Spatial Distribution Of Neurons Research Articles

COVID-19 causes neuronal degeneration and reduces neurogenesis in human hippocampus.

Recent investigations of COVID-19 have largely focused on the effects of this novel virus on the vital organs in order to efficiently assist individuals who have recovered from the disease. In the present study we used hippocampal tissue samples extracted from people who died after COVID-19. Utilizing histological techniques to analyze glial and neuronal cells we illuminated a massive degeneration of neuronal cells and changes in glial cells morphology in hippocampal samples. The results showed that in hippocampus of the studied brains there were morphological changes in pyramidal cells, an increase in apoptosis, a drop in neurogenesis, and change in spatial distribution of neurons in the pyramidal and granular layer. It was also demonstrated that COVID-19 alter the morphological characteristics and distribution of astrocyte and microglia cells. While the exact mechanism(s) by which the virus causes neuronal loss and morphology in the central nervous system (CNS) remains to be determined, it is necessary to monitor the effect of SARS-CoV-2 infection on CNS compartments like the hippocampus in future investigations. As a result of what happened in the hippocampus secondary to COVID-19, memory impairment may be a long-term neurological complication which can be a predisposing factor for neurodegenerative disorders through neuroinflammation and oxidative stress mechanisms.

Hardware-aware liquid state machine generation for 2D/3D Network-on-Chip platforms

The liquid state machine (LSM) is a spiking neural network (SNN) that usually is offline mapped to an NoC-based neuromorphic processor to perform a specific task. The creation of these LSM models does not consider the structure of Network on Chip (NoC) which results in heavy communication pressure on the NoC. This paper proposes a hardware-aware generation framework for the LSM network by considering the spatial distribution of neurons in the NoC. It is the first time for the LSM generation work with combining the characteristics of NoC. This framework also adopts the heuristic algorithm to search the hyperparameter for the LSM networks to achieve state-of-art accuracy. It also reduces the spikes generated by those LSM models. It keeps the communication between neurons within cores as much as possible, which could reduce the communication between cores effectively and improve the performance of NoC, including reducing the traffic flow, reducing the average latency, improving the throughput and reducing the total running time.

Large-scale, cell-resolution volumetric mapping allows layer-specific investigation of human brain cytoarchitecture.

Although neuronal density analysis on human brain slices is available from stereological studies, data on the spatial distribution of neurons in 3D are still missing. Since the neuronal organization is very inhomogeneous in the cerebral cortex, it is critical to map all neurons in a given volume rather than relying on sparse sampling methods. To achieve this goal, we implement a new tissue transformation protocol to clear and label human brain tissues and we exploit the high-resolution optical sectioning of two-photon fluorescence microscopy to perform 3D mesoscopic reconstruction. We perform neuronal mapping of 100mm3 human brain samples and evaluate the volume and density distribution of neurons from various areas of the cortex originating from different subjects (young, adult, and elderly, both healthy and pathological). The quantitative evaluation of the density in combination with the mean volume of the thousands of neurons identified within the specimens, allow us to determine the layer-specific organization of the cerebral architecture.

Effect of Dimethyl Fumarate on the Motor Function and Spatial Arrangement of Primary Motor Cortical Neurons in the Sub-Acute Phase of Stroke in a Rat Model

BackgroundThe therapeutic effects of dimethyl fumarate (DMF) in patients with multiple sclerosis and animal models of neurologic disease were reported. The density and the distribution pattern of motor neurons are important in transmitting the signal and controlling the movement-related functions. The present study evaluated the effects of DMF treatment on the neurological functions, infarct volume, and spatial distribution of the neurons in the primary motor cortex after cerebral ischemia. MethodsThirty-three Sprague-Dawley rats were randomly divided into three groups: The sham group underwent surgery without middle cerebral artery occlusion (MCAO) and drug. The vehicle and treatment groups after MCAO received a vehicle or DMF for three consecutive days. Post-stroke neurological and motor functions were assessed. At the end of the third day, the brains were removed, and the cerebral infarct volume was evaluated. We used cresyl violet staining to analyze the density and the spatial arrangement of motor cortical neurons using Voronoi tessellation. ResultsTreatment of the brain ischemia for three days with DMF could not significantly reduce the neurological and motor function deficits and infarct volume. However, it reduced the neuronal area and death and preserved their spatial distribution in the normal regular pattern. ConclusionCerebral ischemia decreased the neuronal density of the primary motor cortex and changed their distributions to a random pattern. DMF treatment during sub-acute ischemic stroke did not significantly improve the neurological deficit scores. However, it could prevent neuronal swelling and death and preserved the spatial distribution of the cortical neurons in their normal pattern.

