Preferential orientation of stellate cell dendrites in the visual cortex of the dark-reared rat

A Review of Effects of Environment on Brain Size in Insects

Simple Summary

What makes a big brain is fascinating since it is considered as a measure of intelligence. Above all, brain size is associated with body size. If species that have evolved with complex social behaviours possess relatively bigger brains than those deprived of such behaviours, this does not constitute the only factor affecting brain size. Other factors such as individual experience or surrounding environment also play roles in the size of the brain. In this review, I summarize the recent findings about the effects of environment on brain size in insects. I also discuss evidence about how the environment has an impact on sensory systems and influences brain size.

Abstract

Brain size fascinates society as well as researchers since it is a measure often associated with intelligence and was used to define species with high “intellectual capabilities”. In general, brain size is correlated with body size. However, there are disparities in terms of relative brain size between species that may be explained by several factors such as the complexity of social behaviour, the ‘social brain hypothesis’, or learning and memory capabilities. These disparities are used to classify species according to an ‘encephalization quotient’. However, environment also has an important role on the development and evolution of brain size. In this review, I summarise the recent studies looking at the effects of environment on brain size in insects, and introduce the idea that the role of environment might be mediated through the relationship between olfaction and vision. I also discussed this idea with studies that contradict this way of thinking.

Distinct Neocortical Progenitor Lineages Fine-tune Neuronal Diversity in a Layer-specific Manner

How the variety of neurons that organize into neocortical layers and functional areas arises is a central question in the study of cortical development. While both intrinsic and extrinsic cues are known to influence this process, whether distinct neuronal progenitor groups contribute to neuron diversity and allocation is poorly understood. Using in vivo genetic fate-mapping combined with whole-cell patch clamp recording, we show that the firing pattern and apical dendritic morphology of excitatory neurons in layer 4 of the barrel cortex are specified in part by their neural precursor lineage. Further, we show that separate precursors contribute to unique features of barrel cortex topography including the intralaminar position and thalamic innervation of the neurons they generate. Importantly, many of these lineage-specified characteristics are different from those previously measured for pyramidal neurons in layers 2-3 of the frontal cortex. Collectively, our data elucidate a dynamic temporal program in neuronal precursors that fine-tunes the properties of their progeny according to the lamina of destination.

Visual deprivation alters dendritic bundle architecture in layer 4 of rat visual cortex

The effect of visual deprivation followed by light exposure on the tangential organisation of dendritic bundles passing through layer 4 of the rat visual cortex was studied quantitatively in the light microscope. Four groups of animals were investigated: (I) rats reared in an environment illuminated normally--group 52 dL; (II) rats reared in the dark until 21 days postnatum (DPN) and subsequently light exposed for 31 days-group 21/31; (III) rats dark reared until 52 DPN and then subsequently light exposed for 3 days--group 3 dL; and (IV) rats totally dark reared until 52 DPN--group 52 DPN. Each group contained five animals. Semithin 0.5-1-μm thick resin-embedded sections were collected from tangential sampling levels through the middle of layer 4 in area 17 and stained with Toluidine Blue. These sections were used to quantitatively analyse the composition and distribution of dendritic clusters in the tangential plane. The key result of this study indicates a significant reduction in the mean number of medium- and small-sized dendritic profiles (diameter less than 2 μm) contributing to clusters in layer 4 of groups 3 dL and 52 dD compared with group 21/31. No differences were detected in the mean number of large-sized dendritic profiles composing a bundle in these experimental groups. Moreover, the mean number of clusters and their tangential distribution in layer 4 did not vary significantly between all four groups. Finally, the clustering parameters were not significantly different between groups 21/31 and the normally reared group 52 dL. This study demonstrates, for the first time, that extended periods of dark rearing followed by light exposure can alter the morphological composition of dendritic bundles in thalamorecipient layer 4 of rat visual cortex. Because these changes occur in the primary region of thalamocortical input, they may underlie specific alterations in the processing of visual information both cortically and subcortically during periods of dark rearing and light exposure.

