Spine formation and maturation of type 1 synapses on spiny stellate neurons in primate visual cortex

Developmental Changes in Pyramidal Cell Morphology in Multiple Visual Cortical Areas Using Cluster Analysis

Neuronal morphology is characterized by salient features such as complex axonal and dendritic arbors. In the mammalian brain, variations in dendritic morphology among cell classes, brain regions, and animal species are thought to underlie known differences in neuronal function. In this work, we obtained a large dataset from http://neuromorpho.org/ comprising layer III pyramidal cells in different cortical areas of the ventral visual pathway (V1, V2, V4, TEO, and TE) of the macaque monkey at different developmental stages. We performed an in depth quantitative analysis of pyramidal cell morphology throughout development in an effort to determine which aspects mature early in development and which features require a protracted period of maturation. We were also interested in establishing if developmental changes in morphological features occur simultaneously or hierarchically in multiple visual cortical areas. We addressed these questions by performing principal component analysis (PCA) and hierarchical clustering analysis on relevant morphological features. Our analysis indicates that the maturation of pyramidal cell morphology is largely based on early development of topological features in most visual cortical areas. Moreover, the maturation of pyramidal cell morphology in V1, V2, V4, TEO, and TE is characterized by unique developmental trajectories.

Preferential Targeting of Lateral Entorhinal Inputs onto Newly Integrated Granule Cells

Mature dentate granule cells in the hippocampus receive input from the entorhinal cortex via the perforant path in precisely arranged lamina, with medial entorhinal axons innervating the middle molecular layer and lateral entorhinal cortex axons innervating the outer molecular layer. Although vastly outnumbered by mature granule cells, adult-generated newborn granule cells play a unique role in hippocampal function, which has largely been attributed to their enhanced excitability and plasticity (Schmidt-Hieber et al., 2004; Ge et al., 2007). Inputs from the medial and lateral entorhinal cortex carry different informational content. Thus, the distribution of inputs onto newly integrated granule cells will affect their function in the circuit. Using retroviral labeling in combination with selective optogenetic activation of medial or lateral entorhinal inputs, we examined the functional innervation and synaptic maturation of newly generated dentate granule cells in the mouse hippocampus. Our results indicate that lateral entorhinal inputs provide the majority of functional innervation of newly integrated granule cells at 21 d postmitosis. Despite preferential functional targeting, the dendritic spine density of immature granule cells was similar in the outer and middle molecular layers, which we speculate could reflect an unequal distribution of shaft synapses. However, chronic blockade of neurotransmitter release of medial entorhinal axons with tetanus toxin disrupted normal synapse development of both medial and lateral entorhinal inputs. Our results support a role for preferential lateral perforant path input onto newly generated neurons in mediating pattern separation, but also indicate that medial perforant path input is necessary for normal synaptic development.SIGNIFICANCE STATEMENT The formation of episodic memories involves the integration of contextual and spatial information. Newly integrated neurons in the dentate gyrus of the hippocampus play a critical role in this process, despite constituting only a minor fraction of the total number of granule cells. Here we demonstrate that these neurons preferentially receive information thought to convey the context of an experience. Each newly integrated granule cell plays this unique role for ∼1 month before reaching maturity.

Pyramidal cell development: postnatal spinogenesis, dendritic growth, axon growth, and electrophysiology

Here we review recent findings related to postnatal spinogenesis, dendritic and axon growth, pruning and electrophysiology of neocortical pyramidal cells in the developing primate brain. Pyramidal cells in sensory, association and executive cortex grow dendrites, spines and axons at different rates, and vary in the degree of pruning. Of particular note is the fact that pyramidal cells in primary visual area (V1) prune more spines than they grow during postnatal development, whereas those in inferotemporal (TEO and TE) and granular prefrontal cortex (gPFC; Brodmann's area 12) grow more than they prune. Moreover, pyramidal cells in TEO, TE and the gPFC continue to grow larger dendritic territories from birth into adulthood, replete with spines, whereas those in V1 become smaller during this time. The developmental profile of intrinsic axons also varies between cortical areas: those in V1, for example, undergo an early proliferation followed by pruning and local consolidation into adulthood, whereas those in area TE tend to establish their territory and consolidate it into adulthood with little pruning. We correlate the anatomical findings with the electrophysiological properties of cells in the different cortical areas, including membrane time constant, depolarizing sag, duration of individual action potentials, and spike-frequency adaptation. All of the electrophysiological variables ramped up before 7 months of age in V1, but continued to ramp up over a protracted period of time in area TE. These data suggest that the anatomical and electrophysiological profiles of pyramidal cells vary among cortical areas at birth, and continue to diverge into adulthood. Moreover, the data reveal that the “use it or lose it” notion of synaptic reinforcement may speak to only part of the story, “use it but you still might lose it” may be just as prevalent in the cerebral cortex.

