Neurons In Supragranular Layers Research Articles

Hyperphosphorylated tau in Alzheimer's disease disseminates along pathways predicted by the Structural Model for Cortico-cortical Connections.

In Alzheimer´s disease (AD), hyperphosphorylated tau spreads along the cerebral cortex in a stereotypical pattern that parallels cognitive deterioration. Tau seems to spread transsynaptically along cortico-cotical pathways that, according to synaptic tract-tracing studies in nonhuman primates, have specific laminar patterns related to the cortical type of the connected areas. This relation is described in the Structural Model. In the present article, we study the laminar distribution of hyperphosphorylated tau, labeled with the antibody AT8, along temporal cortical types in postmortem human brains with different AD stages to test the predictions of the Structural Model. Brains from donors without dementia had scant AT8-immunorreactive (AT8-ir) neurons in allo-, meso-, and isocortical types. In early AD stages, the mesocortical dysgranular type, including part of the transentorhinal cortex, had the highest AT8 immunostaining and AT8-ir neurons density. In advanced AD stages, AT8 immunostaining increased along the isocortical types until reaching the auditory koniocortex. Regarding laminar patterns, in early AD stages there were more AT8-ir neurons in supragranular layers in each de novo involved neocortical type; in advanced AD stages, AT8-ir neurons were equally distributed in supra- and infragranular layers. These AT8-ir laminar patterns are compatible with the predictions of the Structural Model. In summary, we show that hyperphosphorylated tau initially accumulates in allo-, meso-, and isocortical types, offer a proof of concept for the validity of the Structural Model to predict synaptic pathway organization in the human cerebral cortex, and highlight the relevance of nonhuman primate tract-tracing studies to understand human neuropathology.

A long-range, recurrent neuronal network linking the emotion regions with the somatic motor cortex.

SUMMARYRecurrent neural networks (RNNs) are designed to learn sequential patterns in silico, but it is unclear whether and how an RNN forms in the native networks of the mammalian brain. Here, we report an innate RNN, which is formed by the unidirectional connections from three basic units: input units arriving from emotion regions, a hidden unit in the medial prefrontal cortex (mPFC), and output units located at the somatic motor cortex (sMO). Specifically, the neurons from basal lateral amygdala (BLA) and the insular cortex (IC) project to the mPFC motor-cortex-projecting (MP) neurons. These MP neurons form a local self-feedback loop and target major projecting neurons of the sMO. Within the sMO, the neurons in the infragranular layers receive stronger input than the neurons in supragranular layers. Finally, we show in vivo evidence that the communications from the emotion regions to the sMO are abolished when MP neurons are chemogenetically silenced.

The Effects of NMDA Receptor Blockade on Sensory-Evoked Responses in Superficial Layers of the Rat Barrel Cortex.

Transmission of excitation from L4 to L2/3 is a part of a canonical circuit of cortical sensory signal processing. While synapses from L4 to L2/3 are mediated by both AMPA and NMDA glutamate receptors, previous studies suggested that sensory-evoked excitation of neurons in supragranular layers is almost entirely mediated by NMDA receptors. Here, we readdressed this question using extracellular recordings of sensory-evoked potentials (SEPs) and multiple unit activity (MUA) in the rat barrel cortex. We found that blockade of NMDA receptors using the selective antagonist dAPV profoundly inhibited the late part of L2/3 SEP, the associated sink, and MUA response but did not affect its initial part. Our results indicate that both non-NMDA and NMDA receptors are involved in sensory signal transmission from L4 to L2/3. While non-NMDA receptors mediate fast transmission of sensory signals, NMDA-Rs are importantly involved in the generation of the late phase of the sensory-evoked response in supragranular layers.

Reversible Functional Changes Evoked by Anodal Epidural Direct Current Electrical Stimulation of the Rat Auditory Cortex.

