amygdala volume, after 3 months of age. At birth, the paralaminar nucleus contained a large pool of immature neurons that gradually developed into mature neurons, leading to a late increase in the volume of this nucleus. Our findings revealed that distinct amygdala nuclei exhibit different developmental profiles and that the amygdala is not fully mature for some time postnatally. We identified different periods during which pathogenic factors might lead to the abnormal development of distinct amygdala circuits, which may contribute to different human neurodevelopmental disorders associated with alterations of amygdala structure and functions.

Neuroglialpharmacology: myelination as a shared mechanism of action of psychotropic treatments

Current psychiatric diagnostic schema segregate symptom clusters into discrete entities, however, large proportions of patients suffer from comorbid conditions that fit neither diagnostic nor therapeutic schema. Similarly, psychotropic treatments ranging from lithium and antipsychotics to serotonin reuptake inhibitors (SSRIs) and acetylcholinesterase inhibitors have been shown to be efficacious in a wide spectrum of psychiatric disorders ranging from autism, schizophrenia (SZ), depression, and bipolar disorder (BD) to Alzheimer's disease (AD). This apparent lack of specificity suggests that psychiatric symptoms as well as treatments may share aspects of pathophysiology and mechanisms of action that defy current symptom-based diagnostic and neuron-based therapeutic schema. A myelin-centered model of human brain function can help integrate these incongruities and provide novel insights into disease etiologies and treatment mechanisms. Available data are integrated herein to suggest that widely used psychotropic treatments ranging from antipsychotics and antidepressants to lithium and electroconvulsive therapy share complex signaling pathways such as Akt and glycogen synthase kinase-3 (GSK3) that affect myelination, its plasticity, and repair. These signaling pathways respond to neurotransmitters, neurotrophins, hormones, and nutrition, underlie intricate neuroglial communications, and may substantially contribute to the mechanisms of action and wide spectra of efficacy of current therapeutics by promoting myelination. Imaging and genetic technologies make it possible to safely and non-invasively test these hypotheses directly in humans and can help guide clinical trial efforts designed to correct myelination abnormalities. Such efforts may provide insights into novel avenues for treatment and prevention of some of the most prevalent and devastating human diseases.

Understanding aberrant white matter development in schizophrenia: an avenue for therapy?

Although historically gray matter changes have been the focus of neuropathological and neuroradiological studies in schizophrenia, in recent years an increasing body of research has implicated white matter structures and its constituent components (axons, their myelin sheaths and supporting oligodendrocytes). This article summarizes this body of literature, examining neuropathological, neurogenetic and neuroradiological evidence for white matter pathology in schizophrenia. We then look at the possible role that antipsychotic medication may play in these studies, examining both its role as a potential confounder in studies examining neuronal density and brain volume, but also the possible role that these medications may play in promoting myelination through their effects on oligodendrocytes. Finally, the role of potential novel therapies is discussed.

Statistical comparison of spike responses to natural stimuli in monkey area V1 with simulated responses of a detailed laminar network model for a patch of V1

A major goal of computational neuroscience is the creation of computer models for cortical areas whose response to sensory stimuli resembles that of cortical areas in vivo in important aspects. It is seldom considered whether the simulated spiking activity is realistic (in a statistical sense) in response to natural stimuli. Because certain statistical properties of spike responses were suggested to facilitate computations in the cortex, acquiring a realistic firing regimen in cortical network models might be a prerequisite for analyzing their computational functions. We present a characterization and comparison of the statistical response properties of the primary visual cortex (V1) in vivo and in silico in response to natural stimuli. We recorded from multiple electrodes in area V1 of 4 macaque monkeys and developed a large state-of-the-art network model for a 5 × 5-mm patch of V1 composed of 35,000 neurons and 3.9 million synapses that integrates previously published anatomical and physiological details. By quantitative comparison of the model response to the "statistical fingerprint" of responses in vivo, we find that our model for a patch of V1 responds to the same movie in a way which matches the statistical structure of the recorded data surprisingly well. The deviation between the firing regimen of the model and the in vivo data are on the same level as deviations among monkeys and sessions. This suggests that, despite strong simplifications and abstractions of cortical network models, they are nevertheless capable of generating realistic spiking activity. To reach a realistic firing state, it was not only necessary to include both N-methyl-d-aspartate and GABA(B) synaptic conductances in our model, but also to markedly increase the strength of excitatory synapses onto inhibitory neurons (>2-fold) in comparison to literature values, hinting at the importance to carefully adjust the effect of inhibition for achieving realistic dynamics in current network models.

Alzheimer's disease as homeostatic responses to age-related myelin breakdown

The amyloid hypothesis (AH) of Alzheimer's disease (AD) posits that the fundamental cause of AD is the accumulation of the peptide amyloid beta (Aβ) in the brain. This hypothesis has been supported by observations that genetic defects in amyloid precursor protein (APP) and presenilin increase Aβ production and cause familial AD (FAD). The AH is widely accepted but does not account for important phenomena including recent failures of clinical trials to impact dementia in humans even after successfully reducing Aβ deposits. Herein, the AH is viewed from the broader overarching perspective of the myelin model of the human brain that focuses on functioning brain circuits and encompasses white matter and myelin in addition to neurons and synapses. The model proposes that the recently evolved and extensive myelination of the human brain underlies both our unique abilities and susceptibility to highly prevalent age-related neuropsychiatric disorders such as late onset AD (LOAD). It regards oligodendrocytes and the myelin they produce as being both critical for circuit function and uniquely vulnerable to damage. This perspective reframes key observations such as axonal transport disruptions, formation of axonal swellings/sphenoids and neuritic plaques, and proteinaceous deposits such as Aβ and tau as by-products of homeostatic myelin repair processes. It delineates empirically testable mechanisms of action for genes underlying FAD and LOAD and provides "upstream" treatment targets. Such interventions could potentially treat multiple degenerative brain disorders by mitigating the effects of aging and associated changes in iron, cholesterol, and free radicals on oligodendrocytes and their myelin.