Self-organization of modular network architecture by activity-dependent neuronal migration and outgrowth.

The spatial distribution of neurons and activity-dependent neurite outgrowth shape long-range interaction, recurrent local connectivity and the modularity in neuronal networks. We investigated how this mesoscale architecture develops by interaction of neurite outgrowth, cell migration and activity in cultured networks of rat cortical neurons and show that simple rules can explain variations of network modularity. In contrast to theoretical studies on activity-dependent outgrowth but consistent with predictions for modular networks, spontaneous activity and the rate of synchronized bursts increased with clustering, whereas peak firing rates in bursts increased in highly interconnected homogeneous networks. As Ca2+ influx increased exponentially with increasing network recruitment during bursts, its modulation was highly correlated to peak firing rates. During network maturation, long-term estimates of Ca2+ influx showed convergence, even for highly different mesoscale architectures, neurite extent, connectivity, modularity and average activity levels, indicating homeostatic regulation towards a common set-point of Ca2+ influx.

Network community, clusters and hubs in cortical micro circuits

Networks of cortical neurons are essentially non-random [1]. Although it is known that such networks show interesting structure at multiple temporal and spatial scales [2], almost no experimental work has been done to reveal how structures at these different scales relate to each other. This study aimed to clarify important relations between non-randomness in groups of 3-6 neurons (clusters) and non-randomness in groups of 50-100 neurons (communities) through five steps. First, we recorded spontaneous activity of up to 500 neurons from rodent somatosensory cortex using a 512ch. multi-electrode system over one hour [3]. Second, we reconstructed effective connectivity using transfer entropy [4]. Third, we compared topologies of effective networks at the 3-6 neuron scale (clusters including motifs [Figure1-B]) with topologies of synaptic connections measured from 12 neuron simultaneous patch clamp experiments [5,6]. Fourth, we constructed community or modular structures representing non-randomness from larger groups of neurons. Fifth, we evaluated the extent to which structure at each of these scales was robust. We did this by swapping connections from high degree nodes (hubs) with those from low degree nodes (non-hubs). Figure 1 (A) An example of spatial distribution of neurons and effective connections. Different markers indicate different communities. The biggest two communities are covered by blue and red regions. Upper-right yellow region is an example cluster of 6 neurons. ... We found three things. First, the degree-distribution followed a power-law This demonstrated that hubs could not have been the result of random sampling from a Gaussian distribution. Second, effective networks consisting of hundreds of cortical neurons have distinctive non-random structures of connectivity at two different scales. Third, structure at the cluster level was relatively more fragile than structure at the community level. The difference between non-randomness evaluated by cluster and community will become the important first step to understand multiple different scales of cortical neuronal networks.

Pattern analysis and spatial distribution of neurons in culture

The nervous system is a complex, highly-ordered, integrated network of cells. Dispersed cultures of neurons enable investigations into intrinsic cellular functions without the complexities inherent in the intact nervous system. This culture process generates a homogeneously dispersed population that is assumed to be spatially random. Despite the vast number of studies utilizing dispersed neurons, few studies address the spatial distribution of large populations of neurons, in vitro. We used ink-jet printing and surface chemistry to define patterned areas of poly-lysine adhesion (∼50 μm spots) juxtaposed against a fluorinated-silane background. We quantitatively analysed populations of patterned neurons on printed protein spots, and unpatterned neurons. Using a microarray scanner, we acquired large images (72 mm × 22 mm) of patterns, and neurons with and without patterns. Fast Fourier transformation (FFT) image analysis was used to determine global alignment of neurons to patterns. Through point pattern analysis, we described the spatial organization of dispersed neurons with, or without, patterned substrates. Patterned neurons show spatial organization characteristics reminiscent of printed patterns, with spatial distributions representative of unpatterned neurons. Most notably, both patterned and unpatterned neurons show departure from null models of complete spatial randomness (CSR; a homogeneous Poisson process) at shorter distances with conformity to CSR occurring at longer distances. Cellular morphometrics show that when compared to their unpatterned counterparts, spot-patterned neurons exhibit a significant increase (p < 0.0001) in the mean dendritic circularity and an increase in the number of more circular neurons. Through neurite tracing, we show that dendritic processes are also highly confined to patterned areas, and that they are on average 58% shorter than dendrites of neurons without patterns. Our findings show that patterned areas change the spatial organization of the somata and dendrites of cultured neurons, and that traditional neuronal cultures deviate from CSR.