Rat auditory cortex was subjected to 0.1 mA anodal direct current in seven 10-min sessions on alternate days. Based on the well-known auditory cortex control of olivocochlear regulation through corticofugal projections, auditory brainstem responses (ABRs) were recorded as an indirect test of the effectiveness and reversibility of the multisession protocol of epidural stimulation. Increases of 20–30 dB ABR auditory thresholds shown after epidural stimulation reverted back to control levels 10 min after a single session. However, increases in thresholds revert 4 days after multisession stimulation. Less changes in wave amplitudes and threshold shifts were shown in ABR recorded contralaterally to the electrically stimulated side of the brain. To assess tissue effects of epidural electric stimulation on the brain cortex, well characterized functional anatomical markers of glial cells (GFAP/astrocytes and Iba1/microglial cells) and neurons (c-Fos) were analyzed in alternate serial sections by quantitative immunocytochemistry. Restricted astroglial and microglial reactivity was observed within the cytoarchitectural limits of the auditory cortex. However, interstitial GFAP overstaining was also observed in the ventricular surface and around blood vessels, thus supporting a potential global electrolytic stimulation of the brain. These results correlate with extensive changes in the distribution of c-Fos immunoreactive neurons among layers along sensory cortices after multisession stimulation. Quantitative immunocytochemical analysis supported this idea by showing a significant increase in the number of positive neurons in supragranular layers and a decrease in layer 6 with no quantitative changes detected in layer 5. Our data indicate that epidural stimulation of the auditory cortex induces a reversible decrease in hearing sensitivity due to local, restricted epidural stimulation. A global plastic response of the sensory cortices, also reported here, may be related to electrolytic effects of electric currents.

Layer-Dependent Short-Term Synaptic Plasticity Between Excitatory Neurons in the C2 Barrel Column of Mouse Primary Somatosensory Cortex.

Neurons process information through spatiotemporal integration of synaptic input. Synaptic transmission between any given pair of neurons is typically a dynamic process with presynaptic action potentials(APs) evoking depressing or facilitating postsynaptic potentials when presynaptic APs occur within hundreds of milliseconds of each other. In order to understand neocortical function, it is therefore important to investigate such short-term synaptic plasticity at synapses between different types of neocortical neurons. Here, we examine short-term synaptic dynamics between excitatory neurons in different layers of the mouse C2 barrel column through in vitro whole-cell recordings. We find layer-dependent short-term plasticity, with depression being dominant at many synaptic connections. Interestingly, however, presynaptic layer 2 neurons predominantly give rise to facilitating excitatory synaptic output at short interspike intervals of 10 and 30 ms. Previous studies have found prominent burst firing of excitatory neurons in supragranular layers of awake mice. The facilitation we observed in the synaptic output of layer 2 may, therefore, be functionally relevant, possibly serving to enhance the postsynaptic impact of burst firing.

Transformation of the Radial Glia Scaffold Demarcates Two Stages of Human Cerebral Cortex Development

The classic view of cortical development, embodied in the radial unit hypothesis, highlights the ventricular radial glia (vRG) scaffold as a key architectonic feature of the developing neocortex. The scaffold includes continuous fibers spanning the thickness of the developing cortex during neurogenesis across mammals. However, we find that in humans, the scaffold transforms into a physically discontinuous structure during the transition from infragranular to supragranular neuron production. As a consequence of this transformation, supragranular layer neurons arrive at their terminal positions in the cortical plate along outer radial glia (oRG) cell fibers. In parallel, the radial glia that contact the ventricle develop distinct gene expression profile and "truncated" morphology. We propose a supragranular layer expansion hypothesis that posits a deterministic role of oRG cells in the radial and tangential expansion of supragranular layers in primates, with implications for patterns of neuronal migration, area patterning, and cortical folding.

Differential susceptibility of cortical and subcortical inhibitory neurons and astrocytes in the long term following diffuse traumatic brain injury.

