Dendritic spines of CA 1 pyramidal cells in the rat hippocampus: serial electron microscopy with reference to their biophysical characteristics

Serial electron microscopy and 3-D reconstructions of dendritic spines from hippocampal area CA 1 dendrites were obtained to evaluate 2 questions about relationships between spine geometry and synaptic efficacy. First, under what biophysical conditions are the spine necks likely to reduce the magnitude of charge transferred from the synapses on the spine heads to the recipient dendrite? Simulation software provided by Charles Wilson (1984) was used to determine that if synaptic conductance is 1 nS or less, only 1% of the hippocampal spine necks are sufficiently thin and long to reduce charge transfer by more than 10%. If synaptic conductance approaches 5 nS, however, 33% of the hippocampal spine necks are sufficiently thin and long to reduce charge transfer by more than 10%. Second, is spine geometry associated with other anatomical indicators of synaptic efficacy, including the area of the postsynaptic density and the number of vesicles in the presynaptic axon? Reconstructed spines were graphically edited into head and neck compartments, and their dimensions were measured, the areas of the postsynaptic densities (PSD) were measured, and all of the vesicles in the presynaptic axonal varicosities were counted. The dimensions of the spine head were well correlated with the area of PSD and the number of vesicles in the presynaptic axonal varicosity. Spine neck diameter and length were not correlated with PSD area, head volume, or the number of vesicles. These results suggest that the dimensions of the spine head, but not the spine neck, reflect differences in synaptic efficacy. We suggest that the constricted necks of hippocampal dendritic spines might reduce diffusion of activated molecules to neighboring synapses, thereby attributing specificity to activated or potentiated synapses.     

Dendritic spines of rat cerebellar Purkinje cells: serial electron microscopy with reference to their biophysical characteristics

We have used serial electron microscopy and 3-dimensional reconstructions of dendritic spines from Purkinje spiny branchlets of normal adult rats to evaluate 2 questions about the relationship of spine geometry to synaptic efficacy. First, do relationships between spine geometry and other anatomical indicators of synaptic activity suggest that spine size and shape might be associated with synaptic efficacy? Reconstructed spines were graphically edited into head and neck compartments; the area of the postsynaptic density (PSD) was measured; the volume of spine smooth endoplasmic reticulum (SER) was computed; and all of the vesicles in the axonal varicosities were counted. Spine head volume and the volume of SER contained in the head are well correlated with the area of the PSD and the number of vesicles in the presynaptic axonal varicosity. Spine neck diameter does not fluctuate with PSD area, head volume, or the vesicle number. These results suggest that the dimensions of the spine head, but not of the spine neck, are likely to reflect differences in synaptic efficacy. Second, does the geometry of cerebellar spine necks reduce the transfer of synaptic charge to the recipient dendrite from the theoretical maximum that could be transferred if the synapse were on a dendritic shaft? Comparison of volume to surface area showed that the spine heads are approximately spherical and the necks are approximately cylindrical. Application of results from a biophysical model that assumed these geometrical shapes for spines (Wilson, 1984) showed that the cerebellar spine necks are unlikely to reduce transfer of synaptic charge by more than 5-20% even if their SER were to completely block passage of current through the portion of the neck that it occupies. We suggest that the constricted spine neck diameter might serve to isolate metabolic events in the vicinity of activated synapses by reducing diffusion to neighboring synapses, without significantly influencing the transfer of synaptic charge to the postsynaptic dendrite.     

Constitutive tumor necrosis factor (TNF)-deficiency causes a reduction in spine density in mouse dentate granule cells accompanied by homeostatic adaptations of spine head size