Maturational trajectories of cortical brain development through the pubertal transition: unique species and sex differences in the monkey revealed through structural magnetic resonance imaging

Characterizing normal brain development in the rhesus macaque is a necessary prerequisite for establishing better nonhuman primate models of neuropathology. Structural magnetic resonance imaging scans were obtained on 37 rhesus monkeys (20 Male, 17 Female) between 10 and 64 months of age. Effects of age and sex were analyzed with a cross-sectional design. Gray matter (GM) and white matter (WM) volumes were determined for total brain and major cortical regions using an automatic segmentation and parcellation pipeline. Volumes of major subcortical structures were evaluated. Unlike neural maturation in humans, GM volumes did not show a postpubertal decline in most cortical regions, with the notable exception of the prefrontal cortex. Similar to humans, WM volumes increased through puberty with less change thereafter. Caudate, putamen, amygdala, and hippocampus increased linearly as did the corpus callosum. Males and females showed similar maturational patterns, although males had significantly larger brain volumes. Females had a proportionately larger caudate, putamen, and hippocampus, whereas males had both an absolute and relatively larger corpus callosum. The authors discuss the possible implications of these findings for research using the rhesus macaque as a model for neurodevelopmental disorders.

Linking white and grey matter in schizophrenia: oligodendrocyte and neuron pathology in the prefrontal cortex

Neuronal circuitry relies to a large extent on the presence of functional myelin produced in the brain by oligodendrocytes. Schizophrenia has been proposed to arise partly from altered brain connectivity. Brain imaging and neuropathologic studies have revealed changes in white matter and reduction in myelin content in patients with schizophrenia. In particular, alterations in the directionality and alignment of axons have been documented in schizophrenia. Moreover, the expression levels of several myelin-related genes are decreased in postmortem brains obtained from patients with schizophrenia. These findings have led to the formulation of the oligodendrocyte/myelin dysfunction hypothesis of schizophrenia. In this review, we present a brief overview of the neuropathologic findings obtained on white matter and oligodendrocyte status observed in schizophrenia patients, and relate these changes to the processes of brain maturation and myelination. We also review recent data on oligodendrocyte/myelin genes, and present some recent mouse models of myelin deficiencies. The use of transgenic and mutant animal models offers a unique opportunity to analyze oligodendrocyte and neuronal changes that may have a clinical impact. Lastly, we present some recent morphological findings supporting possible causal involvement of white and grey matter abnormalities, in the aim of determining the morphologic characteristics of the circuits whose alteration leads to the cortical dysfunction that possibly underlies the pathogenesis of schizophrenia.

A structural MRI study of human brain development from birth to 2 years

Brain development in the first 2 years after birth is extremely dynamic and likely plays an important role in neurodevelopmental disorders, including autism and schizophrenia. Knowledge regarding this period is currently quite limited. We studied structural brain development in healthy subjects from birth to 2. Ninety-eight children received structural MRI scans on a Siemens head-only 3T scanner with magnetization prepared rapid gradient echo T1-weighted, and turbo spin echo, dual-echo (proton density and T2 weighted) sequences: 84 children at 2-4 weeks, 35 at 1 year and 26 at 2 years of age. Tissue segmentation was accomplished using a novel automated approach. Lateral ventricle, caudate, and hippocampal volumes were also determined. Total brain volume increased 101% in the first year, with a 15% increase in the second. The majority of hemispheric growth was accounted for by gray matter, which increased 149% in the first year; hemispheric white matter volume increased by only 11%. Cerebellum volume increased 240% in the first year. Lateral ventricle volume increased 280% in the first year, with a small decrease in the second. The caudate increased 19% and the hippocampus 13% from age 1 to age 2. There was robust growth of the human brain in the first two years of life, driven mainly by gray matter growth. In contrast, white matter growth was much slower. Cerebellum volume also increased substantially in the first year of life. These results suggest the structural underpinnings of cognitive and motor development in early childhood, as well as the potential pathogenesis of neurodevelopmental disorders.

DNA methylation in the human cerebral cortex is dynamically regulated throughout the life span and involves differentiated neurons

The role of DNA cytosine methylation, an epigenetic regulator of chromatin structure and function, during normal and pathological brain development and aging remains unclear. Here, we examined by MethyLight PCR the DNA methylation status at 50 loci, encompassing primarily 5' CpG islands of genes related to CNS growth and development, in temporal neocortex of 125 subjects ranging in age from 17 weeks of gestation to 104 years old. Two psychiatric disease cohorts--defined by chronic neurodegeneration (Alzheimer's) or lack thereof (schizophrenia)--were included. A robust and progressive rise in DNA methylation levels across the lifespan was observed for 8/50 loci (GABRA2, GAD1, HOXA1, NEUROD1, NEUROD2, PGR, STK11, SYK) typically in conjunction with declining levels of the corresponding mRNAs. Another 16 loci were defined by a sharp rise in DNA methylation levels within the first few months or years after birth. Disease-associated changes were limited to 2/50 loci in the Alzheimer's cohort, which appeared to reflect an acceleration of the age-related change in normal brain. Additionally, methylation studies on sorted nuclei provided evidence for bidirectional methylation events in cortical neurons during the transition from childhood to advanced age, as reflected by significant increases at 3, and a decrease at 1 of 10 loci. Furthermore, the DNMT3a de novo DNA methyl-transferase was expressed across all ages, including a subset of neurons residing in layers III and V of the mature cortex. Therefore, DNA methylation is dynamically regulated in the human cerebral cortex throughout the lifespan, involves differentiated neurons, and affects a substantial portion of genes predominantly by an age-related increase.