Long-term diffuse traumatic brain injury (dTBI) causes neuronal hyperexcitation in supragranular layers in sensory cortex, likely through reduced inhibition. Other forms of TBI affect inhibitory interneurons in subcortical areas but it is unknown if this occurs in cortex, or in any brain area in dTBI. We investigated dTBI effects on inhibitory neurons and astrocytes in somatosensory and motor cortex, and hippocampus, 8 weeks post-TBI. Brains were labeled with antibodies against calbindin (CB), parvalbumin (PV), calretinin (CR) and neuropeptide Y (NPY), and somatostatin (SOM) and glial fibrillary acidic protein (GFAP), a marker for astrogliosis during neurodegeneration. Despite persistent behavioral deficits in rotarod performance up to the time of brain extraction (TBI = 73.13 ± 5.23% mean ± SEM, Sham = 92.29 ± 5.56%, P < 0.01), motor cortex showed only a significant increase, in NPY neurons in supragranular layers (mean cells/mm2 ± SEM, Sham = 16 ± 0.971, TBI = 25 ± 1.51, P = 0.001). In somatosensory cortex, only CR+ neurons showed changes, being decreased in supragranular (TBI = 19 ± 1.18, Sham = 25 ± 1.10, P < 0.01) and increased in infragranular (TBI = 28 ± 1.35, Sham = 24 ± 1.07, P < 0.05) layers. Heterogeneous changes were seen in hippocampal staining: CB+ decreased in dentate gyrus (TBI = 2 ± 0.382, Sham = 4 ± 0.383, P < 0.01), PV+ increased in CA1 (TBI = 39 ± 1.26, Sham = 33 ± 1.69, P < 0.05) and CA2/3 (TBI = 26 ± 2.10, Sham = 20 ± 1.49, P < 0.05), and CR+ decreased in CA1 (TBI = 10 ± 1.02, Sham = 14 ± 1.14, P < 0.05). Astrogliosis significantly increased in corpus callosum (TBI = 6.7 ± 0.69, Sham = 2.5 ± 0.38; P = 0.007). While dTBI effects on inhibitory neurons appear region- and type-specific, a common feature in all cases of decrease was that changes occurred in dendrite targeting interneurons involved in neuronal integration. J. Comp. Neurol. 524:3530-3560, 2016. © 2016 Wiley Periodicals, Inc.

Sparse, reliable, and long-term stable representation of periodic whisker deflections in the mouse barrel cortex

The rodent whisker system is a preferred model for studying plasticity in the somatosensory cortex (barrel cortex). Contrarily, only a small amount of research has been conducted to characterize the stability of neuronal population activity in the barrel cortex. We used the mouse whisker system to address the neuronal basis of stable perception in the somatosensory cortex. Cortical representation of periodic whisker deflections was studied in populations of neurons in supragranular layers over extended time periods (up to 3months) with long-term two-photon Ca2+ imaging in anesthetized mice. We found that in most of the neurons (87%), Ca2+ responses increased sublinearly with increasing number of contralateral whisker deflections. The imaged population of neurons was activated in a stereotypic way over days and for different deflection rates (pulse frequencies). Thus, pulse frequencies are coded by response strength rather than by distinct neuronal sub-populations. A small population of highly responsive neurons (~3%) was sufficient to decode the whisker stimulus. This conserved functional map, led by a small set of highly responsive neurons, might form the foundation of stable sensory percepts.

Immunolocalization of Dynamin I Protein in Projection Neurons of the Visual System of the Adult Cat

We here report on the immunolocalization of Dynamin I (Dyn I) in neurons of the visual system of the cat. The lateral geniculate nucleus (LGN) complex displayed abundant Dyn I immunoreactivity in typical relay cells of the X-, Y- and W-pathway. The superficial and deep layers of the superior colliculus were also populated by Dyn I-immunoreactive projection neurons of the W- and Y-cell system. In primary visual areas 17 and 18, many densely packed layer VI neurons were intensely stained. A clear Dyn I signal was also demonstrated in pyramidal neurons of supragranular layers II and III, while layer IV displayed low Dyn I immunoreactivity. Additionally, area 18 displayed larger border pyramidal neurons in layer III compared to area 17. Generally, Dyn I was localized to the cell body and dendrites of neurons, to the neuropil and sometimes also to axon bundles. Typically, the Dyn I signal was not always uniformly distributed within the somatodendritic compartment. Based on its widespread distribution mainly in projection neurons Dyn I may play a fundamental role in mature neurons of different cortical and subcortical structures of the adult mammalian brain.

Quantitative Analysis of the Corticocortical Projections to the Middle Temporal Area in the Marmoset Monkey: Evolutionary and Functional Implications

The connections of the middle temporal area (MT) were investigated in the marmoset, one of the smallest primates. Reflecting the predictions of studies that modeled cortical allometric growth and development, we found that in adult marmosets MT is connected to a more extensive network of cortical areas than in larger primates, including consistent connections with retrosplenial, cingulate, and parahippocampal areas and more widespread connections with temporal, frontal, and parietal areas. Quantitative analyses reveal that MT receives the majority of its afferents from other motion-sensitive areas in the temporal lobe and from the occipitoparietal transition areas, each of these regions containing approximately 30% of the projecting cells. Projections from the primary visual area (V1) and the second visual area (V2) account for approximately 20% of projecting neurons, whereas "ventral stream" and higher-order association areas form quantitatively minor projections. A relationship exists between the percentage of supragranular layer neurons forming the projections from different areas and their putative hierarchical rank. However, this relationship is clearer for projections from ventral stream areas than it is for projections from dorsal stream or frontal areas. These results provide the first quantitative data on the connections of MT and extend current understanding of the relationship between cortical anatomy and function in evolution.

