Development of axonal arbors of layer 6 pyramidal neurons in ferret primary visual cortex

The nonclassical MHC class I Qa-1 expressed in layer 6 neurons regulates activity-dependent plasticity via microglial CD94/NKG2 in the cortex

During developmental critical periods, circuits are sculpted by a process of activity-dependent competition. The molecular machinery involved in regulating the complex process of responding to different levels of activity is now beginning to be identified. Here, we show that the nonclassical major histocompatibility class I (MHCI) molecule Qa-1 is expressed in the healthy brain in layer 6 corticothalamic neurons. In the visual cortex, Qa-1 expression begins during the critical period for ocular dominance (OD) plasticity and is regulated by neuronal activity, suggesting a role in regulating activity-dependent competition. Indeed, in mice lacking Qa-1, OD plasticity is perturbed. Moreover, signaling through CD94/NKG2, a known cognate Qa-1 heterodimeric receptor in the immune system, is implicated: selectively targeting this interaction phenocopies the plasticity perturbation observed in Qa-1 knockouts. In the cortex, CD94/NKG2 is expressed by microglial cells, which undergo activity-dependent changes in their morphology in a Qa-1–dependent manner. Our study thus reveals a neuron–microglial interaction dependent upon a nonclassical MHCI molecule expressed in L6 neurons, which regulates plasticity in the visual cortex. These results also point to an unexpected function for the Qa-1/HLA-E (ligand) and CD94/NKG2 (receptor) interaction in the nervous system, in addition to that described in the immune system.

The regulation of cortical neurogenesis

The mammalian cerebral cortex is the pinnacle of brain evolution, reaching its maximum complexity in terms of neuron number, diversity and functional circuitry. The emergence of this outstanding complexity begins during embryonic development, when a limited number of neural stem and progenitor cells manage to generate myriads of neurons in the appropriate numbers, types and proportions, in a process called neurogenesis. Here we review the current knowledge on the regulation of cortical neurogenesis, beginning with a description of the types of progenitor cells and their lineage relationships. This is followed by a review of the determinants of neuron fate, the molecular and genetic regulatory mechanisms, and considerations on the evolution of cortical neurogenesis in vertebrates leading to humans. We finish with an overview on how dysregulation of neurogenesis is a leading cause of human brain malformations and functional disabilities.

Ontogeny of white matter, toll-like receptor expression, and motor skills in the neonatal ferret

Inflammation caused by perinatal infection, superimposed with hypoxia and/or hyperoxia, appears to be important in the pathogenesis of preterm neonatal encephalopathy, with white matter particularly vulnerable during the third trimester. The associated inflammatory response is at least partly mediated through Toll-like receptor (TLR)-dependent mechanisms. Immunohistochemistry, gene expression, and behavioral studies were used to characterize white matter development and determine TLR3 and TLR4 expression and accumulation in the neonatal ferret brain. Expression of markers of white matter development increased significantly between postnatal day (P)1 and P10 (NG2, PDGFRα) or P15 (Olig2), and either remained elevated (NG2), or decreased again at P40 (PDGFRα, Olig2). Olig2 immunostaining within the internal capsule was also greatest at P15. Myelin basic protein (MBP) immunostaining and mRNA expression increased markedly from P15 to P40 and into adulthood, which correlated with increasing performance on behavioral tests (negative geotaxis, cliff aversion, righting reflex, and catwalk gait analysis). TLR4 and TLR3 positive staining was low at all ages, but TLR3 and TLR4 mRNA expression both increased significantly from P1 to P40. Following lipopolysaccharide (LPS) and hypoxia/hyperoxia exposure at P10, meningeal and parenchymal inflammation was seen, including an increase in TLR4 positive cells. These data suggest that the neuroinflammation associated with prematurity could be modeled in the newborn ferret.