Myelin breakdown and iron changes in Huntington's disease: pathogenesis and treatment implications

BACKGROUND

Postmortem and in vivo imaging data support the hypothesis that premature myelin breakdown and subsequent homeostatic remyelination attempts with increased oligodendrocyte and iron levels may contribute to Huntington's Disease (HD) pathogenesis and the symmetrical progress of neuronal loss from earlier-myelinating striatum to later-myelinating regions. A unique combination of in vivo tissue integrity and iron level assessments was used to examine the hypothesis.

METHODS

A method that uses two Magnetic resonance imaging (MRI) instruments operating at different field-strengths was used to quantify the iron content of ferritin molecules (ferritin iron) as well as tissue integrity in eight regions in 11 HD and a matched group of 27 healthy control subjects. Three white matter regions were selected based on their myelination pattern (early to later-myelinating) and fiber composition. These were frontal lobe white matter (Fwm) and splenium and genu of the corpus callosum (Swm and Gwm). In addition, gray matter structures were also chosen based on their myelination pattern and fiber composition. Three striatum structures were assessed [caudate, putamen, and globus pallidus (C, P, and G)] as well as two comparison gray matter regions that myelinate later in development and are relatively spared in HD [Hippocampus (Hipp) and Thalamus (Th)].

RESULTS

Compared to healthy controls, HD ferritin iron levels were significantly increased in striatum C, P, and G, decreased in Fwm and Gwm, and were unchanged in Hipp, Th, and Swm. Loss of tissue integrity was observed in C, P, Fwm, and especially Swm but not Hipp, Th, G, or Gwm. This pattern of findings was largely preserved when a small subset of HD subjects early in the disease process was examined.

CONCLUSIONS

The data suggest early in the HD process, myelin breakdown and changes in ferritin iron distribution underlie the pattern of regional toxicity observed in HD. Prospective studies are needed to verify myelin breakdown and increased iron levels are causal factors in HD pathogenesis. Tracking the effects of novel interventions that reduce myelin breakdown and iron accumulation in preclinical stages of HD could hasten the development of preventive treatments.

Cochlear implants: cortical plasticity in congenital deprivation

Congenital auditory deprivation (deafness) leads to a dysfunctional intrinsic cortical microcircuitry. This chapter reviews these deficits with a particular emphasis on layer-specific activity within the primary auditory cortex. Evidence for a delay in activation of supragranular layers and reduction in activity in infragranular layers is discussed. Such deficits indicate the incompetence of the primary auditory cortex to not only properly process thalamic input and generate output within the infragranular layers, but also incorporate top-down modulations from higher order auditory cortex into the processing within primary auditory cortex. Such deficits are the consequence of a misguided postnatal development. Maturation of primary auditory cortex in deaf animals shows evidence of a developmental delay and further alterations in gross synaptic currents, spread of activation, and morphology of local field potentials recorded at the cortical surface. Additionally, degenerative changes can be observed. When hearing is initiated early in life (e.g., by chronic cochlear-implant stimulation), many of these deficits are counterbalanced. However, plasticity of the auditory cortex decreases with increasing age, so that a sensitive period for plastic adaptation can be demonstrated within the second to sixth months of life in the deaf cat. Potential molecular mechanisms of the existence of sensitive period are discussed. Data from animal research may be compared to electroencephalographic data obtained from cochlear-implanted congenitally deaf children. After cochlear implantation in humans, three phases of plastic adaptation can be observed: a fast one, taking place within the first few weeks after implantation, showing no sensitive period; a slower one, taking place within the first months after implantation (a sensitive period up to 4 years of age); and possibly a third, and the longest one, related to increasing activation of higher order cortical areas.

Postnatal development of GFAP, connexin43 and connexin30 in cat visual cortex

In cat visual cortex, neurons acquire progressively mature functional properties during the first postnatal months. The aim of this study was to analyze the development of astrocytes during this period. The patterns of expression of the glial fibrillary acidic protein (GFAP) as well as of two gap junction proteins expressed in astrocytes, connexin43 (Cx43) and connexin30 (Cx30), were investigated by immunohistochemistry and optical density measurements, in visual cortical areas 17 and 18 at four different ages: 2 weeks (postnatal days 12 to 15, P12-15), 1 month (P27-31), 2 months (P60-62) and beyond 1 year. Since visual experience is a key factor for neural development, the patterns of expression of these three proteins were studied both in normally-reared and monocularly deprived animals. Interestingly, the distribution of GFAP, Cx43 and Cx30 was found to change dramatically but independently of visual experience, during postnatal development, even beyond P60. During the first postnatal month, GFAP and Cx43 were mainly localized in the white matter underlying the visual cortical areas 17 and 18. Then, their distributions evolved similarly with a progressive decrease of their density in the white matter associated with an increase in the cortex. Connexin30 expression appeared only from the second postnatal month, strictly in the cortex and with a laminar distribution which was similar to that of Cx43 at the same age. In adults, a specific laminar distribution was observed, that was identical for GFAP, Cx43 and Cx30: their density was higher in layers II/III and V than in the other cortical layers.