Structural plasticity in the developing visual system

The visual system has been used extensively to study cortical plasticity during development. Seminal experiments by Hubel and Wiesel (Wiesel, T.N. and Hubel, D.H. (1963) Single cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol., 26: 1003-1017.) identified the visual cortex as a very attractive model for studying structural and functional plasticity regulated by experience. It was discovered that the thalamic projections to the visual cortex, and neuronal connectivity in the visual cortex itself, were organized in alternating columns dominated by input from the left or the right eye. This organization was shown to be strongly influenced by manipulating binocular input during a specific time point of postnatal development known as the critical period. Two chapters in this volume review the molecular and functional aspects of this form of plasticity. This chapter reviews the structural changes that occur during ocular dominance (OD) plasticity and their possible functional relevance, and discusses developments in the methods that allow the analysis of the molecular and cellular mechanisms that regulate them.

The Maturation of the Superior Collicular Map of Auditory Space in the Guinea Pig is Disrupted by Developmental Visual Deprivation

In the normal guinea pig a map of auditory space appears, in the deeper layers of the superior colliculus, at 32 days after birth (DAB). The animal is unable to construct this collicular map of auditory space in the absence of developmental visual experience. Auditory receptive fields of animals dark-reared from birth are typically large, occupying most of the contralateral hemifield. There is no topographic relationship between the collicular location of the recording electrode and the spatial position from which auditory stimuli elicit a maximal response. The fields of dark-reared animals resemble, in their tuning parameters, the spatially undifferentiated fields typical of young postnatal normal guinea pigs. To investigate the time-course during which visual experience is required for map emergence, animals received normal visual experience until either 18 or 26 DAB and were then dark-reared until the terminal mapping experiment. Maps developed in neither group. Animals provided with a normal visual environment until 30 DAB, and then placed in the dark did, however, construct topographically organized spatial maps with discrete spatial receptive fields. Maps also failed to emerge in animals receiving normal visual experience both before and after a 4-day period of visual deprivation between 26 and 30 DAB. We conclude that this 4-day period, or part of it, constitutes a 'crucial' period during which visual experience is required for the normal elaboration of the collicular map of auditory space.

Effects of visual or light deprivation on the morphology, and the elimination of the transient features during development, of type I retinal ganglion cells in hamsters

Intracellular injection of Lucifer Yellow (LY) was used to study the detailed morphology of the normal visually deprived, and light-deprived superior colliculus projecting Type I retinal ganglion cells (RGCs) in hamsters. The soma size of the normal Type I cells ranged from 337 to 583 microns 2 with a mean of 436 microns 2. Two to six primary dendrites were observed in these cells. The mean dendritic field diameter was 495 microns and ranged from 309 to 702 microns. The dendritic field diameter of this population of cells exhibited an eccentricity dependence. Quantitative comparisons between the normal and visually deprived or light-deprived Type I RGCs indicated that the morphology of these three groups of cells were similar to each other in terms of the soma size, dendritic field diameter, branching pattern, and total length of the dendrites. During the normal development of cats and hamsters, several transient features, such as exuberant dendritic spines and intraretinal axonal branches, have been observed in the developing RGCs. The complete elimination of these transient features occurs at about 3 and 2 weeks after the opening of the eyes in cats and hamsters, respectively. In the present study, the hypothesis whether visual experience or light stimulation is required for the elimination of these transient features during development was examined. After studying a total of 115 mature Type I RGCs, which included cells from the normal, visually deprived and light deprived animals, no transient feature was observed. We conclude that visual or light deprivation has no effect on the morphological development of superior colliculus projecting Type I RGCs in hamsters, and the elimination of the transient features on the Type I RGCs during development does not depend on visual experience or light stimulation.

The postnatal development of layer VI pyramidal neurons in the cat's striate cortex, as visualized by intracellular Lucifer yellow injections in aldehyde-fixed tissue

The postnatal development of layer VI pyramidal neurons in the cat's striate cortex has been studied by means of intracellular injections of Lucifer yellow in aldehyde-fixed tissue (LYF technique). It is shown that the LYF technique gives results qualitatively and quantitatively similar to results obtained with other techniques (Golgi, marker-injections in viable tissue). Quantitative analysis demonstrated significant increases in soma diameter, number and length of basal dendrites, length of second order apical dendrites and, in particular, in number of spines/unit dendritic length, during the first postnatal month. Maturation of the basal dendritic tree and increase in number of spines continue in the second postnatal month. At later postnatal times soma diameter and number of spines decrease by about 20%. Dendritic varicosities are most frequent during the first postnatal week, and decrease in number steadily from thereon. The late maturation of layer VI pyramidal neurons suggests that these cells might be affected by early peripheral lesions and/or sensory deprivation to which the striate cortex of the cat has been shown to be most susceptible around the end of the first postnatal month.