Dendritic spines and development: towards a unifying model of spinogenesis--a present day review of Cajal's histological slides and drawings

Dendritic spines receive the majority of excitatory connections in the central nervous system, and, thus, they are key structures in the regulation of neural activity. Hence, the cellular and molecular mechanisms underlying their generation and plasticity, both during development and in adulthood, are a matter of fundamental and practical interest. Indeed, a better understanding of these mechanisms should provide clues to the development of novel clinical therapies. Here, we present original results obtained from high-quality images of Cajal's histological preparations, stored at the Cajal Museum (Instituto Cajal, CSIC), obtained using extended focus imaging, three-dimensional reconstruction, and rendering. Based on the data available in the literature regarding the formation of dendritic spines during development and our results, we propose a unifying model for dendritic spine development.

Synaptic integration of adult-generated olfactory bulb granule cells: basal axodendritic centrifugal input precedes apical dendrodendritic local circuits

The adult mammalian olfactory bulb (OB) receives a continuing influx of new interneurons. Neuroblasts from the subventricular zone (SVZ) migrate into the OB and differentiate into granule cells and periglomerular cells that are presumed to integrate into the synaptic circuits of the OB. We have used retroviral infection into the SVZ of mice to label adult-generated granule cells and follow their differentiation and integration into OB circuitry. Using synaptic markers and electron microscopy, we show new granule cells integrating into the reciprocal circuitry of the external plexiform layer (EPL), beginning at 21 d postinfection (dpi). We further show that synapses are formed earlier, beginning at 10 dpi, on the somata and basal dendrites of new cells in the granule cell layer (GCL), before dendritic elaboration in the EPL. In the EPL, elaborate dendritic arbors with spines are first evident at 14 dpi. The density of spines increases from 14 to 28 dpi, and then decreases by 56 dpi. Despite the initial appearance of dendritic spines at 14 dpi in the EPL, no expression of presynaptic or postsynaptic markers is seen until 21 dpi. These data suggest that adult-generated granule cells are first innervated by centrifugal or mitral/tufted cell axon collaterals in the GCL and that these inputs may contribute to their differentiation, maturation, and synaptic integration into the dendrodendritic local circuits found in the EPL.

Non-synaptic dendritic spines in neocortex

A long-held assumption states that each dendritic spine in the cerebral cortex forms a synapse, although this issue has not been systematically investigated. We performed complete ultrastructural reconstructions of a large (n=144) population of identified spines in adult mouse neocortex finding that only 3.6% of the spines clearly lacked synapses. Nonsynaptic spines were small and had no clear head, resembling dendritic filopodia, and could represent a source of new synaptic connections in the adult cerebral cortex.

Passive avoidance training decreases synapse density in the hippocampus of the domestic chick

The bird hippocampus (Hp), although lacking the cellular lamination of the mammalian Hp, possesses comparable roles in spatial orientation and is implicated in passive avoidance learning. As in rodents it can be divided into dorsal and ventral regions based on immunocytochemical, tracing and electrophysiological studies. To study the effects of passive avoidance learning on synapse morphometry in the Hp, spine and shaft synapse densities of 1-day-old domestic chicks were determined in dorsal and ventral Hp of each hemisphere by electron microscopy, 6 and 24 h following training to avoid pecking at a bead coated with a bitter-tasting substance, methyl anthranilate (MeA). The density of asymmetric spine and shaft synapses in MeA-trained birds at 6 h post-training was significantly lower in the dorsal and ventral Hp of the right hemisphere relative to control (untrained) chicks, but by 24 h this difference was absent. A hemispheric asymmetry was apparent in the ventral Hp where the water-trained group showed enhanced shaft and spine synapse density in the left hemisphere, whilst in the MeA-trained group only asymmetric shaft synapses follow the same pattern in relation to the right hemisphere. There were no differences in asymmetric shaft synapses in the dorsal Hp at 6 h post-training, but at 24 h post-training there was a reduction in the density of shaft synapses in the right hemisphere in MeA compared with control birds. These data are discussed in relation to the pruning effects of stress and learning on synapse density in chick Hp.

Differentiation of spinous synapses in the mouse organ of corti

The inner hair cells, the primary auditory receptors, are perceived only as a means for transfer of sound signals via the auditory nerve to the central nervous system. During initial synaptogenesis, they receive relatively few and mainly somatic synapses. However, around the onset of hearing (10-14 postnatal days in the mouse), a complex network of local spinous synapses differentiates, involving inner hair cells, their afferent dendrites, and lateral olivocochlear terminals. Inner hair cell spines participate in triadic synapses between olivocochlear terminals and afferent dendrites. Triadic synapses have not yet been confirmed in the adult. Synaptic spines of afferent dendrites form axodendritic synapses with olivocochlear terminals and somatodendritic synapses with inner hair cells. The latter are of two types: ribbon-dendritic spines and stout dendritic spines surrounded only by a crown of synaptic vesicles. Formation of spinous afferent synapses results from sprouting of dendritic filopodia that intussuscept inner hair cell cytoplasm. This process continues in the adult, indicating ongoing synaptogenesis. Spinous processes of olivocochlear synaptic terminals contact adjacent afferent dendrites, thus integrating their connectivity. They develop about 14 postnatal days, but their presence in the adult has yet to be confirmed. Differentiation of spinous synapses in the organ of Corti results in a total increase of synaptic contacts and in a complexity of synaptic arrangements and connectivity. We propose that spinous synapses provide the morphological substrate for local processing of initial auditory signals within the cochlea.