Heterogeneity of the developmental patterns of neurotrophin protein levels among neocortical areas of macaque monkeys

Based on morphological and physiological characteristics, the mammalian neocortex is divided into various neocortical areas and its diversity is prominent in the primates including humans. These neocortical areas are constructed during development, but the details of the developmental events remain unclear, especially at the molecular level. We measured the mRNA and protein levels of neurotrophins in various neocortical areas of developing rhesus monkeys. The expression patterns of both the neurotrophin-3 (NT-3) mRNA and the protein showed area differences. In the sensory and motor areas, NT-3 mRNA and protein levels had started to decline by a week after birth. In contrast, the levels declined after the third postnatal week in the association neocortical areas. The level of brain-derived neurotrophic factor (BDNF) protein changed in an area-dependent manner during development, but that of mRNA did not. The decline of the BDNF protein level started earlier in the sensory and motor neocortical areas than in the association neocortical areas, suggesting that sensory and motor neocortical areas develop earlier than the association areas in terms of the developmental changes in neurotrophins.

Differential expression of the truncated TrkB receptor, T1, in the primary motor and prefrontal cortices of the adult macaque monkey

A truncated TrkB receptor, T1, which is one of the receptors for brain-derived neurotrophic factor, has been shown to regulate the morphology of neurons and glial cells in primary cultures and/or slices overexpressing T1 in the recent past. However, in vivo localization of T1 at protein level remains unclear. In the present study, we examined the localization of T1 in the primary motor and prefrontal cortices of adult monkeys by using immunohistochemistry. In the primary motor cortex, T1 immunoreactivity was observed mainly in the pyramidal neurons of layers II-VI, especially Betz cells of layer V. The apical and basal dendrites and cell bodies of Betz cells were strongly stained. In addition, we found that the interneurons were also T1-immunopositive and that there were no T1-positive astrocytes. In the prefrontal cortex, we observed strong immunoreactivity of T1 in astrocytes as well as pyramidal neurons of layer V. The pyramidal neurons and interneurons in layers II/III were faintly immunoreactive for T1. Thus, these findings, together with the fact that T1 is involved in morphological control of neurons and glial cells, suggest that the prefrontal cortex might possess a different degree of morphological plasticity than the primary motor cortex in the adult monkey.

Immunohistochemical investigation of the brain of aged dogs. I. Detection of neurofibrillary tangles and of 4-hydroxynonenal protein, an oxidative damage product, in senile plaques

In the aging dog brain lesions develop spontaneously. They share some morphological characteristics with those of Alzheimer 's disease in man. Diffuse and primitive plaques are well known, whereas neuritic plaques rarely develop. Neurofibrillary tangles have not been seen in the canine. The aim of the present investigation was to study major age-related changes of the dog's brain using paraffin sections with respect to cross-immunoreactivity of tau, A beta protein and other immunoreactive components including hydroxynonenal protein, which is a marker for oxidative damage. The occurrence of neurofibrillary tangles and of the protein tau therein was studied in serial brain sections of two dogs with the Gallyas stain and by immunohistochemistry with three different antibodies against tau. Senile plaques were stained with a monoclonal anti-A beta (residues 8-17), polyclonal anti-apolipoprotein E and a monoclonal antibody against 4-hydroxynonenal (HNE). Amyloid deposits and controls were screened by Congo red staining viewed in fluorescent light, followed by polarized light for green birefringence. With the Gallyas stain and one of the antisera against tau, neurofibrillary tangles were revealed in a similar dispersed pattern, whereas the other antitau antisera gave negative results. With the anti-HNE a positive reaction was found in cerebral amyloid deposits and in vascular wall areas where amyloid deposition was confirmed by Congo-red staining, and in perivascular cells and in some neurons. These results indicate that the canine with his tangles and plaques which show oxidative changes, forms a spontaneous modelfor understanding the early changes and their interrelationships in Alzheimer's disease.

Change of expression of full-length and truncated TrkBs in the developing monkey central nervous system

We examined the expression of full-length TrkB (TrkBTK+) and truncated TrkB (TrkBTK-) in the central nervous system (CNS) of the macaque monkey (Macaca fascicularis) using a western blot analysis. At the adult stage, the levels of TrkBTK+ in cerebral cortices were higher than those in other structures of CNS and the expressions of TrkBTK+ in the association cortices (except area PE) were relatively lower than those in the primary cortices. In contrast, TrkBTK- in the hippocampus and the cerebellum was significantly higher than in other structures. In various developing cerebral cortices, TrkBTK+ was detected at the same levels from embryonic day 120 (E120) to the adult period. In contrast, the expression of TrkBTK- increased remarkably after the newborn stage (NB), reached the maximum level at postnatal day 60 (P60) and maintained the same level into adulthood. The peaks of TrkBTK- in the association cortices were more delayed than in the primary cortices. The expression of TrkBTK- occurred at a time that correlates with the elimination of axons and the down-regulation of neuropeptides. The present study suggests that TrkBTK- plays an important role in the axonal remodelling and that it may act as a negative effector of TrkBTK+ in the primate CNS, reducing responsiveness to BDNF and/or NT-4/5.