Connectivity statistics and their implications on activity dynamics in cortical cell cultures

Event Abstract Back to Event Connectivity statistics and their implications on activity dynamics in cortical cell cultures Samora Okujeni1, 2*, Steffen Kandler1, 2 and Ulrich Egert2, 3 1 BCCN Freiburg, Neurobiology and Biophysics, Germany 2 Albert-Ludwigs-University Freiburg , Germany 3 BCCN Freiburg, Department of Microsystems Engineering , Germany In the absence of complex architecture, central parameters of connectivity in neuronal networks are the size of dendrites and axons in conjunction with the spatial distribution of neurons. The homeostatic regulation of neurite fields is crucial for the maintenance of stability in the activity dynamics. We analyze these fundamental properties of neuronal networks in dissociated cortical cell cultures grown on micro-electrode arrays (MEAs). These generic random networks display a self-regulated maturation process that is characterized by neurite outgrowth, synapse over-expression and pruning similar to the critical period in the developing cortex. Within this period of network formation we manipulated neuronal connectivity by pharmacological intervention with structural differentiation processes. Previous studies showed that pharmacological inhibition of protein kinase C (PKC) activity enhances neurite outgrowth [1] and impairs cell migration [2] and network pruning [3] in early neural development. Following these findings, we chronically treated cortical cell cultures with PKC inhibitors and characterized them morphologically and functionally. To capture connectivity statistics we applied a modified Scholl analysis [4] to describe the radial dendritic field density of neurons embedded into the network. The method involves the grouping of neurons into classes of defined local neuron density to account for local differences in the overlap of dendritic fields of spatially non-uniformly distributed neurons. Dendrite profiles across conditions were subsequently compared based on these classes. Blocking PKC activity significantly enhanced radial arborization by +15% at intermediate (DIV14) and about +50% at later stages (DIV40) of development. The evolution of dendritic fields furthermore revealed an ongoing extension in cultures exposed to PKC inhibitors in contrast to an incipient reduction found under control conditions. This suggests an impaired downregulation of neurite outgrowth and pruning with the ending of the initial wiring phase. To further confirm these findings, dendrites of neurons in sparse cultures with negligible dendritic field overlap were traced manually and likewise showed significantly increased arborization and enhanced segment extension. By correlating local and global neuron density [5] we moreover found reduced clustering in the networks developing under inhibited PKC activity, indicating impaired cell migration. Electrophysiological recordings were performed during the development to assess functional consequences of the altered network differentiation. In cultures exposed to PKC inhibitors we had a higher yield of electrodes displaying neuronal activity, which accounts for a higher probability of finding neurons in the vicinity of an electrode in absence of neuronal clustering. The number of active electrodes furthermore remained stable in the course of development in contrast to a decrease found under control conditions, which provides electrophysiological evidence for the non-initiation of pruning processes. The activity stayed organised in network-wide bursting events that characteristically emerge in neuronal cell cultures. Bursts however tended to be more compact, occurred at a higher frequency and were stronger synchronized over the MEA. This suggests a faster propagation of activity through the networks with enhanced inter-neuronal connectivity. In summary we show that inhibition of PKC activity in developing cortical cell cultures increases connectivity and relate the structural changes to functional differences in the activity dynamics.

Voronoi analysis uncovers relationship between mosaics of normally placed and displaced amacrine cells in the thraira retina

Although neuronal dynamics is to a high extent a function of synapse strength, the spatial distribution of neurons is also known to play an important role, which is evidenced by the topographical organization of the main stations of the visual system: retina, lateral geniculate nucleus, and cortex. The coexisting systems of normally placed and displaced amacrine cells in the vertebrate retina provide interesting examples of retinotopic spatial organization. However, it is not clear whether these two systems are spatially interrelated or not. The current work applies two mathematical-computational methods-a new method involving Voronoi diagrams for local density quantification and a more traditional approach, the Ripley K function-in order to characterize the mosaics of normally placed and displaced amacrine cells in the retina of Hoplias malabaricus and search for possible spatial relationships between these two types of mosaics. The results obtained by the Voronoi local density analysis suggest that the two systems of amacrine cells are spatially interrelated through nearly constant local density ratios.