Estimation of numerical density and mean synaptic height in chick hippocampus 24 and 48 hours after passive avoidance training

The effects of passive avoidance learning on synaptic morphology and number in the dorsolateral hippocampus of chick were investigated at 24 and 48 h after training. Chicks of both sexes were used. The numerical density of synapses and mean synaptic height were determined using design-based quantitative electron microscopic techniques. Our results suggest that after training there is a significant increase in synaptic density in the dorsolateral hippocampus of chicks at both 24 and 48 h, and also that the mean synaptic height was significantly different between trained and control groups. The increase in synaptic density was due to shaft (type II) synapses. It is known that during synaptogenesis, shaft synapses are formed first and are then converted to spine synapses. The only hemispheric asymmetry was found in the 24 h water-trained (W-trained) males where the numerical density of spine synapses was significantly higher in the left hippocampus. No significant differences due to gender in either numerical synaptic density or synapse height were observed at either 24 and 48 h. Comparison of the 24 h with 48 h groups showed an increase in shaft synaptic density over time in the W-trained groups, and an increased density of both shaft and spine synapses with time in methylanthranilate-trained (MeA-trained) chicks. These results demonstrate that the dorsolateral hippocampus of the chick shows synaptic changes at both 24 and 48 h after training and implicates this region in the long-term memory process.

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.

Decreased benzodiazepine receptor density in the cerebellum of early blind human subjects

As a first approach to study the effect of early visual deprivation in the GABA-ergic inhibitory system, the distribution of benzodiazepine receptors (BZR) was accurately estimated using [11C]flumazenil ([11C]FMZ). Measurements were carried out in five subjects who became blind early in life and in five sighted control subjects. The interactions between [11C]FMZ and BZR were described using a non-linear compartmental analysis which permitted to estimate the BZR synaptic density independently of other model parameters. The distribution of BZR in the visual areas and other cortical regions of blind subjects was qualitatively and quantitatively similar to that of controls. However, the BZR density in the cerebellum was significantly lower in blind than in control subjects (P<0.01). Our findings suggest that modifications of the cerebellar neural circuitry may be concomitant to the already observed compensatory reorganization in cerebral areas of blind subjects.

The minimal provision principle of functional systems. Neuronal mechanisms

An important characteristic of the developing nerve cell is its ability to function long before maturity. At this time, the neuron has the basic properties of an excitable membrane, an incompletely developed dendritic tree, and a low level of afferent input. All of these factors hinder the initiation of synaptic and spike potentials. It is proposed that the functional potential of neurons at the early stages of development are provided by an adaptive system, which may include: 1) the excess phenomenon, i.e., increases in the numbers of nerve cells, their processes, spines, and synapses; 2) factors increasing the chances of meeting the expected afferentation, i.e., the orientation and growth of dendrites towards the afferent input and the structure and localization of branch points; 3) factors facilitating the initiation of nerve spikes i.e., juvenile channels for ion current generation, electrical interactions between cells. and additional trigger zones. These groups of properties solve a single common problem: that of facilitating cells to respond to single or weak signals. This is the basic condition supporting the operation of interneuron interactions at the early stages of brain development.

Altered GABA neurotransmission and prefrontal cortical dysfunction in schizophrenia

Dysfunction of the dorsolateral prefrontal cortex appears to be a central feature of the pathophysiology of schizophrenia, and this dysfunction may be related to alterations in gamma aminobutyric acid (GABA) neurotransmission. Determining the causes and consequences of altered GABA neurotransmission in schizophrenia, and the relationship of these changes to other abnormalities in prefrontal cortical circuitry, requires an understanding of which of the multiple subpopulations of cortical GABA neurons are affected. The chandelier class of GABA neurons, especially those located in the middle layers of the prefrontal cortex (PFC), have been hypothesized to be preferentially involved in schizophrenia because they 1) receive direct synaptic input from dopamine axons, 2) exert powerful inhibitory control over the excitatory output of layer 3 pyramidal neurons, and 3) undergo substantial developmental changes during late adolescence, the typical age of onset of schizophrenia. Consistent with this hypothesis, the axon terminals of chandelier neurons, as revealed by immunoreactivity for the GABA membrane transporter, are reduced substantially in the middle layers of the PFC in schizophrenic subjects. This alteration appears to be selective for the chandelier class of GABA neurons and for the disease process of schizophrenia. These findings provide insight into the pathophysiologic mechanisms underlying prefrontal cortical dysfunction in schizophrenia, and they reveal new targets for therapeutic intervention in this illness.