Dendritic atrophy in children with Down's syndrome

Dendritic branching was evaluated in the visual cortex of 8 children with Down's syndrome and 10 controls, ranging in age from 4 months to 7 years and divided into infantile, late-infantile, and juvenile groups. Camera lucida drawings of Golgi-impregnated neurons were used for examining the following dendritic aspects: dendritic intersections as a function of distance from the cell body, point of maximum dendritic branching, number of branch orders, total number of branch segments, and total dendritic length. The number of intersections and the total dendritic length were above normal in the infantile period (6 months old or less) and dropped steadily to significantly below normal in the juvenile group (older than 2 years). These reductions contrasted with expanding dendritic arborization in normal children. The results suggest that the dendritic tree atrophies in early childhood in Down's syndrome.

Differential rearing effects on rat visual cortex synapses. I. Synaptic and neuronal density and synapses per neuron

The bulk of the evidence indicating that different experiences can lead to differences in synapse numbers involves inference from measures of postsynaptic surface (spines and dendrites) in Golgi impregnated tissue. The capriciousness of Golgi impregnation and the absence of direct evidence regarding changes in afferents mandate confirmation of synapse changes by electron microscopy. We calculated the ratio of synapses per neuron in layers I-IV of occipital cortex of rats reared in complex (EC), social (SC), or isolated (IC) environments. Synaptic density estimates were derived from electron micrographs of osmium-uranyl-lead stained tissue and neuronal density estimates were derived from toluidine blue stained semithin sections using stereological methods which correct for group differences in the sizes of synapses and neuronal nuclei. The ratio of these densities, synapses per neuron, was highest in complex environment rats, intermediate in socially reared rats and lowest in isolates, in accordance with predictions from prior Golgi studies. The bulk of the differences were attributable to neuronal density, which was highest in IC rats and lowest in ECs. Synaptic density did not differ statistically across groups. These results indicate, at least within this area and paradigm, that differences in dendritic measures in Golgi impregnated tissue reflect differences in the number of synapses per neuron.

A quantitative Golgi analysis of the postnatal maturation of dendrites in the central nucleus of the inferior colliculus of the rat

In this work, we used Golgi impregnation and computer assistance to analyze the quantitative development of the dendrites of neurons in the central nucleus of the inferior colliculus in the rat from 8 (P8) to 20 (P20) days of age, i.e., during the period of functional maturation of the auditory system. The number and length of the dendritic segments and the number of spines of a total of 516 impregnated cells were studied. The variations of both the mean number of peripheral dendritic segments and the mean dendritic domain area suggest that the dendritic tree of these cells is orientated preferentially in the sagittal plane before the onset of functional maturation, but undergoes a reorientation in the frontal plane during this critical period. The length of dendritic segments varies as a function of both the stage of the period of functional maturation and the order of magnitude of these segments. During the first stage, from P8 to P16, only proximal and intermediate segments lengthen, the distal segments increasing their length later from P16 to P20. The lengthening of the dendritic segments is associated with dramatic reabsorption of the dendritic spines on all dendritic segments and seems closely related to the functional maturation of the auditory pathways.

Developmental and experimental changes in dendritic symmetry in n. laminaris of the chick

Early acoustic experience affects the structure of neurons in the brainstem of young chickens. In the binaurally innervated cells of the nucleus laminaris the symmetry of the dorsal and ventral dendritic trees normally increases during the embryonic and early postnatal period. Unilateral ear plugs disrupt that development. This study shows that balanced stimulation plays an important role in the development of symmetrical neuronal structures in the central auditory pathway.

The structure of cerebral cortex in the rat following prenatal administration of 6-hydroxydopamine