Evaluation of nerve cell distribution in cerebral cortex—a proposal for a new method applied to patients with epilepsy

Microdysgenesis is a microscopic malformation of cortical development (MCD) associated with epilepsy, but its significance in epileptogenesis is debated. This is partly since the histopathological aberrations associated with microdysgenesis can also be found in normal brains. We here report a method for objective analysis of one criterion for microdysgenesis, irregular cortical nerve cell distribution. Tissue from the lateral temporal lobe from two epilepsy patients was compared with tissue from two post-mortem controls. An expansion/shrinkage factor was calculated to determine the change in tissue size during cutting and mounting. Neurons were identified and the positions of their nucleoli were marked and stored. The spatial distribution of neurons was analysed using distance to nearest neighbouring neuron and Voronoi tessellation. Specimens from the epilepsy patients expanded markedly during mounting compared with controls. Epilepsy specimens had shorter mean distances to nearest neighbour than controls and smaller Voronoi tessellation areas than controls. Both measurements suggest more densely packed neurons in epilepsy specimens. This pilot study describes a new objective method for identification of cortical neurons and their spatial distribution. Voronoi tessellation and distance to nearest neighbouring neuron might provide robust methods for objective analysis of cortical nerve cell distribution. The yield of such comparisons might be improved if each cortical layer is analysed separately.

Synchronous Phase Clustering in a Network of Neurons with Spatially Decaying Excitatory Coupling

Synchronization is studied in a spatially-distributed network of weekly-coupled, excitatory neurons of Hodgkin–Huxley type. All neurons are coupled to each other synaptically with a fixed time delay and a coupling strength inversely proportional to the distance between two neurons. We found that a robust, noise-resistant phase clustering state occurred regardless of the initial phase distribution. This has not been shown in previous studies where similar clustering states were found only when the coupling was inhibitory. The spatial distribution of neurons in each synchronous cluster is determined by the spatial distribution of the coupling strength. Phase-interaction properties of the model neurons in the network are used to explain why such a clustering state can be robust.

502. Reduced neuropil in the auditory cortex of patients with schizophrenia

It has been hypothesized that there is a reduction in the interneuronal space in the prefrontal cortex in Schizophrenia (Selemon et al., 1995). Because the spatial distribution of neurons in the cortex is focused around the minicolumn, we anticipated that the hypothesis of reduced interneuronal space in schizophrenia could be independently tested by analysis of these units. Initial studies have focused on prefrontal cortex, areas 9 and 46. We investigated a small population of normal controls (n = 13) and schizophrenia patients (n = 7), diagnosed according to DSM III—R criteria. Patients and controls were age and sex matched. Photomicrographs of the posterior portion of the superior temporal gyrus (area Tpt) were taken at 100× magnification. Among the features examined was the amount of neuropil space in the periphery of cell columns where local circuits, synapses, and dendritic branches predominate. We also measured the amount of spacing between cell soma within the cell-dominant portion of the columns, and the distance separating cell columns in the horizontal plane. All the values for spacing distances and neuropil space were less in schizophrenic brains. Significant findings resulted when we combined the data for the right and left sides of both schizophrenics and controls. We conclude that neuropil space is decreased in Tpt in schizophrenia patients, with the greatest reduction occurring within the core of the cell column. The finding infers a generalized distribution of interneuronal space reduction in the schizophrenic brain.

Binaural response characteristics in isofrequency sheets of the gerbil inferior colliculus

The spatial distribution of neurons with different binaural response properties was studied in two isofrequency areas of the inferior colliculus (IC) of the gerbil ( Meriones unguiculatus). Single units were recorded in dorsomedial-to-ventrolaterally oriented electrode penetrations at different rostrocaudal positions, within a low (1–2 kHz; N = 108) and a high (23–27 kHz; N = 90) ‘iso-frequency’ sheet. Response types were not equally represented within the two layers. Within low-frequency regions, 41% of the units were excited following sound stimulation of either ear (EE), 21% received an excitatory input from one and an inhibitory input from the other ear (EI), and 12% were monaural (E0), while within high-frequency regions the distribution was 20% EE, 47% EI and 18% E0. However, the spatial arrangement of EE, EI and E0, within the dorsomedial-to-ventrolateral extent of the isofrequency sheets, was on the whole comparable within the two regions: EE-units were accumulated dorsomedially and EI-units ventrolaterally in the respective isofrequency sheets. E0-units were distributed equally within the low-frequency sheet but occurred more frequently in the middle part of the high-frequency lamina. About equal proportions of the units (low frequencies 9%; high frequencies 7%) had complex binaural response characteristics and were characterized as E/IE. In the rostrocaudal dimension of the nucleus, orderly arrangement of different unit types was less obvious. The results show that there is a tendency for grouping of different binaural unit types within the inferior colliculus. However, these unit types are not strictly segregated. Furthermore, it was shown that, despite the differences found in low- and high-frequency sites, there is a common scheme of ‘binaural representation’ across frequencies within the IC. Provided that EE-, EI-, and monaural response characteristics originate from separate sources, the data point to a convergence of binaural brainstem afferents within the IC.