Synaptogenesis via dendritic filopodia in developing hippocampal area CA1

To determine the role of dendritic filopodia in the genesis of excitatory synaptic contacts and dendritic spines in hippocampal area CA1, serial section electron microscopy and three-dimensional analysis of 16 volumes of neuropil from nine male rat pups, aged postnatal day 1 (P1) through P12, were performed. The analysis revealed that numerous dendritic filopodia formed asymmetric synaptic contacts with axons and with filopodia extending from axons, especially during the first postnatal week. At P1, 22 +/- 5.5% of synapses occurred on dendritic filopodia, with 19 +/- 5.9% on filopodia at P4, 20 +/- 8.0% at P6, decreasing to 7.2 +/- 4.7% at P12 (p < 0.02). Synapses were found at the base and along the entire length of filopodia, with many filopodia exhibiting multiple synaptic contacts. In all, 162 completely traceable dendritic filopodia received 255 asymmetric synaptic contacts. These synapses were found at all parts of filopodia with equal frequency, usually occurring on fusiform swellings of the diameter. Most synaptic contacts (53 +/- 11%) occurred directly on dendritic shafts during the first postnatal week. A smaller but still substantial portion (32 +/- 12%) of synapses were on shafts at P12 (p < 0.036). There was a highly significant (p < 0.0002) increase in the proportion of dendritic spine synapses with age, rising from just 4.9 +/- 4.3% at P1 to 37 +/- 14% at P12. The concurrence of primarily shaft and filopodial synapses in the first postnatal week suggests that filopodia recruit shaft synapses that later give rise to spines through a process of outgrowth.

Axosomatic input to subpopulations of cortically projecting pyramidal neurons in primate prefrontal cortex

Pyramidal cells, the major class of cortical excitatory neurons, can be divided into different subpopulations based upon the target region of their principal axon projection. The activity of pyramidal neurons is regulated in part through inhibitory synaptic inputs to the soma from local circuit neurons. However, little is known about how the density of these axosomatic inputs differs among subpopulations of pyramidal neurons in the prefrontal cortex of primates. In this study, retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used to identify pyramidal neurons in monkey prefrontal cortex (areas 9 and 46), which were labeled via either associational (ipsilateral hemisphere) or callosal (contralateral hemisphere) principal axon projections. Ultrastructural analysis revealed that the relative number of terminals apposed to the somatic membrane did not differ between associational and callosal neurons. However, neurons in the supragranular layers were apposed by a significantly greater number of axon terminals than were neurons in the infragranular layers. These findings suggest that the laminar environment of a neuron may play a more important role than principal axon projection in determining the amount of axosomatic inhibitory input it receives.

Synchronous development of pyramidal neuron dendritic spines and parvalbumin-immunoreactive chandelier neuron axon terminals in layer III of monkey prefrontal cortex

Postnatal development of the primate cerebral cortex involves an initial proliferation and the subsequent attrition of cortical synapses. Although these maturational changes in synaptic density have been observed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific populations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in area 9 and 46 of rhesus monkey prefrontal cortex. These neurons are of particular interest because they play a major role in the flow of information both within and between cortical regions. Quantitative reconstructions of Golgi-impregnated mid-layer III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitatory inputs to these neurons. Relative spine density on both the apical and basilar dendritic trees increased by 50% during the first two postnatal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels were achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these distinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density. These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory inputs during development and, in concert with other data, they emphasize the cellular specificity of postnatal refinements in cortical circuitry.

Ultrastructural evidence for synaptic interactions between thalamocortical axons and subplate neurons

Thalamic axons are known to accumulate in the subplate for a protracted period prior to invading the cortical plate and contacting their ultimate targets, the neurons of layer 4. We have examined the synaptic contacts made by visual and somatosensory thalamic axons during the transition period in which axons begin to leave the subplate and invade the cortical plate in the ferret. We first determined when geniculocortical axons leave the subplate and begin to grow into layer 4 of the visual cortex by injecting 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine (Dil) into the lateral geniculate nucleus (LGN). By birth most LGN axons are still confined to the subplate. Over the next 10 days LGN axons grow into layer 4, but many axons retain axonal branches within the subplate. To establish whether thalamic axons make synaptic contacts within the subplate, the anterograde tracer PHA-L was injected into thalamic nuclei of neonatal ferrets between postnatal day 3 and 12 to label thalamic axons at the electron microscope level. The analysis of the PHA-L injections confirmed the Dil data regarding the timing of ingrowth of thalamic axons into the cortical plate. At the electron microscope level, PHA-L-labelled axons were found to form synaptic contacts in the subplate. The thalamic axon terminals were presynaptic primarily to dendritic shafts and dendritic spines. Between postnatal days 12 and 20 labelled synapses were also observed within layer 4 of the cortex. The ultrastructural appearance of the synapses did not differ significantly in the subplate and cortical plate, with regard to type of postsynaptic profiles, length of postsynaptic density or presynaptic terminal size. These observations provide direct evidence that thalamocortical axons make synaptic contacts with subplate neurons, the only cell type within the subplate possessing mature dendrites and dendritic spines; they also suggest that functional interactions between thalamic axons and subplate neurons could play a role in the establishment of appropriate thalamocortical connections.