The early, prenatal formation of noradrenergic projections to the forebrain has led to the proposition that these axons exert a trophic influence on cerebral cortex during ontogeny. To test this hypothesis, we have examined a number of different structural features of cortical development following prenatal lesions of the ascending noradrenergic axons. The parameters that were analyzed include cytoarchitecture, dendritic morphology, and the distribution of monoaminergic and nonmonoaminergic cortical afferents. Rat fetuses were administered the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) by transuterine, intraperitoneal injection on embryonic day 17. Vehicle-injected controls and fetuses treated with the catecholamine uptake inhibitor desmethylimipramine (DMI) prior to 6-OHDA were prepared. After reaching maturity (200-300 g), the brain of treated and control rats were examined using Nissl and Golgi preparations (for cytoarchitecture and dendritic morphology), histofluorescence (for monoaminergic afferents, especially dopaminergic axons), and serotonin and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Effective lesioning of the ascending noradrenergic system was confirmed in each case, using DBH immunocytochemistry. Prenatal treatment with 6-OHDA resulted in complete and long-lasting destruction of the noradrenergic innervation of the cerebral cortex, along with hyperinnervation of the diencephalon and brain stem. Despite the widespread denervation of cerebral cortex, no significant alterations in cytoarchitecture, dendritic morphology, or spine counts were found in treated brains. In particular, no abnormalities were observed in the apical dendrites of layer VI pyramidal cells, based on qualitative criteria. The distribution, density and morphology of serotonergic and dopaminergic afferents were unaffected. Thalamocortical afferents had developed normally as reflected by the cortical barrels. In 33% of the 6-OHDA-treated fetuses foci of ectopic neurons were found at the cortical surface. The ectopias contain neuronal processes, somata, and synapses interspersed with collagen and other connective tissue elements. While the ectopias may result from selective damage to the noradrenergic neurons, the finding of similar (but smaller) malformations in DMI-protected animals is equally consistent with a non-specific effect of 6-OHDA upon non-adrenergic cells. The examination of intervening stages will be needed to resolve this question. Based on the parameters of cortical structure analyzed in this study we conclude that the neocortex develops normally even in the absence of the noradrenergic system.

Organization and development of brain stem auditory nuclei of the chicken: dendritic development in N. laminaris

Nucleus laminaris (NL) is a third-order auditory nucleus in the avian brain stem which receives spatially segregated binaural inputs from the second-order nuclei magnocellularis. The examination of the development of dendritic structure in NL revealed a number of events: In the initial period of dendritic growth (E 8--9) there is no gradient of dendritic morphology or apparent size. Starting about E 9--10, there is a spatiotemporal gradient of proliferation of numerous fine dendritic processes, from rostromedial to caudolateral, corresponding with the morphological lamination of NL, and possibly with the onset of cell death. This is followed by a spatiotemporal gradient of the elimination of the overproliferated processes, from rostromedial to caudolateral possibly coinciding with the cessation of cell death. A result of the spatiotemporal gradients of dendritic process proliferation and elimination is a spatial gradient in the morphology (extension, branching) of the remaining "mature" dendrites. At E 15 there is only a slight spatial gradient of total dendritic size across NL; this gradient is larger at E 19, and by P 25 there is a 13-fold change in dendritic size from rostromedial to caudolateral. Regression analyses suggest the size gradient begins to form when NL activity becomes driven by cochlear activity, at about E 14. The progressive formation of the size gradient is largely the result of two factors: the growth of dendritic trees, and the loss or primary dendrites. The growth rate of the dendritic trees of NL cells was found to be very highly correlated with the intensities of the sound frequencies to which the cells respond. From E 15 to P 25 there is a 50% loss of the "mature" primary dendrites of NL neurons. The separate dorsal and ventral dendritic size gradients seen at E 15 realign to coincide at E 19, and the moderate correlation of dorsal and ventral dendritic sizes seen at E 15 and E 19 is significantly increased at P 25, indicating a developmental process of sharpening in the relationship of the dorsal and ventral dendritic organizations in the nucleus. The data suggest that a key element in the regulation of dendritic size and structure in n. laminaris may be the activity of the afferents to the cells.

Postnatal modifications of the dendritic tree of cells in the inferior colliculus of the cat. A quantitative Golgi analysis

Postnatal modifications of dendrites have been quantitatively studied by network analysis of the dendritic tree in the two central nuclei of the inferior colliculus in the cat. This analysis revealed cells with two types of branching patterns suggesting two different modes of growth. The predominant pattern is characterized by dichotomous branching on random segments (DR cells). However, a purely collateral branching pattern is particular to certain cells (CB cells). These two branching patterns were found in both nuclei of the IC in adult and young cats, but the exact significance of these two cell types remains unclear. The dendritic trees of cells in kittens differed from those of the adult cat. Also, the types of modification were different in the two functionally distinct nuclei of the inferior colliculus that we studied. The most dramatic modifications were observed in the dendritic tree of DR cells in the central nucleus, which receives fibers from the auditory nuclei in the brainstem. Two parameters were modified: the mean number of terminal segments and the mean total length of segments. Both parameters increased in the sagittal plane and decreased in the frontal plane. These modifications indicate a reorientation of the dendritic tree in the sagittal plane, along the incoming axons from the auditory nuclei. As these afferents become functionally mature only after birth, this spatial remodeling of the dendrites seems closely related to functional maturation of secondary auditory axons. In the dorsomedial nucleus that receives fibers from the auditory cortices, the dendritic tree of DR cells also undergoes spatial reorientation. This is more evident in the horizontal plane and with respect to the incoming axons. Our results suggest that the characteristic orientation of the dendritic tree of cells observed in the inferior colliculus of the adult cat is established only after the first postnatal weeks. This orientation seems to result from an active process of remodeling concomitant with the functional maturation of afferents, a fact already established for various cell types in the nervous system.