G1 Phase Regulation, Area-Specific Cell Cycle Control, and Cytoarchitectonics in the Primate Cortex

We have investigated the cell cycle-related mechanisms that lead to the emergence of primate areas 17 and 18. These areas are characterized by striking differences in cytoarchitectonics and neuron number. We show in vivo that (1) area 17 precursors of supragranular neurons exhibit a shorter cell cycle duration, a reduced G1 phase, and a higher rate of cell cycle reentry than area 18 precursors; (2) area 17 and area 18 precursors show contrasting and specific levels of expression of cyclin E (high in area 17, low in area 18) and p27Kip1 (low in area 17, high in area 18); (3) ex vivo up- and downmodulation of cyclin E and p27Kip1 show that both regulators influence cell cycle kinetics by modifying rates of cell cycle progression and cell cycle reentry; (4) modeling the areal differences in cell cycle parameters suggests that they contribute to areal differences in numbers of precursors and neuron production.

Early specification of the hierarchical organization of visual cortical areas in the macaque monkey.

The laminar organization of cortico-cortical projection neurons (expressed by the percentage of supragranular projecting neurons - SLN%) characterizes cortical pathways as feedforward (FF) or feedback (FB) and determines the hierarchical ranking of cortical areas. There is evidence of a developmental reduction in SLN% of pathways to area V1. Here, by analyzing pre- and postnatal projections to area V4, we have been able to address whether developmental reductions of SLN% impact on information processing in the immature cortex. FB pathways to area V4 exhibit 28-84% reduction of SLN%. This contrasts with the FF projections, which show little or no SLN% reduction. However, SLN% values in the immature cortex allocated cortical areas to the same hierarchical levels as in the adult. The developmental reduction of SLN% is a widespread phenomenon in the neocortex and is a distinctive feature of FB pathways. Two mechanisms contribute to developmental changes in SLN%: (i) delayed ingrowth of axons into the cortical target from infragranular layer neurons and (ii) prolonged developmental reduction of the divergence of projections from supragranular layer neurons. The present results show that FF and FB projections exhibit different developmental processes and patterns of connections linking cortical areas and their hierarchical relations are established prenatally, independently of regressive phenomena.

Laminar Distribution of Neurons in Extrastriate Areas Projecting to Visual Areas V1 and V4 Correlates with the Hierarchical Rank and Indicates the Operation of a Distance Rule

The directionality of corticocortical projections is classified as feedforward (going from a lower to higher hierarchical levels), feedback (interconnecting descending levels), and lateral (interconnecting equivalent levels). Directionality is determined by the combined criteria of the laminar patterns of the axon terminals as well as the cells of origins and has been used to construct models of the visual system, which reveals a strict hierarchical organization (Felleman and Van Essen, 1991; Hilgetag et al., 1996a). However, these models are indeterminate partly because we have no indication of the distance separating adjacent levels. Here we have attempted to determine a graded parameter describing the anatomical relationship of interconnected areas. We have investigated whether the precise percentage of labeled supragranular layer neurons (SLN%) in each afferent area after injection in either visual areas V1 or V4 determines its hierarchical position in the model. This shows that pathway directionality in the Felleman and Van Essen model is characterized by a range of SLN% values. The one exception is the projection of the frontal eye field to area V4, which resembles a feedforward projection. Individual areal differences in SLN% values are highly significant, and the number of hierarchical steps separating a target area from a source area is found to be tightly correlated to SLN%. The present results show that the hierarchical rank of each afferent area is reliably indicated by SLN%, and therefore this constitutes a graded parameter that is related to hierarchical distance.

Development of neuronal activity and activity-dependent expression of brain-derived neurotrophic factor mRNA in organotypic cultures of rat visual cortex.