Dendritic spine density in the lobus parolfactorius of the domestic chick is increased 24 h after one-trial passive avoidance training

One to three day old chicks spontaneously peck at small objects. When presented with a chrome bead coated with the bitter tasting substance methyl anthranilate (MeA), chicks peck once, display a characteristic disgust response and subsequently avoid a similar bead. Chicks that are trained on a water coated bead continue to peck a similar bead on retrial. Twenty four hours after training on this one-trial passive avoidance paradigm, chicks were tested for retention. The brains of chicks displaying the correct behavioural response (> 90%) were removed and the lobus parolfactorius from each hemisphere was dissected from the brain and impregnated using a rapid Golgi technique. Analysis of large multipolar neurones by centrifugal dendritic branch order showed that there were significantly more spines on all orders examined in the left hemispheres of MeA-trained chicks compared to water-trained control chicks. Significantly higher spine densities were also found on 4th and 5th order branches of neurones in the right lobus parolfactorius of MeA-trained chicks compared to water-trained chicks. No significant difference in dendritic length was observed. These results suggest that substantial plasticity occurs in post-synaptic structures in the lobus parolfactorius following passive avoidance training. It is suggested that this plasticity is related to processes involved in long term information storage.

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

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

Neuronal organization and plasticity in adult monkey visual cortex: immunoreactivity for microtubule-associated protein 2

Immunocytochemical staining for microtubule-associated protein 2 (MAP 2) was used to examine the morphology of neurons, the organization of neuronal groups, and the neurochemical plasticity of cells in adult monkey area 17. MAP 2-immunostained neurons are present through the depth of area 17 but are most intensely immunoreactive in layers IVB and VI. From layer IVB, separate groups of MAP 2-positive cells invade layers IVA and IVC alpha. Clusters of cells protrude upward from superficial layer IVB and occupy the central core regions of the cytochrome oxidase (CO)-stained honeycomb in layer IVA, while large neurons typical of layer IVB are distributed in irregular clusters in the subjacent layer IVC alpha. The somata in the layer IVA honeycomb cores give off immunostained dendrites which remain largely within the core regions. Patches of MAP 2-positive neurons are also present in layers II and III, where they coincide with the CO-stained puffs. Intravitreal injections of tetrodotoxin (TTX) into one eye of adult monkeys produce stripes of alternating light and dark MAP 2 immunostaining in layer IVC. Stripes of light immunostaining coincide with stripes of light CO staining, and correspond to reduced MAP 2 immunoreactivity within cortical neurons dominated by the TTX-injected eye. In layers II and III, the MAP 2 immunostaining of patches overlying the injected-eye columns is similarly reduced. No change occurs in the MAP 2 immunostaining of layer IVA. These data suggest that the anatomical and physiological heterogeneity of layers IVA and IVC alpha arises from the periodic invasion of neurons characteristic of layer IVB, that the neurons in layer IVA have dendrites confined to thalamocortical-recipient or nonrecipient zones and that the expression of MAP 2 changes in adult cortical neurons following the loss of retinal input.

Development of synapses in macaque monkey striate cortex

A quantitative electron-microscopic (EM) analysis of the development of synaptic density (number of synapses/100 microns2 neuropil) has been done in primary visual cortex (striate, area 17) of the Old World monkey Macaca nemestrina. A comparative EM morphological study of developing synaptic contacts also was done in the same tissue. We find that a few immature synaptic contacts are present at fetal (F) 75 days either in the marginal zone, which becomes layer 1, or in the deepest portion of the cortical plate, the future layer 6. At F90-140 days synaptic contacts are found throughout the cortical plate, but their density remains higher in lower cortical layers. By F140 days synaptic density averaged for all layers (10.9) is three times higher than at F90 days. Just before and after birth, synaptic density rises very rapidly to peak at postnatal (P) 12 weeks (63) and then declines slowly to reach adult values (37.7) between 2-6 years. This pattern was further tested by comparing synaptic density in layer 2 which contains the last cells generated in the striate cortex to that in layer 6 which contains the first cells generated in the striate cortex. Layer 6 contained the first synapses, and had a higher density up to F140 days (an "inside-to-outside" distribution). Synaptic density was equal in the two layers at F152 days and P2 days, but by P12 weeks synaptic density in layer 2 was 27% higher than that in layer 6 (an "outside-to-inside" distribution). After P12 weeks, the synaptic density declined 51% in layer 2 and 21% in layer 6 so that both layers achieved similar densities by P6 years. A light and EM comparison of neuropil and synaptic contact morphology finds that, at each age up to birth, synapses in layer 2 are generally less mature than those in layer 6, but these differences disappear shortly after birth. Between P6-24 weeks, synaptic contacts throughout the cortex acquire a mature morphology that clearly differentiates between asymmetric and symmetric types, although asymmetric contacts continue to acquire more postsynaptic density until adulthood. This complex developmental pattern suggests a sequence for synaptic developments which is more related to neuron birthdate than to the arrival of extrinsic pathways or developmental events occurring in specific laminae.