Dendritic plasticity in mouse barrel cortex following postnatal vibrissa follicle damage

Neonatal damage to a row of mystacial vibrissae in the mouse causes cytoarchitectonic alterations in the contralateral SmI barrel Cortex. The region for the appropriate row of barrels develops as a smaller homogeneous zone while barrels in adjacent rows are expanded. To investigate the effects of this phenomenon on the morphology of individual neurons, adult mice in which Row-C vibrissae (the middle row) had been cauterized on days 1--5 following birth were processes by the Golgi-Cox method. All neurons in layer IV of the Row-C zones, of the Row-C barrels of a control hemisphere, and some neurons in the adjacent enlarged Row-B barrels were measured with a computer-assisted microscope. Their location with respect to cytoarchitectonic boundaries was determined from a Nissl counterstain. Data from 239 cells are presented. For each cell, measures of dendritic length and branching were obtained. The orientation of the dendritic trees with respect to the barrel sides was also measured. The measures of dendritic lengths and branching did not show any differences between control and experimental animals or between animals damaged on different days. Measures of orientation did show changes related to the age at the time of damage. In animals damaged on postnatal day (PND)-3 or earlier, many cells in the Row-C zone were observed with dendrites orienting toward the adjacent Rows-B or -D. "Putative" Row-C cells in the expanded parts of Rows-B and -D were strongly oriented toward barrels in those rows. These results suggest that dendritic length and branching may be determined intrinsically but that the orientation of the dendritic trees appears to be strongly influenced by the pattern of extrinsic afferent inputs from the thalamus. In the case of the whisker-damaged animals, the orientation of the Row-C neuron dendritic trees toward the "functional" thalamocortical inputs in Rows-B and -D contributes strongly to the resultant cytoarchitectonic changes. The implications of these results for normal developmental processes and their relationship to functional studies of the cortex are considered.

Contextual memory in the molecular domain

Contextually rich recall of past events and actions indicates the formation of complex memory traces in which many items of information are integrated. The speed of this process and the inference that large numbers of cortical neurons are involved argue against synaptic transmission of all of the information required. The intercellular electromagnetic field giving rise to the EEG may function as an additional carrier of information essential to contextual processing. Recent experiments have led to models of the neural membrane that show very great sensitivities to the intercellular field. Changes of arrayed molecular conformations in this membrane due to cooperative effects in the intercellular field may provide a contextual memory located within the dense dendritic network of the cortex. Integrative effects within the volume structure of complex electromagnetic fields may thus provide a means of high-speed contextual processing and discrimination.

Determinants of cell shape and orientation: a comparative Golgi analysis of cell-axon interrelationships in the developing neocortex of normal and reeler mice

Patterns of dendritic development in the neocortex of normal and reeler E15-17 mouse embryos are studied in Golgi impregnations. Interactions between dendrites and axon-rich strata appear to be critical determinants of dendritic morphology in both genotypes. Firstly, axon-dendrite proximity appears to stimulate dendritic sprouting, elongation and branching. Secondly, the position of the axon-rich strata with respect to the differentiating cell appears to determine the direction of dendritic growth and thereby the ultimate configuration of the dendritic arbor. With regard to specific cell configurations, a multipolar form is generated when the cell is embedded in an axon-rich zone. A monopolar or bipolar configuration is achieved when the cell lies in the axon-poor cortical plate and addresses and axon-rich stratum with one or both radially extended migratory processes. Such variations in the configuration of neurons with polar dendritic systems may be observed uniquely in the mutant cortex because axon-rich zones are stratified anomalously at multiple levels in the cortical plate. As a consequence, polar dendritic systems develop from either the superior, the inferior or both somatic poles of postmigratory cells. Pyramidal cells may, therefore, develop a normal upright or an abnormal "upside-down" disposition. Regardless of the orientation of the polar dendritic system, the axon emerges from the inferior aspect of the cell suggesting that there has been no rotation of the original migratory axis of the cell.