The majority of excitatory synapses terminating on cortical neurons are found on dendritic spines. The geometry of spines, in particular the size of the spine head, tightly correlates with the strength of the excitatory synapse formed with the spine. Under conditions of synaptic plasticity, spine geometry may change, reflecting functional adaptations. Since the cytokine tumor necrosis factor (TNF) has been shown to influence synaptic transmission as well as Hebbian and homeostatic forms of synaptic plasticity, we speculated that TNF-deficiency may cause concomitant structural changes at the level of dendritic spines. To address this question, we analyzed spine density and spine head area of Alexa568-filled granule cells in the dentate gyrus of adult C57BL/6J and TNF-deficient (TNF-KO) mice. Tissue sections were double-stained for the actin-modulating and plasticity-related protein synaptopodin (SP), a molecular marker for strong and stable spines. Dendritic segments of TNF-deficient granule cells exhibited ～20% fewer spines in the outer molecular layer of the dentate gyrus compared to controls, indicating a reduced afferent innervation. Of note, these segments also had larger spines containing larger SP-clusters. This pattern of changes is strikingly similar to the one seen after denervation-associated spine loss following experimental entorhinal denervation of granule cells: Denervated granule cells increase the SP-content and strength of their remaining spines to homeostatically compensate for those that were lost. Our data suggest a similar compensatory mechanism in TNF-deficient granule cells in response to a reduction in their afferent innervation.     

Serial reconstructions of granule cell spines in the mammalian olfactory bulb

The morphology of olfactory bulb granule cell spines and their dendrodendritic synaptic relations with mitral and tufted cell dendrites were examined using serial electron micrographs and 3D computer reconstructions. Most granule cell spines were pedunculated with large elliptical heads and necks (stems) longer than those described for exclusively postsynaptic spines elsewhere in the nervous system. The spines typically contained a mitochondrion, which most likely reflects the metabolic requirements of the presynaptic functions of these spines. In several cases multiple spine heads were observed connected to the parent dendritic trunk via a common neck. In addition, dendritic varicosities making synaptic connections were noted. In the data set sampled, all of the reconstructions supported the hypothesis of divergence of granule cell connectivity: in no instance was a granule cell found to contact repeatedly the same mitral or tufted cell dendrite. Examination of the topological organization of reciprocal dendrodendritic synaptic connections with mitral/tufted cell dendrites revealed parallel rows of spine heads on mitral/tufted secondary dendrites separated by intervening zones of several microns in which no synaptic appositions were found. The results provide evidence regarding rules of connectivity underlying the function of local circuits in mediating lateral inhibition in the external plexiform layer of the olfactory bulb.     

Reduction in spine density associated with long-term potentiation in the dentate gyrus suggests a spine fusion-and-branching model of potentiation

Approximately 2,700 dendritic spines in Golgi-impregnated hippocampal granule cells were quantified via image analysis 24 h after the unilateral induction of long-term potentiation in seven rats. Stereological corrections were made using a tilting disector and analytical unfolding technique. In the potentiated hemisphere the mean spine density along dendrites was reduced by ∼20%. The relative frequency of shorter, thicker spines was increased in potentiated tissue. Physiological consequences of two morphological changes leading to a reduction in spine density (retraction or fusion of spines) were examined using a compartmental model of a simplified granule cell. The model was constructed in the NEURON modeling environment and included a realistic population of 60 dendritic spines (with dual-component synapses and active Ca²⁺-dependent mechanisms). Simulations demonstrated that potentiation of postsynaptic responses was compatible with fusion (with branching) of a proportion of spines with their neighbors but was not compatible with retraction of spines. This result held over wide variations of model parameters as long as dendritic membranes were assumed to be excitable. Hippocampus 1997;7:489–500. © 1997 Wiley-Liss, Inc.     

Organization of mitochondria in olfactory bulb granule cell dendritic spines.

In contrast to dendritic spines with only postsynaptic functions, the spines of olfactory bulb granule cells subserve both pre- and postsynaptic roles. In single sections these spines were previously seen to contain mitochondria, most likely needed to provide energy for presynaptic functions, but their frequency and distribution were unknown. In order to understand the organization of mitochondria in these specialized dendritic appendages, we have studied the geometry and cytoplasmic organization of granule cell spines with computer-assisted reconstructions of serial electron micrographs. The spine heads were seen to be elliptical in shape with a single pair of reciprocal synapses on the concave face apposed to the mitral/tufted cell dendrite. Mitochondria were found localized in the spine neck as well as the spine head and often extended between the two compartments. Based on their variable distribution it seems reasonable to suggest that these mitochondria are motile and move in and out of spine compartments from the parent dendrite. Spine apparatus was apparent in most of the spines as membrane bound cisterns of smooth endoplasmic reticulum located close to mitochondria. The possible role of spine apparatus in facilitating the movement of mitochondria in the necks and heads of granule cell spines in the absence of microtubules is discussed.     