Spatial distribution of neurons in tissue culture wells: implications for sampling methods to estimate population size.

Many laboratory procedures require the counting of cells in culture. While many cultured cells may be counted by automated methods, neuronal cultures often require manual cell counting methods that are prohibitively time-consuming. This paper examines methods of sampling from tissue culture wells for estimating total cell counts. Performance of sampling and estimation schemes will depend in part on how the cells distribute themselves within a well. Spatial statistical analysis techniques are applied to the known total number and distribution of neurons in two wells counted in a grid scheme to demonstrate some important features of the neuron distributional patterns. Based on these two wells and simulated realizations from other point processes, a new sampling and estimation technique using open wedge-shaped sampling regions radiating from the centre of the well is proposed. This method is shown to result in more accurate estimates of the total number of neurons in the well than standard methods.

Responses in the diagonal band of Broca, adjacent septal nuclei and the islands of Calleja of cats to stimulation of the subcallosal fornix, medial basal hypothalamus and medial forebrain bundle

The projection of neurons in the septal nuclei and the insula magna of the islands of Callaja (IC) was explored together with their response to stimulation of the fornix. The septal nuclei all contained neurons projecting in the medial forebrain bundle (MFB). Only the diagonal band of Broca (DBB) and the lateral septal nucleus (LS) contained many neurons projecting toward the medial basal hypothalamus (MBH). The spatial distribution of neurons excited by stimulation of the fornix in the DBB was almost identical with the distribution of neurons projecting toward the MBH and there was considerable overlap (10/28 cells). In the medial septal nucleus the spatial distribution of neurons excited by stimulation of the fornix and neurons projecting in the MFB was similar and there was considerable overlap (6/21 cells). The connectivity of the IC resembled that of the MS but there was little overlap between the neurons excited by fornix stimulation and those projecting in the MFB (1/27 cells). In the LS there were almost equal numbers of neurons projecting in the MFB and toward the MBH but there was very little input from the fornix. Neurons were significantly more often excited by stimulation of the lateral fornix, carrying axons from the subiculum, than they were by medial stimulation exciting axons from Ammon's horn. Axons projecting toward the MBH or in the MFB had conduction velocities less than 1 m/s.

The origins of the cochlear efferent bundle of Rasmussen in primates

The spatial distribution of neurons emitting fibers of the olivocochlear bundle was determined by injecting horseradish peroxidase into the round window in a series of new and old world monkeys. Animals were perfused 24–28 h later. Sections through the pons and medulla were reacted with tetramethyl benzidine and counterstained with thionin and/or neutral red. Perikarya containing reaction product numbered over 800 in one case. The greatest concentration of labeled cells was located in the dorsal periolivary area intermediate between the dorsal margins of the lateral and medial superior olivary nuclei. Approximately 400 cells were ipsilateral to the injection, opposed to 100 contralaterally. The dorsolateral periolivary area contained over 100 labeled cells ipsilaterally, and only about one-third as many contralaterally. While 95 labeled cells occupied the region of the contralateral medial preolivary nucleus, about two-thirds as many labeled cells were found in the comparable ipsilateral nucleus. Although a few labeled neurons were present bilaterally in the lateral preolive and the lateral superior olivary nucleus, no labeled cells were found in the region of the dorsomedial or ventromedial periolivary nuclei (medial to the medial superior olive). [Work supported by the Deafness Research Foundation.]