Molecular analysis of neocortical layer structure in the ferret

Molecular markers that distinguish specific layers of rodent neocortex are increasingly employed to study cortical development and the physiology of cortical circuits. The extent to which these markers represent general features of neocortical cell type identity across mammals, however, is unknown. To assess the conservation of layer markers more broadly, we isolated orthologs for 15 layer-enriched genes in the ferret, a carnivore with a large, gyrencephalic brain, and analyzed their patterns of neocortical gene expression. Our major findings are: 1) Many but not all layer markers tested show similar patterns of layer-specific gene expression between mouse and ferret cortex, supporting the view that layer-specific cell type identity is conserved at a molecular level across mammalian superorders; 2) Our panel of deep layer markers (ER81/ETV1, SULF2, PCP4, FEZF2/ZNF312, CACNA1H, KCNN2/SK2, SYT6, FOXP2, CTGF) provides molecular evidence that the specific stratifications of layers 5 and 6 into 5a, 5b, 6a, and 6b are also conserved between rodents and carnivores; 3) Variations in layer-specific gene expression are more pronounced across areas of ferret cortex than between homologous areas of mouse and ferret cortex; 4) This variation of area gene expression was clearest with the superficial layer markers studied (SERPINE2, MDGA1, CUX1, UNC5D, RORB/NR1F2, EAG2/KCNH5). Most dramatically, the layer 4 markers RORB and EAG2 disclosed a molecular sublamination to ferret visual cortex and demonstrated a molecular dissociation among the so-called agranular areas of the neocortex. Our findings establish molecular markers as a powerful complement to cytoarchitecture for neocortical layer and cell-type comparisons across mammals.

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Organizing principles of cortical layer 6

Neurons in the deepest layer of mammalian cerebral cortex are morphologically and physiological diverse and are situated in a strategic position to modulate neuronal activity locally and in other structures. The variety of neuronal circuits within which layer 6 neurons participate differs across species and cortical regions. However even amidst this diversity, common organizational features emerge. Examination of the anatomical and physiological characteristics of different classes of layer 6 neuron, each specialized to participate in distinct circuits, provides insight into the functional contributions of layer 6 neurons toward cortical information processing.

Functional excitatory microcircuits in neonatal cortex connect thalamus and layer 4

The functional connectivity of the cerebral cortex is shaped by experience during development, especially during a critical period early in life. In the prenatal and neonatal cortex, transient neuronal circuits are formed by a population of subplate neurons (SPNs). However, SPNs are absent in the adult cortex. While SPNs are crucial for normal development of the cerebral cortex and of thalamocortical synapses, little is known about how they are integrated in the developing thalamocortical circuit. We therefore investigated SPNs in vitro in thalamocortical slices of A1 and medial geniculate nucleus (MGN) in mouse from postnatal day 1 (P1) to P13. We found that SPNs can fire action potentials at P1 and that their intrinsic membrane properties are mature after P5. We find that SPNs receive functional excitatory inputs from the MGN as early as P2. The MGN projections to SPNs strengthen between P2 and P13 and are capable of inducing action potentials in SPNs. Selective activation of SPNs by photostimulation produced EPSCs in layer 4 neurons, demonstrating a functional excitatory connection. Thus, SPNs are tightly integrated into the developing thalamocortical circuit and would be a reliable relay of early spontaneous and sound-evoked activity. The role of SPNs in development likely results from their strong excitatory projection to layer 4, which might function to regulate activity-dependent processes that enable mechanisms required for the functional maturation and plasticity of the developing cortex and thereby contribute to the development of normal cortical organization.

The development of cortical columns: role of Fragile X mental retardation protein

Neuronal circuits in the brain are complex and precise. Here, I review aspects of the development of cortical columns in the rodent barrel cortex, focusing on the anatomical and functional data describing the maturation of ascending glutamatergic circuits. Projections from layer 4 to layer 3 develop into cortical columns with little macroscopic refinement. Depriving animals of normal sensory experience induces long-term synaptic depression but does not perturb this pattern of development. Mouse models of mental retardation can help us understand the mechanisms of development of cortical columns. Fmr1 knock-out (ko) mice, a model for Fragile X syndrome, lack Fragile X mental retardation protein (FMRP), a suppressor of translation present in synapses. Because FMRP expression is stimulated by neuronal activity, Fmr1-ko mice provide a model to survey the role of sensory input in brain development. Layer 4 to layer 3 projections are altered in multiple ways in the young mutant mice: connection rate is low and layer 4 cell axons are spatially diffuse. Sensory deprivation rescues the connection rate phenotype. The interaction of FMRP and neuronal activity in the development of cortical circuits is discussed.