Granule cell dispersion in temporal lobe epilepsy is associated with changes in dendritic orientation and spine distribution

Granule cell dispersion is a characteristic feature of Ammon's horn sclerosis in temporal lobe epilepsy. It was recently shown that granule cell dispersion is associated with decreased expression of the extracellular matrix protein Reelin. Reelin controls neuronal lamination and the differentiation of dendrites and spines. Here, we studied dendritic orientation and the distribution of dendritic spines on granule cells in surgical specimens of patients suffering from temporal lobe epilepsy. In this material, we compared granule cells in dentate areas showing granule cell dispersion with granule cells in areas exhibiting a normal, densely packed granule cell layer. Granule cells (GC) were Golgi-stained and analyzed using a computer-based camera lucida system and were categorized as being located proximal or distal to the hilus (GCprox, GCdist). We found that GCprox in a densely packed granule cell layer exhibited a mainly vertically oriented dendritic arbor with a small bifurcation angle (17°) between branching dendrites. In contrast, GCdist in a densely packed granular layer showed a wider bifurcation angle (35°), suggesting a widening of the dendritic field during the migratory process to superficial positions. Granule cells in the dispersed granule cell layer showed an even wider bifurcation angle of their apical dendrites (GCprox: 40°; GCdist: 58°) and also exhibited recurrent basal dendrites. Spine density on dendrites of GCprox in dispersed areas was increased compared to GCprox in the normal, compact granule cell layer. In contrast, dendrites of GCdist extending into the molecular layer showed a reduced spine density in dispersed areas. The results are discussed in view of other findings on neuronal reorganization in the epileptic dentate gyrus.     

Postnatal development of dendritic spines on olfactory bulb granule cells in rats

Postnatal morphological changes in granule cell dendritic spines and filopodia (collectively referred to as "spines/filopodia") were examined in the rat main olfactory bulb to characterize the development of the neural circuitry for olfaction. Granule cells were labeled with a membrane dye and confocal laser scanning microscope images of labeled spines/filopodia were acquired in the following three dendritic domains: apical dendrites in the external plexiform layer, those in the granule cell layer, and basal dendrites. In all three domains the proportion of typical spines slightly increased during development, with a concomitant decrease in the proportion of "stubby" spines lacking a neck; the proportion of filopodia remained unchanged, accounting for 20-40% of all protrusions. The mean diameter and length of the spine/filopodium population were nearly constant throughout development. On the other hand, the developmental pattern of the spine/filopodium density varied markedly, depending on the domain of the dendrites. In the external plexiform layer, the density did not change remarkably during development. The density in apical dendrites in the granule cell layer increased during the initial 2 postnatal weeks, then gradually decreased. The spine/filopodium density in basal dendrites, however, continued to increase until 4 weeks of age, and then began to decrease. These results suggest that a substantial amount of input-specific synaptic remodeling occurs in granule cells during development, which proceeds from superficial dendritic domains to deeper ones, occurring most prominently in the basal dendrites.     

Olfactory learning-induced morphological modifications in single dendritic spines of young rats

Learning-related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague-Dawley rats. We have previously shown that olfactory discrimination rule-learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning-induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser-scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule-learning enhance the cortical network ability to enter into a 'learning mode', in which memories of new odours can be acquired rapidly and efficiently.     

Morphological variability is a characteristic feature of granule cells in the primate fascia dentata: A combined Golgi/electron microscope study