Postnatal changes in the numerical density and total number of asymmetric and symmetric synapses in the hypoglossal nucleus of the rat

Morphometric analyses were performed to investigate the progressive and regressive phases of synaptogenesis in the hypoglossal nucleus of the rat during normal postnatal development. The total volume of the hypoglossal nucleus and both the numerical density (NV, contacts per mm3) and total number of synapses were measured at 5-day intervals from birth to postnatal day 30 and in young adults. Values of NV were calculated separately for asymmetric and symmetric synapses as well as for axospinous, axodendritic and axosomatic contacts. The volume of the hypoglossal nucleus increased significantly from birth to postnatal day 30 (414%) with no further increase in the adult. The NV of all synapses increased significantly from birth to day 20 (131%), followed by a significant decreases in adults (45%). The total number of synapses increased significantly from birth to day 20 (843%), followed by a significant decrease in adults (30%). Similar developmental changes in density and total number were observed for asymmetric and symmetric synapses. The magnitude of synapse elimination, occurring after day 20, was approximately 30% for both morphological types. During postnatal development the vast majority of synapses in the hypoglossal nucleus were found to form axodendritic contacts (85-95%). Synapse elimination was observed for axospinous, axodendritic and axosomatic contacts. These findings indicate that the progressive and regressive phases of synaptogenesis occur earlier in the hypoglossal nucleus than in the cerebral cortex of the rat, suggesting a caudal-to-rostral gradient. Synapse elimination was not restricted on the basis of morphological type or postsynaptic target site.

Monocular deprivation alters the morphology of glial fibrillary acidic protein-immunoreactive astrocytes in the rat visual cortex

Monocular deprivation was used to examine the experience-dependent structural plasticity of astrocytes in Oc1M and Oc1B visual cortex of young and adult rats. Stereological techniques were employed to assess the numerical density (Nv) of cells and surface density (Sv) of processes immunoreactive for glial fibrillary acidic protein in laminae II/III, IV, V and VI in the hemisphere opposite the deprived eye. In one group of pups eyelids were sutured on postnatal day 12 (P12) and maintained until P80 (MD), while a second group had the sutures removed at P75 followed by 5 days of light exposure (MD + L). An unoperated light experienced group was used for comparisons (L). The Sv of astrocytic processes in lamina IV but not laminae II/III, V and VI was significantly decreased in the MD group. The ratio of Sv to the Nv of neurons, an estimate of the amount of astrocytic membrane per neuron, was also significantly decreased in layer IV. The Nv of astrocytes was not significantly different among the three groups. In adults that were monocularly deprived for 5, 10 and 30 days the Nv of astrocytes and Sv of their processes were not significantly altered in layer IV. There was however an increase in the Nv of all types of glial cells combined in layer IV following 10 and 30 days. These results indicate that the structure of astrocytes is influenced by visual experience during development whereas merely altering the level of visually-driven activity in the adult was not sufficient to induce astrocytic structural change.

Postnatal development of glial fibrillary acidic protein, vimentin and S100 protein in monkey visual cortex: evidence for a transient reduction of GFAP immunoreactivity

In the cerebral cortex of some species, the gradual appearance of glial fibrillary acidic protein (GFAP) is often interpreted as reflecting the parallel maturation of neuronal connectivity. We studied the postnatal maturation of astrocytes in the primary visual cortex of Callithrix jacchus using antibodies against GFAP, vimentin and S100 protein as immunohistochemical markers. In the cortical grey matter of this species, the overall GFAP-immunoreactivity (IR) as measured by image analysis is high at birth (130% of the adult value), decreases until about 3 months (80%) and increases again towards adult values (100%). Vimentin-IR was high at birth, and declined towards 3 months and later. In contrast, S100-IR augmented postnatally in neuropil, and showed a laminar shift of maximum IR from layer IV to supragranular layers during ontogenesis. The decrease of GFAP-IR is predominantly due to changes in density of GFAP-positive (+) astrocytes within cortical tissue (newborn: 18,600 GFAP+astrocytes/mm3; 1 month: 11,600/mm3; 3 months: 5,700/mm3; adult: 10,200/mm3), while the overall number of astrocytes remained relatively constant as shown by the number of S100-positive astrocytic cell bodies. At times of low GFAP-IR a reduced area density of intermediate filaments was found in astrocytes by electron microscopy. The period of reduced GFAP-expression coincides with the time of prominent synapse remodeling in the visual cortex of marmosets. These data suggest that GFAP-expression may depend on functional conditions rather than time-dependent maturation.

Developmental changes in the distribution of acetylcholinesterase in the extrastriate visual cortex of the monkey

In the fetal and neonatal monkey, periodically organized regions of high activity of acetylcholinesterase were found in the visual cortical area V2 (Area 18). The acetylcholinesterase bands, like the thin and thick stripes of cytochrome oxidase, were found to run orthogonal to the area 17/18 border. During neonatal development these bands progressively narrow and finally disappear shortly after four months of age.

Cytochemical redistribution of 5'-nucleotidase in the developing cat visual cortex

The adenosine-producing ectoenzyme 5'-nucleotidase has recently been shown to undergo a marked redistribution during development of the cat visual cortex and to be involved in the remodelling of ocular dominance columns (Schoen et al., J. Comp. Neurol., 296, 379-392, 1990). Using an enzyme-cytochemical technique, we now investigate the developmental redistribution of 5'-nucleotidase activity in area 17 of kittens at the ultrastructural level. Between postnatal days 35 and 42, when 5'-nucleotidase is concentrated in layer IV, enzyme reaction product occupies the clefts of asymmetrical synapses within the neuropil. During later development (9th and 13th postnatal weeks), when 5'-nucleotidase spreads over all cortical laminae, the enzyme disappears from its synaptic localization and becomes increasingly associated with astrocytic membranes. The transient appearance of 5'-nucleotidase at synapses parallels the time-course and laminar profile of the synaptic remodelling which takes place during the critical period of visual cortex development. This suggests that synapse-bound 5'-nucleotidase activity plays a role in synaptic malleability, whereas its later association with glial profiles is likely to reflect other functions of the enzyme.