We have analyzed in organotypic rat visual cortex cultures the way in which expression of brain-derived neurotrophic factor (BDNF) mRNA depends on synaptically generated spontaneous bioelectric activity (SBA) as monitored by recordings of pyramidal cells. SBA was initially low, but from the fourth week onwards 83% of the neurons fired action potentials at 0.2-1.2 impulses/s in a well-balanced state of excitation and inhibition. BDNF mRNA expression increased during the second week to a level surprisingly similar to the adult visual cortex in vivo, despite the fact that activity rates in vitro were approximately 10-fold lower than rates reported in vivo. Thus, SBA generated by a cortical neuronal network in the absence of sensory input is sufficient to elicit and maintain BDNF expression. The transient BDNF peak occurring after eye opening in vivo did not occur in vitro. A blockade of SBA seems not to alter the expression of neurotrophin (NT)-3 and -4/5, and tyrosine kinase receptor C and B mRNA. However, BDNF expression remained extremely low. A recovery of SBA after a period of blockade concurred with a transient hyperexcitability. BDNF immediately increased, driven by calcium influx through voltage-gated channels in synergy with NMDA receptors. Expression transiently reached high levels in neurons of supragranular layers. Infragranular neurons, although firing action potentials, recovered BDNF expression much slower. After 5 days in vitro recovery, the network had de novo established a balanced state of excitation and inhibition. Distribution and expression level of BDNF mRNA had returned to control. Even in 'adult' cultures an acute blockade of SBA downregulated BDNF, and a subsequent recovery of SBA restored BDNF expression. We conclude that BDNF mRNA expression depends on and responds with a fast kinetic to changes of the SBA. Steady-state levels do not depend on the absolute levels of activity, but more likely on the balance between excitation and inhibition, suggesting a role for BDNF in activity homeostasis.

Area-specific laminar distribution of cortical feedback neurons projecting to cat area 17: quantitative analysis in the adult and during ontogeny.

Corticocortical pathways can be classified as feedback and feedforward, in part according to the laminar distribution of the parent cell bodies. Here, we have developed exhaustive sampling procedures to determine unambiguously this laminar distribution. This shows that individual extrastriate areas in the adult cat have highly stereotyped proportions of supragranular layer neurons with respect to the total population of neurons back-projecting to area 17. During development, these adult laminar patterns emerge from an initially uniform radial distribution through a process of selective reorganization, which is highly specific to each area. Injections of fluorescent retrograde tracers were made in area 17. In areas 19, 20, posteromedial lateral suprasylvian area, and anteromedial lateral suprasylvian area, we defined a projection zone as the region containing retrogradely labeled neurons. In the neonate, counts of labeled neurons throughout the projection zones show constant percentages of 40% in the supragranular layers. During development, there is an area-specific reduction in the percentage of supragranular labeled neurons generating the laminar distributions characteristic of each area. Numbers of labeled neurons were estimated at different eccentricities of the projection zone. This finding indicates that during development there is a relative decrease in the numbers of labeled neurons of the periphery of the projection zone in the supragranular layers but not in the infragranular layers. This decrease is accompanied by a relative decrease in the dimensions of the supragranular projection zone with respect to the infragranular projection zone. These findings suggest that each extrastriate area precisely adjusts the proportions of supragranular layer neurons back-projecting to striate cortex in part by developmental changes in the divergence-convergence values of individual neurons. This shaping of corticocortical connectivity occurs relatively late in postnatal development and could, therefore, be under epigenetic control.

Visual activity is required to maintain the phenotype of supragranular NPY neurons in rat area 17.

Visual activity governs the functional maturation of the mammalian visual cortex. We report here, that visual experience is required for stabilizing the phenotype of a subset of cortical interneurons. Neurons expressing neuropeptide Y mRNA (NPY neurons) display a transiently higher expression in the early postnatal visual areas 18a and 17 that is followed by a phenotype restriction during the second postnatal month: about 50% of the NPY neurons in supragranular and infragranular layers of area 18a, and in infragranular layers of area 17 gradually stop the NPY expression. In contrast, the expression remains unchanged in supragranular layers of area 17. Dark rearing rats from birth to up to 100 days does neither prevent the developmental onset of NPY mRNA expression, nor does it prevent the phenotype restriction from occurring. In contrast, in dark reared animals NPY neurons in supragranular layers of area 17 now also undergo a phenotype restriction. Returning animals to light after variable periods of darkness results in an upregulation of NPY mRNA expression selectively in neurons in supragranular layers of area 17. These neurons acquire a constitutive expression during the second postnatal month. This suggests that the phenotypic specification of a distinct subset of cortical interneurons is regulated by visual experience which thus influences on the maturation of the neurochemical architecture of area 17.