Postnatal development of thalamic recipient neurons in the monkey striate cortex: I. Comparison of spine acquisition and dendritic growth of layer 4C alpha and beta spiny stellate neurons

A quantitative study has been made from Golgi impregnations of the maturation of dendrites and their spines on spiny stellate neurons in the macaque monkey primary visual cortex. The neurons studied lay within either the alpha or the beta division of lamina 4C; previous workers have shown the alpha division neurons to be contacted by thalamic axon terminals arising from the magnocellular division of the lateral geniculate nucleus (LGN) of the thalamus and the beta division neurons to be contacted by parvocellular LGN inputs. Most thalamic terminals and perhaps the majority of other type 1 (Colonnier, '81), presumed excitatory, inputs to these cells make synaptic contacts on the tips of their dendritic spines. Measurement was made of relative changes in the total number of spines on these alpha and beta spiny neurons over age by measuring both spine density along the dendrites and dendritic arbor size in single 90-microns sections from Golgi rapid preparations. Our previous work (Lund et al., '77; Boothe et al., '79) showed a marked proliferation and attrition of spines and dendritic branches to occur in the early postnatal weeks; Rakic et al. ('86) have since proposed that there is a cortexwide synchrony of synapse acquisition and loss during this same period. However, different visual capacities channelled via the magnocellular and parvicellular geniculate relays show different maturational rates (Harwerth et al., '86). This study indicates that the anatomical maturation of spines on the alpha and beta neurons is not temporally coincident from birth to 30 weeks. During this period, phases of spine acquisition and loss on alpha neurons precedes similar phases on beta neurons. The alpha neurons carry a peak spine population at 5-8 weeks postnatal, whereas the beta neurons carry their peak spine populations between 8 and 24 weeks postnatal. At all ages prior to 30 weeks, the two sets of neurons carry quite different total spine populations. Close to 30 weeks of age, the total spine coverage has fallen on both sets of neurons and becomes identical between the alpha and beta neurons. In animals aged 30 weeks to adult, spine coverage per neuron is maintained at a common figure for the alpha and beta neurons despite further growth and disparate dendritic arbor sizes and different local spine densities in the two groups; this suggests that some common sampling paradigm between pre- and postsynaptic elements is adopted by the alpha and beta neurons and also suggests the development of a close functional correlation between the two sets of neurons.

Development of the calcium-binding protein parvalbumin and calbindin in monkey striate cortex

The development of immunoreactivity for the calcium-binding proteins parvalbumin (PV) and calbindin-D28K (Cal) was studied in Macaca nemestrina striate cortex from fetal (F) 60 days to postnatal (P) 5 + years. We correlated changes in PV and Cal staining patterns with the well-documented developmental sequence for primate striate cortex neuron generation and maturation, synaptogenesis, and thalamocortical axon interactions in an attempt to deduce a functional role for these proteins. Our major findings is that Cal and PV have diametrically opposed developmental patterns except in layer 1. At F60 days both are present only in neurons of layer 1 and the number of labeled cell bodies and processes increases up to F125 days. Almost all Cal+ and PV+ cells in layer 1 disappear by P12 weeks. Cal is present by F113 days in pyramidal and stellate neurons, particularly layers 4-6. The numbers and staining density of cells in layers 2-6 increases up to birth and then both decline by P9-12 weeks. Supragranular layers show a second increase in Cal labeling from P20-36 weeks, and then there is a slow decline to the adult pattern which is reached by P1-2 years. Cell bodies in layers 4A, 4C alpha, and deep 4C beta are heavily Cal+ during pre- and early post-natal periods, but upper 4C beta remains unlabeled. PV is not seen until F155-162 days in layers 2-6. Large stellate and a few pyramidal cells appear first in layers 5/6 and 4C alpha, but PV+ stellate neurons are found in all layers except 4C beta by P6 weeks. Layer 4C beta contains a few PV+ cell bodies at P3 weeks, and light neuropile staining at P6 weeks, but then PV labeling rapidly increases so that by P12 weeks the density of 4C beta exceeds that of 4C alpha. Striate cortex has an adult pattern of cell number and neuropile density by P20 weeks. These developmental patterns suggest that the highest density of Cal cell body staining does not correlate with synaptogenesis, or the postnatal critical period of visually driven, binocular interactions. Rather Cal appears when lateral geniculate axons arrive in cortex, persists over the entire span of thalamocortical interactions, and disappears during the decline of cortical plasticity. The appearance of PV is highly correlated with the onset of complex visually driven activity at birth, while both the number of PV+ cell bodies and the density of PV+ neuropile reach adult levels coincident with the completion of thalamocortical connections.

Arborisation pattern and postsynaptic targets of physiologically identified thalamocortical afferents in striate cortex of the macaque monkey

The monosynaptic targets of different functional types of geniculocortical axons were compared in the primary visual cortex of monkeys. Single thalamocortical axons were recorded extracellularly in the white matter by using horseradish-peroxidase-filled pipettes. Their receptive fields were mapped and classified as corresponding to those of parvi- or magnocellular neurons in the lateral geniculate nucleus. The axons were then impaled and injected intraaxonally with horseradish peroxidase. Two magnocellular (MA) and two parvicellular (PA) axons were successfully recovered and reconstructed in three dimensions. The two MA axons arborised mainly in layer 4C alpha, as did the two PA axons in layer 4C beta. Few collaterals formed varicosities in layer 6. Both MA axons had two large, elongated clumps of bouton (approx. 300-500 x 600-1,200 microns each) and a small clump. One PA axon had two clumps (each with a core appr. 200 microns in diameter); the other had only one (appr. 150-200 microns in axon had 1,380; one MA axon had 3,200 boutons; and those of the more extensive MA axon were not counted. The distribution of postsynaptic targets as well as the number of synapses per bouton has been established for a sample of 150 PA boutons and 173 MA boutons from serial ultrathin sections. The MA axons made on average 2.1 synapses per bouton compared to 1.79 for one PA axon and 2.6 for the other. The sample of boutons taken from the two physiological types of axons contacted similar proportions of dendritic spines (52-68%), shafts (33-47%), and somata (0-3%). The postsynaptic elements were further characterized by immunostaining for GABA. All postsynaptic perikarya and some of the dendrites (4.5-9.5% of all targets) were positive for the amino acid. Near the thalamic synapse GABA-negative dendritic shafts frequently contained lamellar bodies, an organelle identical in structure to spine apparatus. Dendritic shafts and spines postsynaptic to the thalamocortical boutons frequently received an adjacent synapse from GABA-immunoreactive boutons. The similarity between the magno-and parvicellular axons in their targeting of postsynaptic elements, including the GABAergic neurons, suggests that the structural basis of the physiological differences between 4C alpha and 4C beta neurons should be sought in other aspects of the circuitry of layer 4C, such as local cortical circuits, or in the far greater horizontal extent of the thalamocortical and GABAergic axons in layer 4C alpha compared to those in the beta subdivision.