Organization and development of brain stem auditory nuclei of the chicken: dendritic gradients in nucleus laminaris

Nucleus laminaris (NL) is a third-order auditory nucleus in the avian brain stem which receives spatially-segregated binaural inputs from the second-order magnocellular nuclei. The organization of dendritic structure in NL was examined in Golgi-impregnated brains from hatchling chickens. Quantitative analyses of dendritic size and number were made from camera lucida drawings of 135 neurons sampled from throughout the nucleus. The most significant results of this study may be summarized as follows: (1) The preponderant neuron in n. laminaris may be characterized as having a cylindrical-to-ovoid cell body, about 20 micrometer in diameter. The neurons comprising NL were found to be nearly completely homogeneous in issuing their dendrites in a bipolar fashion: one group of dendrites is clustered on the dorsal surface of the cells, the other group on the ventral. The dendrites of NL are contained within the glia-free neuropil surrounding the nucleus. From the rostromedial to the caudolateral poles of NL there is a gradient of increasing extension of the dendrites, increasing number of tertiary and higher-order dendrites, and increasing distance from the somata of the occurrence of branching. (2) The total dendritic size (sum of the dorsal) and ventral dendritic lengths of the cells) increases 3-fold from the rostromedial to the caudolateral poles of NL. About 50% of the variance in dendritic size is accounted for by the position of the cells in NL, and the gradient of dendritic size increase has the same orientation across NL as the tonotopic gradient of decreasing characteristic frequency in NL. (3) From the rostromedial pole to the caudolateral pole of NL there is an 11-fold decrease in the number of primary dendrites along a gradient coinciding with the length and frequency gradients. Sixty-six percent of the variance in dendrite number is accounted for by position in the nucleus. (4) The correlation of dorsal and ventral dendritic size on a cell-by-cell basis is not high (r = 0.47), indicating a fair amount of variability on the single-cell level. On the other hand, the average dorsal dendritic length within an isofrequency band in NL correlates very highly with the average ventral dendritic length. Thus, on an areal basis, the amount of dendritic surface area offered to the dorsal and ventral afferents is tightly regulated. (5) The dorsal and ventral dendrites have separate gradients of increasing length and number across NL. The dorsal gradients are skewed toward the rostrocaudal axis, while the ventral dendritic gradients are skewed mediolaterally. (6) There was no correlation between either dendritic size or number of primary dendrites and the size of the somata in NL, which remains relatively constant throughout the nucleus. Several hypotheses about the ontogenetic control of dendritic structure are examined in light of the above data. Of these, the hypotheses that the ontogeny of dendritic size and number is largely under afferent control receives a great deal of circumstantial support.

Somatosensory cortex: acetylcholinesterase staining of barrel neuropil in the rat

Acetylcholinesterase (AchE) staining of layer IV of rat somatosensory (SmI) cortex was studied. In the barrel field of SmI, there are periodic, intensely AchE staining foci in the pattern and with the dimensions of barrels. The onset of this staining, at age 3 days, corresponds with the arrival of thalamocortical input to the barrels. Undercutting of SmI prevents staining in layer IV. We conclude that there are AchE-rich zones in layer IV that coincide with the specific thalamocortical projection to SmI.

A Golgi study of the early postnatal development of the visual cortex of the hooded rat

Although neuroanatomical plasticity has been demonstrated in the rat visual cortex, no systematic data on the dendritic development of the area are available. In the present study, the visual cortex of hooded rats at 1, 3, 5, 7, 10 and 15 postnatal days of age (P1-P15) was impregnated with the rapid Golgi method. The cortex was divided into the superficial layers, II-IV, and the middle layer V. At P1, pyramidal neurons had apical shafts and the beginning of the apical terminal arch. Analysis of both basilar and oblique dendritic number showed that pyramidal neurons of the middle layer developed more quickly than those in the superficial layers. The number of lower order basilar dendritic branches reached asymptote over the examined time period, whereas the higher order branches were still increasing in number but at a decelerating rate by P15. Dendrites at all ages exhibited varicosities which were especially prominent on the thin dendritic branches of the earlier ages. Some thin, filamentous processes, termed protospines, were found on dendrites and cell bodies at P1 to P5. They seemed to decrease by P7, when a few mature spines appeared. Spines increased in number on days P10 and P15. A comparison of the data from this study with quantified Golgi studies in adult rats indicates that by P10 and P15 the number of basilar branches is in the range seen in the adult.