Areal Differences of NPY mRNA‐expressing Neurons are Established in the Late Postnatal Rat Visual Cortex In Vivo, but not in Organotypic Cultures

In order to learn about the factors regulating the postnatal development of neocortical peptidergic neuron populations, we have analysed neurons expressing neuropeptide Y (NPY) by immunohistochemistry and in situ hybridization in developing and adult rat visual cortical areas 17 and 18a in vivo, and in organotypic slice cultures of rat visual cortex. For quantitative analysis, the percentage of NPY mRNA-expressing neurons was determined in supragranular layers I-IV, in infragranular layers V and VI and in the white matter. In vivo, this percentage increased in visual areas 17 and 18a until postnatal day 21 in supra- and infragranular layers. Initially, in both areas the neurons were about equally distributed in supra- and infragranular layers (a ratio of 1:1). During the second postnatal month, the percentage of NPY mRNA-expressing neurons in area 18a declined by approximately 50% in both supra- and infragranular layers, so that the ratio of 1:1 remained constant. In contrast, in area 17 the percentage of neurons in supragranular layers remained fairly constant, but it declined to 50% in infragranular layers, so that by postnatal day 70 the ratio was gradually shifted to 2:1. Throughout development, area 18a contained significantly more NPY mRNA-expressing neurons than area 17. In organotypic slice cultures, a high density of NPY mRNA-expressing neurons had appeared by 10 days in vitro. A much higher percentage of neurons expressed NPY mRNA. The ratio of labelled neurons in supra- versus infragranular layers was 1:1. Both ratio and percentage remained constant from 10-85 days in vitro. The decline in vivo was not caused by an elimination of transient cell types. All cell types persisted into adulthood. Four NPY peptide-immunoreactive neuronal types were classified by axonal morphology in organotypic slice cultures and in vivo; they include (i) cells in layer VI/white matter with horizontal axons and ascending collaterals, (ii) cells in layers V/VI with descending axon and horizontal collaterals, (iii) Martinotti cells in layers V/VI with ascending axons, and (iv) cells in layers III-V with columnar axons. Two further types, bipolar cells with axons descending from dendrites and small basket cells with short horizontal axons, both found in vivo in layers II/III, could not be unequivocally identified in organotypic slice cultures. The NPY-immunoreactive neuron types had already formed a dense innervation of the cultures by 10 days in vitro, which remained stable for up to 85 days in vitro, and resembled the innervation observed in vivo. NPY peptide-immunoreactive neurons in organotypic slice cultures and in vivo were distributed in cortical layers II/III, V and VI and the white matter, but rarely in layers I and IV, which corresponded to the distribution of NPY mRNA-expressing neurons. However, with in situ hybridization more neurons were detectable, especially in layers II/III. A majority of NPY mRNA-expressing neurons co-localized NPY peptide, somatostatin and calbindin. We conclude that intrinsic cues were sufficient to drive the molecular expression of the NPY phenotype, the morphological differentiation and the stabilization of an organotypic NPY innervation in organotypic slice cultures. However, the area- and lamina-specific changes observed in vivo were not observed under monoculture conditions.

Developmental remodeling of primate visual cortical pathways.