A calcium-stimulated serine protease from monkey brain degrades the beta-amyloid precursor protein

Amyloid deposition, a histopathological feature of Alzheimer's disease brain, may be the underlying cause of this disease. The isolation of enzymes involved in both the normal and aberrant or alternative processing of the beta-amyloid precursor protein may lead to an understanding of how beta-protein, the major component of amyloid deposits, is formed in the brain parenchyma and vasculature of Alzheimer's disease patients and aged humans. As the same kind of deposits is also found in aged primates, the use of primates will undoubtedly help to understand the mechanisms of amyloid deposition, both spatially and temporally. Here we report the partial purification from adult monkey brain of a calcium-activated serine protease that is immunoreactive with antibodies against cathepsin G and is potentially involved in the abnormal degradation of the beta-amyloid precursor protein. Moreover, immunoreactivity with cathepsin G antibodies was localised to astrocytes in both adult and aged monkey cortex, suggesting that our protease may be expressed in astrocytes.

Effect of dark rearing on the volume of visual cortex (areas 17 and 18) and number of visual cortical cells in young kittens

The surface area, total volume, and total number of neurons of areas 17 and 18 in one hemisphere of dark-reared (DR), dark-reared and light-exposed (DRL), and normally reared (NR) kittens were studied at the age of 6 weeks. The thickness of the visual cortex was lower by 13% and 11% (area 17) and by 17% and 16% (area 18) in DR and DRL groups, respectively, when compared with similar cortical areas in NR kittens. The surface area values of area 17 were nearly the same in DR and DRL kittens, both being, however, 37% smaller than in NR animals. The surface area of area 18 was significantly smaller than that of area 17 in each group, and was also lower in DR (by 27%) and DRL (by 21%) groups when compared with the NR group. As a consequence of dark rearing, the numerical density of cortical neurons in area 17 amounted to about double of the value observed in normally reared kittens and was also significantly higher in area 18. The numerical density of nerve cells of DRL kittens fell between the DR and NR groups. The total cortical volume of area 17 was similar in DR and DRL groups but it was by 46% (DR) and by 44% (DRL) smaller than in NR kittens. In each experimental group, the total volume of area 18 was significantly smaller than that of area 17. The cortical volume of area 18 was also smaller than in the NR group by 39% and 34% in DR and DRL groups, respectively. In DR and NR kittens, the total numbers of neurons in areas 17 (DR = 26.4 million, NR = 25.7 million) and 18 (DR = 8.5 million, NR = 9.0 million) were essentially similar. In the DRL groups a significantly smaller number of cortical neurons was found both in area 17 (21.5 million) and in area 18 (6.8 million). It is concluded that, in spite of considerable differences in the cortical thickness, surface area, numerical density, and total cortical volume, the absolute numbers of neurons in area 17 and 18 of visually deprived (DR) and NR kittens do not differ at 6 weeks of age. The main deficit in cortical organization following dark rearing, therefore, appears to be confined mainly to the neuropil, as a result of an underdevelopment of neuronal processes and of depressed synaptic organization.(ABSTRACT TRUNCATED AT 400 WORDS)

Astrocytes in cat visual cortex studied by GFAP and S-100 immunocytochemistry during postnatal development

A monoclonal antibody to glial fibrillary acidic protein (GFAP) and a polyclonal antiserum to the S-100 protein were used to study the expression of these astrocytic proteins in the postnatal visual cortex of the cat. Three changes in antigen expression of these astroglial markers could be distinguished over development. First, the density of cells in the white matter, which are heavily labelled with both antibodies from birth until adulthood, diminishes after the third postnatal weeks. By intracellular filling with Lucifer Yellow the reduction of the cell density can be attributed to the disappearance of large astrocytes with a morphology of transforming radial glia, present only in early postnatal development. Second, heavily labelled, large cells present in the grey matter at the seventh postnatal day have disappeared by the fifth postnatal week. On the basis of their morphology these cells can also be classified as radial glial cells. Finally, astroglial cells of the adult-like stellate form appear to be labelled in the cortical layers between the third and seventh postnatal weeks. While the density of these cells and the S-100 immunoreactivity of the cell bodies is adult-like at the fourth postnatal week, there is a gradual increase of the staining intensity with the GFAP antibody up to the seventh postnatal week. This developmental period is paralleled by the appearance of S-100-positive astrocytic processes. The gradual expression of GFAP immunoreactivity and the increased expression of S-100 is interpreted as reflecting the time course of astrocytic maturation. A possible relation of the maturation of astrocytes and cortical development, both of which are prominent in the time period between the third and seventh postnatal week, is discussed.

Spatial relationships between astrocytes and classical plaque components

Four biopsies of the brain cortex of patients with Alzheimer's disease were used for ultrastructural studies. Three-dimensional reconstruction and morphometric studies of three classical plaques were carried out to determine the distribution of the astrocytes and their processes, and spatial relationships between astrocytes and plaque components. Morphologic and morphometric studies showed marked ultrastructural changes in the astrocytes located in the vicinity of the plaque. Proliferating processes of the two or three astrocytes nearest to the plaque become part of the plaque. Processes of hypertrophic astrocytes touch neighboring vessels, neurons, and amyloid deposits. The volume of the peripheral amyloid deposits in the classical plaques examined exceeds the volume of the amyloid star by about 26%. Amyloid of the classical plaque periphery appears as amyloid wisps. Most amyloid wisps are isolated between astrocytic processes proliferating and penetrating into the plaque. Proliferation and penetration of astrocytic processes into the plaque lead to fragmentation of amyloid deposits and their dispersion. Reduction in the size of amyloid wisps isolated between astrocytic processes toward the plaque periphery and changes in the morphology of amyloid fibers in this plaque region suggest that astrocytes participate in gradual degradation of at least part of the amyloid fibers.