Temporal properties of feedforward and feedback pathways between the thalamus and visual cortex in the ferret

This study examines the temporal properties of geniculocortical and corticogeniculate (CG) pathways that link the lateral geniculate nucleus (LGN) and primary visual cortex in the ferret. Using electrical stimulation in the LGN to evoke action potentials in geniculocortical and CG axons, results show that conduction latencies are significantly faster in geniculocortical neurons than in CG neurons. Within each pathway, axonal latency and visual physiology support the view of sub-classes of neurons. By examining the timing of visual responses and the latency of CG feedback, estimates indicate that visual information can reach the cortex and return to the LGN as early as 60 msec following the onset of a visual stimulus. These findings place constraints on the functional role of corticogeniculate feedback for visual processing.

Development of layer-specific axonal arborizations in mouse primary somatosensory cortex

In the developing neocortex, pyramidal neurons use molecular cues to form axonal arbors selectively in the correct layers. Despite the utility of mice for molecular and genetic studies, little work has been done on the development of layer-specific axonal arborizations of pyramidal neurons in mice. We intracellularly labeled and reconstructed the axons of layer 2/3 and layer 5 pyramidal neurons in slices of primary somatosensory cortex from C57Bl6 mice on postnatal days 7-21. For all neurons studied, the development of the axonal arborizations in mice follows a pattern similar to that seen in other species; laminar specificity of the earliest axonal branches is similar to that of mature animals. At P7, pyramidal neurons are very simple, having only a main descending axon and few primary branches. Between P7 and P10, there is a large increase in the total number of axonal branches, and axons continue to increase in complexity and total length from P10 to P21. Unlike observations in ferrets, cats, and monkeys, two types of layer 2/3 pyramidal neurons are present in both mature and developing mice; cells in superficial layer 2/3 lack axonal arbors in layer 4, and cells close to the layer 4 border have substantial axonal arbors within layer 4. We also describe axonal and dendritic arborization patterns of three pyramidal cell types in layer 5. The axons of tall-tufted layer 5 pyramidal neurons arborize almost exclusively within deep layers while tall-simple, and short layer 5 pyramidal neurons also project axons to superficial layers.

Precise Development of Functional and Anatomical Columns in the Neocortex

Sensory cortex is ordered into columns, each tuned to a subset of peripheral stimuli. To identify the principles underlying the construction of columnar architecture, we monitored the development of circuits in the rat barrel cortex, using laser-scanning photostimulation analysis of synaptic connectivity, reconstructions of axonal arbors, and in vivo whole-cell recording. Circuits impinging onto layer 2/3 neurons from layers 4 and 2/3 developed in a monotonic, precise progression, with little evidence for transient hyperinnervation at the level of cortical columns. Consistent with this, synaptic currents measured in layer 2/3 neurons at PND 8, just after these neurons ceased to migrate, revealed already spatially well-tuned receptive fields.

Decline of the critical period of visual plasticity is concurrent with the reduction of NR2B subunit of the synaptic NMDA receptor in layer 4

The specific composition of NMDA receptor subunits is thought to underlie the developmental plasticity of the cortex revealed by unbalanced binocular stimulation. However, evidence that NR2 subunits change in correlation with the critical period at locations that are relevant to visual plasticity has been missing. Using preembedding and postembedding immunostaining, as well as electron microscopy, we quantified the volumetric densities of NR1-, NR2A-, and NR2B-containing synapses in layers 4 and 2/3 of the ferret visual cortex at different postnatal ages. Before eye opening, NR2A is encountered infrequently at postsynaptic sites in layer 4, but it increases sharply by postnatal day 34. In the subsequent weeks, postsynaptic NR2A labeling increases gradually in both layers 4 and 2/3 to become the most prevalent subunit in the adult animal. The NR2B subunit is the more prevalent subunit at the onset of the critical period of cortical plasticity. However, it displays different developmental patterns in layers 4 and 2/3. Although no change occurs in synaptic NR2B density in layer 2/3, in layer 4, NR2B maintains its high levels through the peak of the critical period and then becomes significantly reduced by the end of the peak of the critical period. This low level is maintained throughout adulthood. Our results demonstrate a correlation between the loss of NR2B subunits from layer 4 synaptic sites and the decline of the critical period, suggesting that the presence of NR2B subunits at synaptic sites could be a permissive factor regulating the ocular dominance plasticity of the developing cortex.