Changes in synaptic density in motor cortex of rhesus monkey during fetal and postnatal life

The density and proportion of synaptic contacts in the primate motor cortex (Brodmann area 4) were determined in 21 rhesus monkeys ranging in age from embryonic day 41 (E41) to 20 years. Two to 4 vertical electron microscopic probes, each consisting of 150-250 overlapping micrographs traversing the thickness of the cortex, were prepared for each specimen. Synapses were categorized according to their morphology (symmetrical or asymmetrical), cellular location (on spines, shafts or soma), number, and ratio of laminar distribution. The density of synapses was expressed per unit area and volume of neuropil (excluding neuronal and glia cell bodies, myelin sheath, blood vessels and extracellular space). The first synapse in the area of the emerging motor cortex were observed at E53 in the marginal zone (prospective layer I) and in the transient subplate zone situated beneath the developing cortical plate. Around midgestation (E89) synapses were observed over the entire width of the cortical plate, and their density was about 5/100 microns 3 of neuropil. During the last two months of gestation synaptic density increased 8-fold across all layers to reach about 40/100 microns 3 at the time of birth (E165). Synaptic production continued postnatally and by the end of the second postnatal month attained a level of 60/100 microns 3 neuropil which is two times higher than in the adults. This level decreased at a slow rate until sexual maturity (3 years of age) and then more rapidly to the adult level which is characterized by relative stability of about 30/100 microns 3. The decline in synaptic density after the peak in infancy occurs predominantly at the expense of asymmetric synapses situated on dendritic spines; the population of symmetric synapses on dendritic shafts remains relatively constant. The development of synaptic connections in the motor cortex of non-human primates involves initial overproduction followed by selective elimination and structural alterations.

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.

Development of neurons in the ectostriatum of normal and monocularly deprived zebra finches: a quantitative Golgi study

The postnatal development of the main neuron type in the ectostriatum, the telencephalic station of the tectofugal pathway, was followed in normally reared and monocularly deprived zebra finches by using the Golgi method. Three parameters were investigated: dendritic field radius, branching index, and spine density. The results show that all three exhibit the same developmental trend--namely, an increase from day 5 until day 20, followed by a subsequent reduction until adulthood (greater than 100 days). Monocular deprivation from birth until day 20, 40, or at least 100 does not seem to interfere with the development of the dendritic field radius or branching index. Clear changes in spine density result from depriving the birds for at least 40 days. In these birds, neurons in the deprived hemisphere bear significantly fewer spines than those in the nondeprived hemisphere, which is mainly due to a lack of normally occurring spine reduction in the nondeprived hemisphere rather than to spine reduction in the deprived hemisphere.

Formation of cholinergic synapses by intrahippocampal septal grafts as revealed by choline acetyltransferase immunocytochemistry

The ultrastructural features of the contacts established by intrahippocampal grafts of foetal septal/diagonal band neurones in the dentate gyrus and the CA1 region of the previously denervated host hippocampus have been analysed with electron microscopic immunocytochemistry using a monoclonal antibody to choline acetyltransferase (ChAT). The results show that the grafted ChAT-positive neurones are capable of forming extensive synaptic contacts with neuronal targets in areas of the dentate gyrus and CA1 which normally receive such innervation. While all types of contacts normally found in association with the granule and pyramidal cell layers were also present in the graft-reinnervated specimens, the quantitative relationship between somatic and dendritic synapses was abnormal. Thus, the ChAT-immunoreactive synapses on cell bodies, which amounted to only a few percent in the normal animal, constituted over 60% in the grafted animals. Conversely, synapses on dendrites which constituted over 90% in the normal dentate were reduced to less than 40% in the grafted animals. The postsynaptic targets of the graft-derived cholinergic synapses included dendrites and cell bodies of dentate granule cells and CA1 pyramidal cells. This supports previous electrophysiological studies and indicates that the septal grafts may be able to modulate host hippocampal function via direct efferent connections onto the granule and pyramidal neurons in the host hippocampal formation.