Ocular dominance plasticity and developmental changes of 5'-nucleotidase distributions in the kitten visual cortex

The distribution of the adenosine-producing ecto-enzyme 5'-nucleotidase was investigated histochemically in the visual cortex of normally reared and monocularly deprived kittens and cats. In normally reared kittens aged between 11 to 44 days, 5'-nucleotidase activity formed a band of intense neuropil staining throughout cortical layer IV of areas 17 and 18. The other layers were almost devoid of reaction product. Between the 4th and 6th week, this band had a patchy appearance in area 17, the center-to-center spacing of 5'-nucleotidase patches being approximately 1 mm. Monocular enucleation accentuated these patches of enhanced 5'-nucleotidase activity or made them reappear at developmental stages at which they had normally faded. Simultaneous visualization of ocular dominance columns by transneuronal transport of intraocularly injected 3H-proline showed that the patches of enhanced 5'-nucleotidase activity coincided with the territories of afferents from the intact eye. With increasing age and normal visual development, the patches disappeared and 5'-nucleotidase activity spread to the supra- and infragranular layers. The adult pattern was characterized by dense staining of all cortical laminae in both areas 17 and 18 and was established at about 8 weeks of age. At approximately 7 weeks of age, when the patches in layer IV had disappeared in the course of normal development, monocular enucleation caused a reappearance of the discontinuous pattern of 5'-nucleotidase activity in layer IV. These results reveal a close relation between the distribution of 5'-nucleotidase and the time course of the developmental phase during which the visual cortex is susceptible to experience-dependent alterations. As suggested by the correlation between sites of enzyme activity and eye dominance columns, the expression of 5'-nucleotidase patches in layer IV appears to be associated with the remodelling of ocular dominance territories that occurs both in normal development and after manipulation of afferent retinal input. Thus, 5'-nucleotidase is likely to serve a function in activity-dependent modifications of cortical circuitry. Moreover, 5'-nucleotidase activity is the only endogenous marker known to date that exhibits a columnar pattern in cat visual cortex.

Cerebral aging: a quantitative study of gliosis in old nude mice

Morphometric glial fibrillary acidic protein (GFAP) studies of the brains of 11 old (18-29 months) female, outbred athymic mice demonstrated astrocytic gliosis (increase in GFAP-positive astrocytes; GFAP-PA) in all mice with a consistent distribution pattern. Specific areas such as the central white matter, hippocampus, diencephalon, gray matter at the floor of the 4th ventricle, and posterior colliculi showed the change most conspicuously, revealing GFAP-PA both interstitially and perivascularly. There was no apparent demyelination in the affected white matter. In addition, there was an increase in GFAP-PA in the external limiting membrane surrounding the diencephalon and base of brain stem, but only to a minor degree over the cerebral hemispheres. The cerebral and cerebellar cortices and hypothalamus showed no significant increase. In contrast, all of the 2-month-old control animals showed only minor amounts of GFAP-PA, seen in the external limiting membrane and a trace in the cerebral white matter. The present data suggest that astroglial sclerotic change in various regions of the brain is an important morphological expression of cerebral aging. In view of the lack of other demonstrable histological changes (i.e., silver and amyloid stains were negative) or significant atrophy, the cause of the observed gliosis in BALB/c mice might represent a genuine aging change. As an incidental finding, aggregates of PAS-positive granules were noted in the Ammon's horn of most old animals, while none were seen in the young controls.

Distribution of neurons and glia in the visual cortex (area 17) of the adult albino rat: a quantitative description

The neuronal and glial cell composition of the rat visual cortex (area 17) has been determined quantitatively using stereological techniques. The volume numerical densities (number of cells per mm3 of cortex) of neurons and of the principal glial cell types (astroglia, oligodendroglia, and microglia) were calculated from tangential semithin resin sections spaced at regular intervals 50 micron apart throughout the entire depth of the visual cortex. From measurements of cortical and laminar thickness the separate volume numerical densities of neurons and glial cells were derived for each lamina in the cortex. In addition, the absolute numbers of cells in each lamina under 1 mm2 of cortical surface were calculated. The mean cortical volume numerical density of neurons was 60,020 +/- 3840/mm3 (mean +/- SEM; n = 8), and 49,040 +/- 2610/mm3 for the combined glial cell types. Astroglia, oligodendroglia, and microglia were present in a ratio of 6:3:1 respectively. It was determined from neuronal and glial somatic volume estimates that the somata of these cells occupied approximately 13.5% of unit cortical volume, with 81.3% of the unit volume being occupied by cortical neuropil. Using previously published reports that described the laminar composition of neurons in terms of the relative proportions of pyramidal and non-pyramidal cells, the laminar volume numerical densities for these neuronal categories have been derived. In addition, it has been estimated that under 1 mm2 of cortical surface there are 79,500 pyramidal and 7790 non-pyramidal neurons distributed throughout layers 1-6 of the rat visual cortex.

Quantitative changes in morphological parameters in the developing visual cortex of the marmoset monkey

Several quantitative morphological parameters were measured during postnatal development in area 17 of the marmoset monkey (Callithrix jacchus). In a series of 14 animals, at ages from birth to adulthood, we studied changes in the thickness, surface area and volume of area 17, as well as the neuronal and glial numerical densities, and total numbers. We found evidence for a rapid increase in thickness, area and volume, culminating between 6 weeks and 6 months postnatally, and then decreasing. The adult values are close to those observed in one-month-old animals. The overshoot in thickness and volume is greatest in layers II, III, IVa and IVc. The neuronal density shows a trend which is opposite to that of volume, and therefore the total number of neurons is constant postnatally, ca. 38 million neurons in area 17 of one hemisphere. The number of glial cells approximately doubles during the first postnatal month and remains stable afterwards, so that in the adult, there is one glial cell for two neurons. Morphological development of area 17 in this New World monkey is similar to that reported in Old World monkeys, as are the adult values for neuronal and glial densities.