Short-term dynamics of thalamocortical and intracortical synapses onto layer 6 neurons in neocortex

Layer 6 is the main source of neocortical connections back to specific thalamic nuclei. Corticothalamic (CT) systems play an important role in shaping sensory input, but little is known about the functional circuitry that generates CT activity. We recorded from the two main types of neurons in layer 6, regular-spiking (RS; pyramidal neurons) and fast-spiking (FS; inhibitory interneurons) cells and compared the physiological properties of different excitatory inputs. Thalamic stimulation evoked two monosynaptic inputs with distinct properties: suspected thalamocortical (TC) synaptic events had short latencies, short-term synaptic depression, and paired-pulse responses that suggested subnormal axonal conduction. A second group of synaptic responses likely originated from intracortical collaterals of CT cells that were antidromically activated from the thalamus. These intracortical responses had longer latencies, short-term synaptic facilitation, and were transmitted by axons with supernormal conduction. Suspected TC inputs to FS cells had significantly larger amplitudes than those onto RS cells. Dual recordings from neighboring neurons in layer 6 revealed both facilitating and depressing synaptic connections; the depressing synapses were probably formed by layer 6 cells that do not project to the thalamus, and thus were not sampled by thalamic stimulation. We conclude that layer 6 neurons integrate a variety of inputs with distinct temporal dynamics that are determined by the presynaptic cell type.

Functional synaptic projections onto subplate neurons in neonatal rat somatosensory cortex

Subplate neurons (SPn) play an important role in the formation of thalamocortical connections during early development and show glutamatergic and GABAergic spontaneous synaptic activity. We characterized these synaptic inputs by performing whole-cell recordings from SPn in somatosensory cortical slices of postnatal day 0-3 rats. At -70 mV, electrical stimulation of the thalamocortical afferents elicited in 68% of the SPn a monosynaptic CNQX-sensitive postsynaptic current (PSC). These fast PSCs were mediated by AMPA receptors, because they were prolonged by cyclothiazide and blocked by GYKI 52466. On membrane depolarization, thalamocortical stimulation elicited in 50% of the cells an additional slow monosynaptic component mediated by NMDA receptors. Stimulation of the cortical plate evoked in 72% of SPn a monosynaptic AMPA receptor-mediated PSC with an additional NMDA component at depolarized membrane potentials and in 40% of the investigated cells polysynaptic responses, depending on GABA(A) and NMDA receptors. Stimulation of the subplate elicited in 67% of SPn a monosynaptic dual-component PSC mediated by AMPA and NMDA receptors activated at -70 mV and in 12% of SPn a monosynaptic single-component PSC mediated by AMPA receptors with an additional NMDA component activated at depolarized membrane potentials. A monosynaptic GABAergic response could be observed in 68% of SPn after stimulation of the subplate. In gramicidin-perforated patch recordings, bath application of GABA caused membrane depolarization to -40 mV and elicited action potentials. These results demonstrate that SPn receive distinct functional synaptic inputs arising from the thalamus, cortical plate, and subplate, indicating that SPn are capable of integrating and processing information from cortical and subcortical regions.

Reorganization of exuberant axonal arbors contributes to the development of laminar specificity in ferret visual cortex

Layer-specific cortical axons are believed to develop precisely from the outset without making exuberant branches in incorrect cortical layers. We tested this hypothesis by following the development of axonal arbors of layer 2/3 pyramidal neurons in ferret visual cortex, which in the adult have dense axonal arbors in layers 2/3 and 5 and not in layer 4. We began our studies at earlier ages and sampled at closer intervals than in previous studies. Our studies reveal that between postnatal day (P) 14 and P18 the initial growth of axonal arbors lacks specificity for layers 2/3 and 5 and involves the formation of a small number of branches incorrectly in layer 4. After P18 the number of incorrect axonal branches in layer 4 decreased, whereas there was extensive axonal growth specifically in layers 2/3 and 5. To further study the mechanisms involved in directing the layer-specific growth and elimination of axonal branches, we studied the development of layer 2/3 neurons in slices kept in culture for 5 d. In these studies the initial imprecise growth at P14-18 followed by precise growth after P18 was mimicked in vitro, but the elimination of incorrect axons from layer 4 did not occur. These studies reveal that most axonal arbors grow precisely in the correct layers, but the initial growth involves the formation of a small number of incorrect axonal branches that are later eliminated. Therefore the mechanisms that regulate axonal growth are not as precise initially as later, and mechanisms must exist to allow selective elimination of axon collaterals from incorrect layers.