This study analyzes the structural variability of granule cells in the monkey fascia dentata. The hippocampi of three adult rhesus monkeys (Macaca mulatta) and two 1-year-old female baboons (Papio anubis) were used for a combined Golgi/electron microscope (EM) study. The results were compared with other Golgi/EM studies on dentate granule cells in small laboratory animals. Whereas the granule cells in small rodents form a relatively uniform population of neurons, we observed a much greater variability of granule cell morphology in monkeys. This variability concerned the size of the cell body, the length and thickness of apical dendrites, the spine density, and the occasional occurrence of basal dendrites. The dendritic length of individual granule cells largely depended on their position in the highly convoluted granular layer. These convolutions caused significant variations in the thickness of the molecular layer and consequently in the length of individual granule cell dendrites. Granule cells with thick dendrites densely covered with spines could be differentiated from those exhibiting much thinner dendritic processes and moderate spine numbers. About 10% of granule cells in the monkey fascia dentata exhibited basal dendrites. Occasionally in the hilus ectopic granule cells were observed that gave rise to long apical dendrites traversing the granular layer. The axons of granule cells, the mossy fibers, entered the hilus, where they gave off several collaterals. In contrast to the light microscopic variability, subtypes of granule cells revealed similar fine structural characteristics, i. e., a round nucleus lacking indentations, a thin rim of cytoplasm, and characteristic spine formations. Large complex spines and smaller, “stubby” spines were observed on apical as well as basal dendrites. This suggests that characteristic spine formations were not induced by specific afferent fibers. All synaptic contacts on spines were of the asymmetric type, whereas both symmetric and asymmetric synapses occured on cell bodies and dendritic shafts. Unlike in rodents, we found a large variability of granule cells in the primate fascia dentata. This variability has to be considered in neropathological studies of this cell type.     

Observations on the commissural projection to the dentate gyrus in the reeler mutant mouse

The commissural projection to the displaced granule cells of the dentate gyrus in Reeler mutant mice has been examined with autoradiography, and light and electron microscopy. Commissural terminals in Reeler are confined to the hilar region, in contrast to normal littermates in which this projection is restricted to the inner part of the molecular layer. Granule cell somata in Reeler, but only exceptionally in normal littermates, are invested with spines, which have postsynaptic specializations, but no spine apparatus, and are contacted by presynaptic terminals. Between 20 and 30 h after destruction of the commissural fibres in Reeler, degenerating terminals can be found contacting both somatic and dendritic spines in the hilus; 30 h after decommissuration the number and length of spines on the somata and proximal dendrites of Golgi impregnated cells is greatly reduced, while spines on the distal parts of the dendritic tree are unaffected. A similar pattern of degeneration after decomissuration is found in the inner molecular layer of normal littermates. These results are discussed in terms of factors controlling the normal development of afferent projections. The paper concludes with an analysis of a potential methodological hazard. A change in mean spine length will of itself result in a change in the number of visible spines in Golgi material. A quantitative assessment of the relation between spine length and the number of visible spines is developed for spherical cell bodies and cylindrical dendrites.     

Intracerebral transplants of the rat fascia dentata: A Golgi/electron microscope study of dentate granule cells

In the present study we describe the morphological characteristics of dentate granule cells in intracerebral allografts of the rat fascia dentata. Blocks of hippocampal tissue containing the fascia dentata were taken from late embryonic and newborn rats and transplanted to the hippocampal region of other newborn and young adult rats. After survival periods of several months the recipient brains were fixed by perfusion and serially sectioned on a Vibratome. Some sections were stained with thionin to determine the localization and general histological organization of the transplants, while others were Golgi stained with a modification of the section Golgi technique. Well-impregnated transplant granule cells were gold-toned and deimpregnated thus allowing a correlated, light and electron microscopic analysis of identified neurons to be done. At the light microscopic level the morphology of the dentate granule cells in the transplants was very similar to Golgi-impregnated, gold-toned granule cells in the fascia dentata of normal rats (controls). A few irregular, more obliquely curved dendrites occurred, but basal dendrites passing into the hilar region were never observed. Following an initial spine-free segment granule cell dendrites were densely covered with spines. The axon, the mossy fiber, originated as usual from the basal pole of the cell body. In the electron microscope, both small and larger complex spines (v and w types) were seen to emerge from the gold-toned dendrites of the identified granule cells. The thin unmyelinated granule cell axons gave rise to giant mossy fiber boutons in the dentate hilus, but in addition numerous aberrant mossy fiber terminals were found innermost in the dentate molecular layer just above the granule cell layer. The results demonstrate that dentate granule cells that have gone through the major part of their differentiation-after transplantation develop characteristic dendritic and axonal elements very similar to those of granule cells in the fascia dentata in situ. The minor changes observed correspond to the redistribution of intrinsic connections that results from the absence of major extrinsic afferents.     