Accurate three-dimensional reconstruction of neuronal distributions in brain: Reconstruction of the rat nucleus locus coeruleus

A technique is described which permits construction of accurate, quantified 3-dimensional maps of the distribution of neuronal cell groups in brain. The cartesian coordinates of landmarks and individual neurons are obtained from serial histological sections utilizing a computer-linked digitizing microscope. The digitized images of these sections are displayed on a computer graphics picture system where they are aligned so that spatial relationships within the nucleus are essentially identical to those of the intact brain. This is accomplished using information about landmarks obtained from photomicrotomy. As a consequence of the alignment procedure, each neuron is assigned a 3-dimensional coordinate representing its position in the reconstituted nucleus, and the reconstruction is oriented in a stereotaxic coordinate system. Nuclei from different brains can then be registered to one another, assigned coordinates relative to this standard coordinate space, and be compared statistically. Differences between nuclei in the spatial distribution of neurons in toto, or in the distribution of anatomically or physiologically defined subpopulations of neurons, can then be visualized with greater accuracy and in more detail than that permitted by traditional techniques. In addition, such comparisons can easily be quantified and statistically evaluated using, for example, analysis-of-variance techniques. For illustrative purposes, the technique is applied to the rat nucleus locus coeruleus as reconstructed from serial Nissl-stained sections.

Binaural response-specific bands in primary auditory cortex (AI) of the cat: Topographical organization orthogonal to isofrequency contours

The spatial distribution of neurons with different binaural response properties has been studied within the three dimensions of the primary auditory cortex (AI) in the cat. Using dichotic stimulation, 92% of neurons encountered could be classified into either the excitatory/excitatory (EE) or excitatory/inhibitory (EI) interaction class. In nearly all of almost 800 penetrations introduced along radial axes, all neurons encountered along a given penetration were of the same binaural response class. Neurons of different binaural interaction classes were spatially segregated within the plane of the cortex. Electrode penetrations made parallel to isofrequency contours traversed the mediolateral extent of AI through the middle layers of the cortex. A sharp segregation of units by binaural response class was observed in these penetrations, i.e. sequences of neurons that were all of the EE class alternated with sequences of EI neurons. The regions of uniform response to binaural stimulation formed radially organized topographical subunits that were elongated along the rostrocaudal dimension of AI. These binaural interaction bands intersect the lines of re-representation of the cochlear sensory epithelium (‘isofrequency contours’) and, thus, create subdivisions of AI that each contain a representation of the entire audible frequency domain. The implications of these results for the concept of AI as a unitary element in auditory processing are discussed.

Three-Dimensional Computer Reconstruction of Neurons and Neuronal Assemblies

In the past few years growing numbers of investigators have begun to use computer techniques in the determination and analysis of biological structures in three dimensions. These applications have ranged from the study of various properties of molecules, especially macromolecules ( 14, 27), to the study of the angle between cleavage planes during embryonic development (5 1,52). Particularly fruitful and extensive efforts have taken place in the area of neuroanatomy; these efforts are the primary focus of this review. A variety of neuroanatomical data have been obtained and analyzed with the aid of computers. First, small computers have been coupled to light microscopes to record, from thick sections or whole mounts, three­ dimensional (3-D) information such as the branching pattern, length, and thickness of dendrites of individual neurons stained by Golgi methods or by dye injection (2, 4, 5, 8, 1 7, 23, 35, 49, 66). Second, computers with drawing tablets have been used to measure, mainly from micrographs or prints, the size, shape, and distribution of neurons (7, 1 1 , 1 3, 67, 70), the distribution of silver grains in autoradiographs (1 , 2, 62), and the spatial distribution of neurons that have taken up dye at some distal injec­ tion site in the brain ( 12). Third, computers have been interfaced with film transport and projection systems to reconstruct, from film strips of aligned serial section electron or light micrographs, the 3-D anatomy of

The number, size and spatial distribution of neurons in lamina IV of the mouse SmI neocortex.

We located the corresponding barrel in Layer IV of the mouse SmI cortex in eleven cerebral hemispheres sectioned in a plane tangential to the pia overlying SmI and in one sectioned and prepared by a combined Golgi-Nissl method. In the section in which barrel C-1 could be optimally visualized each neuronal soma was outlined with a camera lucida and the cross-sectional area measured with the aid of a small computer. In all, nearly 7,000 neurons were measured. We estimate that on average barrel C-1 contains about 2,000 neurons. The mean cross-sectional area of the perikarya is 62.51 mu2 (S.D. plus or minus 14.51 mu2) and the size distribution of the neurons is unimodal and positively skewed. There is no segregation of cells within the barrel on the basis of size. The spatial distribution of cells in the barrel is fairly constant, from specimen to specimen, and the charactieristic cytoarchitectonic appearance of the barrel can be related to regional neuronal packing density since there are at least 1.6 as many neurons in the sides of the barrel as the hollow. The constancy of the cellular composition of the barrels indicates that the mechanisms responsible for the development of the mouse SmI cortex are fairly rigidly determined, and that the barrel field should lend itself well to further quantitative, developmental and physiological analysis.