Inhibitory mechanism by polysialic acid for lamina-specific branch formation of thalamocortical axons

During development, thalamocortical axons form arbors primarily in layer 4 of the neocortex. This lamina-specific branch formation was studied in cultures of rat thalamic explants grown next to chemically fixed cortical slices. After a week in vitro, thalamic axons formed branches specifically in the target layer of fixed cortical slices, regardless of the orientation of the ingrowth. This in vitro system permits a direct assessment of contributions of membrane-associated molecules to thalamic axon branch formation. To this end, the present study uses three enzymatic perturbations: chondroitinase, phosphatidylinositol phospholipase C, or the polysialic acid (PSA)-specific endoneuraminidase (endo N). With endo N pretreatment of cortex, the number of branch points was increased significantly, whereas branch tip length was decreased. In addition, the localization of branch points to the target layer was weakened considerably. These features of branch formation were not altered by the other two enzymatic treatments, except that branch tips were shortened by chondroitinase treatment to the same extent as in endo N treatment. These results suggest that membrane-bound components are involved in lamina-specific branch formation of thalamocortical axons, and in particular that PSA moieties contribute to laminar specificity by inhibiting branch emergence in inappropriate layers.

Development of visual cortical axons: layer-specific effects of extrinsic influences and activity blockade

During normal cortical development, individual pyramidal neurons form intracortical axonal arbors that are specific for particular cortical layers. Pyramidal neurons within layer 6 are able to develop layer-specific projections in cultured slices of ferret visual cortex, indicating that extrinsic influences, including patterned visual activity, are not required (Dantzker and Callaway [1998] J Neurosci 18:4145-4154). However, when spontaneous activity is blocked in cultures with tetrodotoxin, layer 6 pyramidal neurons fail to preferentially target their axons to layer 4. To determine whether mechanisms that regulate the development of layer 6 pyramidal neuron arbors can be generalized to pyramidal neurons in other layers, we examined the development of layer 5 and layer 2/3 pyramidal neurons in cultured slices of ferret visual cortex prepared on postnatal day 14 or 15. Layer 5 pyramidal neurons developed layer-specific axonal arbors during 5-7 days in vitro. However, unlike layer 6 pyramidal neurons, layer 5 pyramidal neurons formed layer-specific axonal arbors in the presence of tetrodotoxin. In contrast to layer 5 and layer 6 pyramidal neurons, layer 2/3 pyramidal neurons did not form appropriate layer-specific projections during 5-7 days in vitro. Taken together, these data suggest that the development of layer-specific axons is regulated by different mechanisms for neurons in different layers and cannot be categorically classified as either activity-dependent or independent. Instead, the type of pyramidal neuron, the layers targeted, and the type of activity must be considered.

Altered development of visual subcortical projections following neonatal thalamic ablation in the hamster

Previous research has demonstrated that precise patterns of axonal connectivity often develop during a series of stages characterized by pathfinding, target recognition, and address selection. This last stage involves the focusing of projections to a precisely defined region within the target. Because thalamic projections begin to innervate cortex before the latter stages are reached, these projections may be important in the establishment of adult-like patterns of cortical connectivity. To address this issue, we examined the mature corticopontine and corticospinal projections of visual cortex deprived of early thalamic input by visual thalamic ablation. Although ablations on the day of birth in hamsters did not disrupt the targeting of appropriate subcortical structures by visual cortical axons, they did alter the organization of projections within the basilar pons and spinal cord. The density and spread of visual corticopontine connections in lesioned animals was greatly increased relative to unlesioned animals, suggesting that thalamic afferents are required during address selection, when the topographic specificity of projections is established. To determine whether early visual thalamic ablation increases connectivity by stabilizing an exuberant developmental projection, we examined the normal development of visual corticopontine connections in hamsters ages postnatal days 1-17 (P1-P17). From the earliest ages, visual cortical axons innervate the pontine nucleus in regions specific to their adult projection zones and show progressive growth within these zones. At no time during development do projections exist that are equivalent to the projections found after thalamic ablation, suggesting that removal of thalamic input does not simply stabilize a developmental projection.