Running induces widespread structural alterations in the hippocampus and entorhinal cortex

Physical activity enhances hippocampal function but its effects on neuronal structure remain relatively unexplored outside of the dentate gyrus. Using Golgi impregnation and the lipophilic tracer DiI, we show that long-term voluntary running increases the density of dendritic spines in the entorhinal cortex and hippocampus of adult rats. Exercise was associated with increased dendritic spine density not only in granule neurons of the dentate gyrus, but also in CA1 pyramidal neurons, and in layer III pyramidal neurons of the entorhinal cortex. In the CA1 region, changes in dendritic spine density are accompanied by changes in dendritic arborization and alterations in the morphology of individual spines. These findings suggest that physical activity exerts pervasive effects on neuronal morphology in the hippocampus and one of its afferent populations. These structural changes may contribute to running-induced changes in cognitive function.     

Inhibitory contacts on dendritic spines of the dentate fascia

GABA-containing axon terminals were observed in the distal two-thirds of the dentate molecular layer to contact spines and dendrites of the granule cells. These contacts have the morphological characteristics of inhibitory synapses: they contain pleomorphic vesicles and have symmetrical junctional specializations. Convergence of an asymmetrical, non-GABAergic and a symmetrical, GABAergic synapse on one spine was often observed.     

Differentiation of granule cell dendrites in the dentate gyrus of the rhesus monkey: A quantitative golgi study

The differentiation of granule cell dendrites in the dentate gyrus of the hippocampal region was studied in a series of developing fetal and postnatal rhesus monkeys whose brains were processed by the rapid Golgi method. The total combined lengths of dendrites, the total number of dendritic spines, and their density on the proximal, middle, and distal thirds of the dendritic shafts were determined at embryonic days 58, 95, 120, 153, term (165), postnatal days 3, 20, 60, 150, 365, and adults. At all ages examined, granule cells exhibited various levels of maturation with the more differentiated cells being situated in the superficial strata of the granular layer and the less mature cells lying in progressively deeper positions, thus conforming to the outside-to-inside spatiotemporal gradient of their genesis. Quanti-tative analysis shows that, in this primate, hippocampal granule cells dif-ferentiate mainly in the second half of gestation with all measured param-eters attaining mature values by the time of birth. However, the analysis also reveals a transient phase of exuberant postnatal development which involves excessive dendritic branching, regional changes in dendritic length, overproduction of dendritic spines, and redistribution of spines within the molecular layer. After reaching peak values around the middle of the first year of life, these parameters decrease and in adult monkeys fall back to the neonatal level.     

Time-lapse imaging of granule cells in mouse entorhino-hippocampal slice cultures reveals changes in spine stability after entorhinal denervation

Principal neurons that are partially denervated after brain injury remodel their synaptic connections and show biphasic changes in their dendritic spine density: during an early phase after denervation spine density decreases and during a late phase spine density recovers again. It has been hypothesized that these changes in spine density are caused by a period of increased spine loss followed by a period of increased spine formation. We have tested this hypothesis, which is based on data from fixed tissues, by using time-lapse imaging of denervated dentate granule cells in organotypic entorhino-hippocampal slice cultures of Thy1-GFP mice. Our data show that nondenervated granule cells turn over spines spontaneously while keeping their spine density constant. Denervation influenced this equilibrium and induced biphasic changes in the spine loss rate but not in the rate of spine formation: during the early phase after denervation the spine loss rate was increased and during the late phase after denervation the spine loss rate was decreased compared with nondenervated control cultures. In line with these observations, time-lapse imaging of identified spines formed after the lesion revealed that the stability of these spines was decreased during the early phase and increased during the late phase after the lesion. We conclude that biphasic changes in spine loss rate and spine stability but not in the rate of spine formation play a central role in the reorganization of dentate granule cells after entorhinal denervation in vitro.     