Changes in cortical dendritic branching subsequent to partial social isolation in stumptailed monkeys

Stumptailed monkeys were reared from 1 week after birth to 6 months of age in either a colony condition with the mother or in partial social isolation that allowed visual contact with the colony animals, but not physical contact. At 6 months of age the animals were killed and selected areas of the neocortex stained by the Golgi-Cox method. Relatively nonspiney cells of Layer IV were drawn and analyzed for complexity of dendritic branching. Isolation-reared animals had significantly decreased branching complexity in Motor I cortex when compared to the control animals. A transform of the data that related the number of branches to the number of previous branches showed a slight rearing effect in Somatosensory I cortex with the deprived animals having a lower rate of branching than the controls. We conclude that social isolation also includes a motoric deprivation that could account for these data.

The effects of dark rearing on the development of the visual cortex of the rat

The effects of dark rearing on the development of the visual cortex has been studied in Wistar rats, as have the effects of subsequent light exposure on recovery. Five groups of animals were used: (1) light exposed until 30-40 days post partum (dpp) (2) dark reared until 30-40 ddp (3) dark reared until 80-120 dpp (4) dark reared to 21 dpp, then light exposed until 40 dpp (5) light exposed to 21 dpp and then dark reared until 40 dpp. Golgi-Cox impregnations of layer IV stellate cell dendritic fields were analysed and total neuronal and glial counts were also done within layer IV of the primary visual cortex. Normal visual stellate cell dendritic fields were radially organised, with the highest dendritic density being recorded below the soma. In short term visually deprived animals and in the exposed only for 21 dpp and then reared in light until 40 dpp the radial distribution of dendrites was maintained but the peak density shifted to above the soma. In all other experimental groups this abnormal polarisation was still present but not as marked. Measurement of branching indices suggested that these field changes resulted from increased branching and growth in the superficial domain and not from the reorientation of dendrites. Differential glial counts revealed a significantly higher number of microglia in dark reared animals than in controls. Neuronal numbers were not affected.

The study of auditory deprivation from birth

This paper reviews the research published on auditory deprivation from birth. The extent of the damage caused to animals by this is discussed in relation to humans, deprived of hearing at birth, but later provided with some auditory input through a fitted hearing aid. Lack of research findings prevents a conclusive statement but there appear to be strong grounds for expecting some resulting permanent deficiency at a cortical level in the hearing of complex sounds such as speech.

The growth of the dendritic trees of Purkinje cells in irradiated agranular cerebellar cortex

The heads of noenatal Wistar rats were irradiated with 200 rads daily from birth to the 10th day post-partum. Ten litters each containing 5 animals were killed at 30 days post-partum and their brains treated by the Golgi-Cox technique. The dendritic trees of 24 Purkinje cells were analysed using the quantitative technique of network analysis, and comparisons made between parameters obtained from 20 normal Purkinje cells. All dendritic trees in agranular irradiated cortex were markedly reduced in size (as indicated by total dendritic length and total number of segments) although mean path lengths were normal. Segment lengths were normal over proximal branches, but uniformly increased over distal branches. Abnormal appendages, called 'giant spines' were observed on many dendrites. They were often some 10 mum in length and their presence effectively reduced segment lengths, increased the frequency of trichotomy and deviated growth from the normal random terminal pattern so that long collateral branching topologies were formed. Nevertheless, trichotomy was uniformly reduced in those trees without 'giant spines' and the distribution of branching patterns suggested that growth had proceeded by random terminal dichotomy. These results demonstrate that the development of dendritic trees is retarded in the agranular irradiated cerebellum, where synaptogenesis is very greatly reduced below normal. The quantitative changes in segment lengths, size of trees, and trichotomy accord with those predicted by the filopodial synaptogenic hypothesis of dendritic growth formulated by Vaughn et al. 99, whilst the results of the topological analysis suggest that branching is established by a degree of non-random interaction between growing dendrites and their substrate. 'Claw-like' dendritic complexes within some Purkinje cell trees may have been induced by aberrent fibre bundles of few surviving granule cells.