The critical period for ocular dominance plasticity in the Ferret's visual cortex

Microelectrode recordings and optical imaging of intrinsic signals were used to define the critical period for susceptibility to monocular deprivation (MD) in the primary visual cortex of the ferret. Ferrets were monocularly deprived for 2, 7 or >14 d, beginning between postnatal day 19 (P19) and P110. The responses of visual cortical neurons to stimulation of the two eyes were used to gauge the onset, peak, and decline of the critical period. MDs ending before P32 produced little or no loss of response to the deprived eye. MDs of 7 d or more beginning around P42 produced the greatest effects. A rapid decline in cortical susceptibility to MD was observed after the seventh week of life, such that MDs beginning between P50 and P65 were approximately half as effective as those beginning on P42; MDs beginning after P100 did not reduce the response to the deprived eye below that to the nondeprived eye. At all ages, 2 d deprivations were 55-85% as effective as 7 d of MD. Maps of intrinsic optical responses from the deprived eye were weaker and less well tuned for orientation than those from the nondeprived eye, with the weakest maps seen in the hemisphere ipsilateral to the deprived eye. Analysis of the effects of 7 d and longer deprivations revealed a second period of plasticity in cortical responses in which MD induced an effect like that of strabismus. After P70, MD caused a marked loss of binocular responses with little or no overall loss of response to the deprived eye. The critical period measured here is compared to other features of development in ferret and cat.

Opposing roles for neurotrophin-3 in targeting and collateral formation of distinct sets of developing cortical neurons

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.

Layer VI in cat primary auditory cortex: Golgi study and sublaminar origins of projection neurons

The organization of layer VI in cat primary auditory cortex (AI) was studied in mature specimens. Golgi-impregnated neurons were classified on the basis of their dendritic and somatic form. Ipsilateral and contralateral projection neurons and the corticogeniculate cells of origin were labeled with retrograde tracers and their profiles were compared with the results from Golgi studies. Layer VI was divided into a superficial half (layer VIa) with many pyramidal neurons and a deeper part (layer VIb) that is dominated by horizontal cells. Nine types of neuron were identified; four classes had subvarieties. Classical pyramidal cells and star, fusiform, tangential, and inverted pyramidal cells occur. Nonpyramidal neurons were Martinotti, multipolar stellate, bipolar, and horizontal cells. This variety of neurons distinguished layer VI from other AI layers. Pyramidal neuron dendrites contributed to the vertical, modular organization in AI, although their apical processes did not project beyond layer IV. Their axons had vertical, intrinsic processes as well as corticofugal branches. Horizontal cell dendrites extended laterally up to 700 microm and could integrate thalamic input across wide expanses of the tonotopic domain. Connectional experiments confirmed the sublaminar arrangement seen in Nissl material. Commissural cells were concentrated in layer VIa, whereas corticocortical neurons were more numerous in layer VIb. Corticothalamic cells were distributed more equally. The cytological complexity and diverse connections of layer VI may relate to a possible role in cortical development. Layer VI contained most of the neuronal types found in other layers in AI, and these cells form many of the same intrinsic and corticofugal connections that neurons in other layers will assume in adulthood. Layer VI, thus, may play a fundamental ontogenetic role in the construction and early function of the cortex.