The pre- and postnatal developmental changes of the cortical afferents to area 17 were studied in the macaque monkey. Paired injections of the retrograde tracers fast blue and diamidino yellow were made in area 17. Quantitative techniques were used to examine the spatial patterns of labeling in three distinct locations of the extrastriate cortex that correspond to known visual areas. In the adult, each cortical region has a characteristic laminar distribution. In the fetus the proportion of supragranular layer neurons in all cortical regions was much higher than in the adult. The present study shows that despite the very high levels of labeled supragranular layer neurons, there is some early areal specialization so that the adult configuration does not emerge from a uniform distribution. The developmental decline in the proportion of labeled supragranular neurons is complete by 1 month after birth. Each injection of tracer gave rise in each cortical area to dense labeling in a restricted region (projection zone). Areal measurements of projection zones in the supra- and infragranular layers showed that the developmental decrease in the proportion of labeled supragranular layer neurons is accompanied by a relative change of the dimensions of supra- and infragranular projection zones: the supragranular projection zone in the fetus is larger than the infragranular projection zone and vice versa in the adult. In the fetus, the two projection zones corresponding to each of the two tracers overlap in the supragranular layers whereas they are largely separated in the infragranular layers. During development there is a progressive decrease in the overlap of the supragranular projection zones and an increase in the overlap in the infragranular layers. Again, the adult configuration is achieved 1 month after birth. This developmental inversion of the areal dimensions of the projection zones in supra- and infragranular layers is accompanied by a drastic decrease in the proportion of double-labeled neurons located in supragranular layers. These results clearly show that early in development, axonal projections to area V1 are modified in very different ways according to whether they originate from supra- or infragranular layers. This developmental process lasts for about 80 d. These findings show that in the primate there is a prolonged remodeling of axonal projections that is a highly characteristic feature of this species.

Segregation of callosal and association pathways during development in the visual cortex of the primate

The segregation of callosal and association pathways in the developing visual cortex of the monkey was studied using the retrograde tracers fast blue and diamidino yellow. Quantitative analysis of the laminar distribution of labeled callosal and association neurons made it possible to reveal the shifting pattern of connections that characterizes the development of these two pathways. In the adult, callosal neurons are restricted to supragranular layers, where they are concentrated at the bottom of layer 3. Association neurons are located both in infra- and supragranular layers. Supragranular layer association neurons are concentrated in layer 2, with limited spread into layer 3 so that there is little overlap with callosal neurons. In the immature brain, callosal neurons are characterized by a tangential distribution that is more widespread than in the adult, while their laminar distribution undergoes little developmental change. Association neurons show two types of changes in their laminar distribution: (1) in the early fetus, there is a large excess of association neurons in supragranular layers, the adult distribution being achieved some time after birth; and (2) during maturation there is a selective elimination of at least 50% of the projections originating from the lower part of layers 2/3. Hence, the adult radial segregation of association and callosal pathways is achieved in part by regressive phenomena. The developmental reduction of bihemispheric projections is largely independent of changes in the organization of association neurons. Quantitative analysis of the morphology and spatial location of neurons sending axon collaterals to both hemispheres suggests that they constitute a subset of callosal neurons and that their frequency is determined by factors that regulate directly this population. These results are discussed with respect to the specification of visual cortical pathways during ontogenesis.

Organization of reciprocal connections between area 17 and the lateral suprasylvian area of cat visual cortex

The lateral suprasylvian (LS) area (or Clare-Bishop area) is a region of visual cortex in the cat which has been defined as an isolated projection zone of area 17 (V1 or striate cortex) within the suprasylvian sulcus. We have studied the overall topography and detailed pattern of connection between these two visual areas following injections of WGA-HRP into one or the other. The projection from area 17 to LS is formed largely (approximately 90%) from supragranular layer neurons that are distributed, in the coronal plane, in multiple regularly spaced patches. These patches are especially prominent in regions of area 17 representing central vision along and around the horizontal meridian. In reconstructions of serial coronal sections, and in flatmounts of the same region, the patches are seen to align so that in the plane tangential to the cortical surface they appear as a system of parallel bands whose main axis of elongation is rostro-ventral to caudo-dorsal, or near parallel to the area 17/18 border. The mean periodicity of the bands is about 1.0 mm. The projection from area 17 terminates mainly in layers 4, 3, and 2 of area LS, and also appears patchy in the coronal plane. Reconstruction of the cortical surface view again reveals a system of rostrocaudal bands, but with a mean periodicity of 2 mm. The back projection is less periodically organized, arising predominantly (approximately 80%) from a continuous sheet of infragranular neurons in area LS and terminating mainly in layer 1 of area 17, across the underlying patch and interpatch zones of the supragranular projection cells. However, neurons in layers 2 and upper 3 of area LS, which form the minority origin of the back projection, are mostly located in columnar registration with the patches of area 17 terminals. The bands of supragranular layer neurons projecting to area LS are aligned obliquely to the iso-orientation domains of area 17, indicating a further component to its organization. It is suggested that this may correspond to a segregation of the X and Y channels in area 17, with outputs to area LS selectively arising from the Y pathway, in accordance with previous reports.