The morphology and synaptic connections of spiny stellate neurons in monkey visual cortex (area 17): a Golgi-electron microscopic study

Based on a gold-toning, Golgi-electron microscope examination of 12 small and medium-sized spiny stellate neurons in laminae 4A, 4B, and 4C of the monkey visual cortex (area 17), the ultrastructure of the cell somata, dendrites, and axons of these neurons is described. Particular attention is paid to the synapses involving the surface of different parts of these neurons. Only symmetric synapses occur on the somata of spiny stellate neurons, and these occur with a frequency of 11.0-15.9 synapses/100 microns2 perikaryal surface. Symmetric synapses also occur on dendritic shafts and, occasionally, on dendritic spines. Asymmetric synapses are occasionally present along the dendritic shafts of spiny stellate neurons, but the majority of asymmetric synapses (75-95%) occur on their dendritic spines. The initial axon segments of the smallest spiny stellate neurons possess no axo-axonal synapses, but several symmetric synapses are present along the initial segment of a medium-sized, spiny stellate neuron in layer 4B. Fifty-three synapses made by boutons of the axons of these spiny stellate neurons have been identified, and all are asymmetric. Sixty per cent of the synapses are formed by boutons en passant and the remainder by the terminal swellings of spine-like axonal appendages, boutons terminaux. Of the synapses formed by the axons of spiny stellate cells, axo-spinous synapses outnumber axo-dendritic synapses two to one, and axo-dendritic synapses involve both spinous and aspinous dendrites. Evidence is presented which suggests that many of the axon terminals forming asymmetric synapses with the dendritic shafts and spines of spiny stellate neurons are derived from other spiny stellate neurons.

Cytodifferentiation and synaptogenesis in the neostriatum of fetal and neonatal rhesus monkeys

Cytodifferentiation and synaptogenesis in the neostriatum (caudate nucleus and putamen) were analyzed by the Golgi impregnation method and electron microscopy in 14 fetuses and 8 postnatal rhesus monkeys. During the second fetal month the neostriatum consists primarily of simple, mostly bipolar, immature cells and a small number of undefined profiles ending with growth cones. The first morphologically defined synapses appear in the putamen at embryonic day 60 ( E60 ) and in the head of the caudate nucleus at E65 . Synaptic density in both structures is less than one per 1000/micron2 of neuropil at this stage; synapses are characterized by asymmetric junctions between axonal profiles and immature dendritic shafts, accumulation of an intermembrane web and aggregation of round clear vesicles in presynaptic profiles. During the third fetal month neuronal cell bodies and glial cells enlarge, and axonal and dendritic processes in Golgi preparations become more complex. Although the basic morphology of synapses remains unchanged, their density increases to 9/1000 micron2 in the putamen and 3.7/1000 micron2 in the caudate. During the fourth fetal month the four principal cell classes of the neostriatum emerge. Spines on the shafts of dendrites are followed closely by the appearance of axospinous synapses. Synaptic density in the putamen is still significantly higher (10.1/1000 micron2) than in the caudate (5.4/1000 micron2), but by the end of the fifth fetal month ( E150 ) it is the same (80/1000 micron2) in both structures. A dramatic increase in synaptic density to 125/1000 micron2 occurs before term ( E165 ) with the emergence of the first asymmetric synapses as well as symmetric synapses with flat or pleomorphic vesicles that terminate predominately on dendritic shafts. Synaptic density continues to increase after birth, reaching a plateau of approximately 190/1000 micron2 at the end of the first postnatal month. Throughout postnatal development the proportions of symmetric and asymmetric synapses on the smooth dendritic shafts undergo systematic fluctuations which may reflect the ingrowth of various afferents as well as local cytological differentiation including the formation of cellular compartments.

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

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

Developmental changes in the relationship between type 2 synapses and spiny neurons in the monkey visual cortex

This study continues an exploration of synaptic development in the primary visual cortex of the monkey (Macaca nemestrina). In a prior study (Mates and Lund, '83a), we observed that type 2 synapses on the cell bodies of spiny stellate neurons of lamina 4C appeared not only to increase in number during early postnatal development but also subsequently decreased during maturation. Using quantitative, stereological electron microscopic methods, we examined the maturation of this synapse population from embryonic day 159 to adult, on spiny stellate neurons of 4C alpha and beta and, for comparison, on pyramidal neurons in upper and lower lamina 6. Tissue was also taken for comparison from two animals reared to 8 weeks of age with binocular eyelid closure from birth. We confirmed that a marked increase and subsequent decrease occurred in this somal type 2 synapse population on both neuron populations. However, due to the infrequency of the smooth dendritic neurons (approximately 5% of the neuron population) giving rise to the type 2 contacts, and due to expansion of the neuropil during maturation increasing intercell distances against constant volume of the type 2 axon arbors, it is concluded that the decrease in type 2 somal synapses may represent a redistribution to dendrites rather than loss from the neuropil. Cells of lamina 4C beta (receiving input from the parvocellular lateral geniculate nucleus-LGN) show a slower initial accumulation of type 2 contacts compared to neurons of lamina 4C alpha (receiving input from magnocellular LGN), or to pyramidal neurons of lamina 6.(ABSTRACT TRUNCATED AT 250 WORDS)