Dual action of a ligand for Eph receptor tyrosine kinases on specific populations of axons during the development of cortical circuits

The structural basis of cortical columns are radially oriented axon collaterals that form precise connections between distinct cortical layers. During development, these connections are highly specified from the initial outgrowth of collateral branches. Our previous work provided evidence for positional cues confined to individual layers that induce and/or prevent the formation of axon collaterals in specific populations of cortical neurons. Here we demonstrated with in situ hybridization techniques that mRNA of the Eph receptor tyrosine kinase EphA5 and one of its ligands, ephrin-A5, are present in distinct cortical layers, at a time when intrinsic connections are being formed in the cortex. Axonal guidance assays indicate that ephrin-A5 is a repellent signal for a populations of axons that in vivo avoid the cortical layer expressing ephrin-A5. In contrast to its established role as a repulsive axonal guidance signal, ephrin-A5 specifically mediates sprouting of those cortical axons that target the ephrin-A5-expressing layer in vivo. These results identify a novel function of ephrin-A5 on axonal arbor formation. The laminar distribution and the dual action on specific populations of axons suggest that ephrin-A5 plays a role in the assembly of local cortical circuits.

The development of local, layer-specific visual cortical axons in the absence of extrinsic influences and intrinsic activity

The laminar specificity of vertical connections in the primary visual cortex (area 17) develops precisely from the outset, leading to the hypothesis that layer-specific axonal targeting is attributable to molecular cues intrinsic to the cortex (Lund et al., 1977; Katz and Callaway, 1992). However, alternative factors that could influence axonal development have not been investigated. This study examines the roles of intrinsic cortical activity and extrinsic influences that could arise from earlier-formed connections with outside cortical and subcortical areas. Organotypic slice cultures were prepared from ferret area 17 before the formation of local axonal connections and were incubated for 5-7 d to allow initial, local axonal arbors to form in the absence of extrinsic influences. Additionally, some slices were cultured in the presence of the Na+ channel blocker tetrodotoxin to block spontaneous action potentials within the slice. Individual neurons were labeled intracellularly with biocytin, and their patterns of local axonal arborizations were reconstructed. This study focuses on the development of layer 6 pyramidal neurons, the axons of which in vivo bypass an incorrect target, layer 5, before specifically arborizing in their local target, layer 4. We found that axonal arbors developing in vitro preferentially arborized in layer 4 versus layer 5. However, inhibition of spontaneous activity within the cortical slice decreased this specificity, resulting in similar numbers of axonal branches in layers 4 and 5. Thus, although cortical axons do not require influences from outside areas, intrinsic spontaneous activity is required for specific axonal arborization in correct laminar targets.

Prenatal development of layer-specific local circuits in primary visual cortex of the macaque monkey

Previous studies have demonstrated that axonal arbors specific for the four main cortical layers - 2/3, 4, 5, and 6 - develop precisely from the outset using activity-independent cues. In macaque primary visual cortex (V1), layer 2/3 is subdivided into layers named 2/3A, 3B, 4A, and 4B, and layer 4 is subdivided into 4Calpha and 4Cbeta. Individual neurons in V1 of mature macaques have axonal arbors that are highly specific for these sublayers. We have studied the prenatal development of laminar and sublaminar specificity of local circuits in macaque V1. Two-hundred thirty-eight neurons were labeled intracellularly in living brain slices prepared from V1 of five prenatal macaque monkeys aged 100 to 145 d postconception (E100-E145). Axonal and dendritic arbors of labeled neurons were reconstructed to assess their relationships to the cortical layers. We find that developing spiny neurons in layers 2-4B and layer 5 specifically target superficial and deep layers without forming "incorrect" branches in layer 4C. Similarly, layer 6 pyramidal neurons that target layer 4C do not form "incorrect" branches in layer 5. These results indicate that specific projections to the main cortical layers develop with a high degree of selectivity, as in other species. However, the development of sublayer-specific projections was not always precise from the outset. Unlike postnatal animals, axons of some prenatal layer 4Cbeta spiny neurons branch in layer 4B. At similar ages, many pyramidal neurons in the upper half of layer 6 have axonal branches in layer 4Calpha as well as 4Cbeta; these projections are specific for 4Cbeta in more mature animals. Also, there is similar "exuberance" in axonal arbors of other layer 6 cell types. Transient projections were also observed in the subplate and to the white matter for cells from all layers, except 4Cbeta. These observations indicate that at least some sublayer-specific projections emerge by elimination of exuberant axonal branches and suggest that they may use activity-dependent mechanisms to identify "correct" target layers. Such cues could be provided by laminar differences in the patterns of spontaneous prenatal activity in the retino-geniculo-cortical network.