Astroglial development in microencephalic rat brain after fetal methylazoxymethanol treatment

Treatment of pregnant rats on gestation day 15 with methylazoxymethanol (MAM) leads to a marked microencephaly in the offspring with a considerable atrophy in cerebral cortex, hippocampus and striatum. The development of the astrocytic populations in these atrophic regions was studied by means of immunohistochemistry using an antiserum against glial fibrillary acidic protein (GFA). The distribution and density of GFA-positive structures were not notably altered in the parietal cortex, hippocampal formation and striatum after prenatal MAM-treatment as compared to control. Also the individual astrocytes were morphologically similar in experimental and control animals in all regions analyzed. We suggest that an adjustment of the astrocytic development has occurred in response to the changed neuronal environment. Alternatively, MAM-treatment may affect neuronal and glial precursor cells leading to a seemingly normal astrocytic cell density.

A stereological study of neocortical maturation in the precocial mouse, Acomys cahirinus

The development of neocortical area 17 of Acomys cahirinus was studied by quantifying changes in the numerical density of neurons and glia, and by examinations of subjects treated on the day of birth with tritiated thymidine. Unlike most laboratory species, Acomys is born in a relatively advanced state, with open and functional ears and eyes and coordinated locomotor abilities. Rapid neocortical growth occurred during the first 60 postnatal days and was accompanied by a 65% decrease in neuronal packing density. No evidence of neuronal proliferation was found in the neocortex, although labelling was found in the dentate gyrus and olfactory bulb. Glial densities remained relatively constant through the same time period, a consequence of continued proliferation. While previous studies have demonstrated that Acomys and the laboratory mouse undergo quite different patterns of olfactory bulb and hippocampal formation growth, the present study indicates that patterns of neocortical maturation are very similar in the two species. These findings suggest large variations both within and between species in patterns of brain growth.

Neuronal composition and development in lamina 4C of monkey striate cortex

Lamina 4C (Lund, '73) of the monkey, Macaca nemestrina, visual striate cortex occupies a key position as a principal recipient zone of axons from the lateral geniculate nucleus (LGN). Synaptic maturation in lamina 4C is of particular interest since it involves a competitive interaction between thalamic axons for postsynaptic territory: an interaction which is strongly influenced by afferent activity (Hubel et al., '77). As an initial step toward understanding the normal process of synapse maturation in 4C, this study examines Golgi material to define the adult neuron populations of subdivisions 4C alpha (receiving afferents from magnocellular LGN) and 4C beta (receiving afferents from parvocellular LGN). Three groups of spine-bearing neurons are described--one relatively confined to either alpha or beta subdivision, the other two bridging the depth of 4C; four groups of smooth dendritic neurons interact with the spine-bearing population. Electron microscopy of normal and Golgi-impregnated tissue is used to define key features of synapse populations on these neurons. From embryonic day 159 through adulthood the smooth and spiny neurons occur in the same constant proportions in the neuropil (5% smooth, 95% spiny). Changes in the distribution of synapses on the spiny neurons are analyzed qualitatively; type 1 axon terminals (asymmetric apposition--round vesicles) shift from dendritic shafts to spine tips during maturation. Each spine is found to bear a type 1 contact at all ages; these results allow us to conclude that the figures of Boothe et al. ('79) on changes in spine populations during maturation can now be interpreted as changes in type 1 synapse populations. It is shown that type 2 synapses (symmetric appositions--pleomorphic vesicles) arise from axons of the smooth dendritic neurons. These synapses are found to increase in number on the spiny cell somata in early postnatal development, and this is followed by a decrease in number to the adult level.

A laminar analysis of the number of neurons, glia, and synapses in the adult cortex (area 17) of adult macaque monkeys

The number of neurons, glia, and synapses in the visual cortex of adult macaque monkeys has been estimated by stereological methods. The data is presented separately for 13 sublaminae. For the total cortical thickness, the average numerical density of neurons is approximately 120,000 per mm3 of tissue. This density increases and decreases in the different sublaminae in the direction that one would expect from the classical descriptions of Nissl-stained material (e.g., 137,000/mm3 in IVC alpha; 211,000/mm3 in IVC beta). There are about 200,000 neurons under 1 mm2 or cortical surface: 28% are in layers I-III, 45% are in layer IV, and 27% are in layers V and VI. The total number in area 17 of one hemisphere is close to 160,000,000. For the total cortical thickness the numerical density of synapses is 276,000,000 per mm3 of tissue. Laminae with higher neuronal densities tend to have higher synaptic densities but the correlation is not perfectly concordant. Moreover, the changes in synaptic densities are not as great as those in neuronal densities so that laminae with higher neuronal densities have a lower synapse to neuron ratio and laminae with lower neuronal densities have a higher synapse to neuron ratio. For the total cortical thickness this ratio is 2,300 synapses per neuron (1,400 in layer IVC beta and 2,800 in layers II and III). There are 478 million synapses under 1 mm2 cortical surface: 40% are in layers I-III, 35% are in layer IV, and 25% are in layers V and VI. The numerical densities of astrocytes, oligodendrocytes, and microcytes are 38,000, 17,000, and 4,000 per mm3, respectively. The overall glia/neuron ratio is 0.49.