Preservation of dendrites with the presence of reorganized mossy fiber collaterals in hippocampal dentate granule cells in patients with temporal lobe epilepsy

Dendritic morphology was studied in human hippocampal dentate granule cells (DGCs) by intracellularly-injecting biocytin in slice preparations that were obtained from temporal lobe epilepsy patients who underwent a surgical treatment for medically-intractable seizures. These DGCs had a fan-shaped dendritic domain of 54.1°±4.1 S.E.M. with 13.8±1.1 branch points and an estimated total dendritic length of 11535.6 μm±3045.4. Dendritic spines were counted, and spine density was calculated to be 0.25 spines/μm±0.16 S.E.M.. However, when the cells were categorized into two groups based on the presence or absence of the aberrant mossy fiber collaterals, the number of dendritic branches was significantly lower and spine density was significantly higher in DGCs that had aberrant collaterals. In particular, in the proximal dendrite, the spine density was 5 times higher in DGCs whose own mossy fibers were reorganized sending aberrant collaterals to this dendritic region (0.750 spines/μm±0.203 S.E.M.: P<0.01) than the DGCs without such collaterals (0.082 spines/μm±0.021 S.E.M.). These results suggest that the axonal reorganization may have an effect on the morphology of DGC dendrites directly or indirectly in such a way that dendritic structure and spines could be protected from seizure-induced excitotoxic cell damage.     

Selective localization of high concentrations of F-actin in subpopulations of dendritic spines in rat central nervous system: a three-dimensional electron microscopic study

Dendritic spines differ considerably in their size, shape, and internal organization between brain regions. We examined the actin cytoskeleton in dendritic spines in hippocampus (areas CA1, CA3, and dentate gyrus), neostriatum, and cerebellum at both light and electron microscopic levels by using a novel high-resolution photoconversion method based in the high affinity of phalloidin for filamentous (F)-actin. In all brain regions, labeling was strongest in the heads of dendritic spines, diminishing in the spine neck. The number of labeled spines varied by region. Compared with the cerebellar molecular layer and area CA3, where nearly every dendritic spine was labeled, less than half the spines were labeled in CA1, dentate gyrus, and neostriatum. Serial section reconstructions of spines in these areas indicated that phalloidin labeling was restricted to the largest and most morphologically diverse dendritic spines. The resolution of the photoconversion technique allowed us to examine the localization and organization of actin filaments in the spine. The most intense staining for actin was found in the postsynaptic density and associated with the spines internal membrane system. In mushroom-shaped spines, F-actin staining was particularly strong between the lamellae of the spine apparatus. Three-dimensional reconstruction of labeled spines by using electron tomography showed that the labeled dense material was in continuity with the postsynaptic density. These results highlight differences in the actin cytoskeleton between different spine populations and provide novel information on the organization of the actin cytoskeleton in vivo.     

Analysis of dendritic spines in rat CA1 pyramidal cells intracellularly filled with a fluorescent dye

The dendritic branching pattern and the distribution of dendritic spines were studied in hippocampal neurones with an improved technique. In slices taken from adult Wistar rats, CA1 pyramidal cells were filled with Lucifer yellow and examined under a laser-scanning confocal microscope. The basal dendrites were found evenly distributed inside a regular cupola-shaped volume. Their total length was about 4,500 μm. The branches divided between one and three times, with the initial segments comprising less than 2%, and the long terminal segments (mean length, 119 μm) including more than 80% of the total length of the basal dendrites. The apical dendritic branches emerged obliquely from the main shaft, ran for a distance of 50 to 250 μm, and made up a total length of about 5,100 μm in stratum radiatum and between 1,100 and 3,200 μm in stratum lacunosum-moleculare. The mean total length of the dendritic tree was 11,900 μm. All values were corrected for shrinkage. Shrinkage was measured in three dimensions and was 20.2% in the horizontal (x/y) plane and 40.9% in the vertical (z) plane. Both the basal and the apical dendritic branches were covered by regularly spaced spines. When corrected for dehydration-induced shrinkage and for hidden spines, the density was 1.80 and 2.00 spines/μm dendritic length for the basal and apical dendritic branches, respectively. Apart from the initial parts of the branches, which had few or no spines, the spines were remarkably evenly spaced. In particular, the distance between spine heads was significantly different from a random distribution, suggesting a regulatory process for the spacing of spines. © 1995 Wiley-Liss, Inc.     

Dendritic development of dentate granule cells in the absence of their specific extrinsic afferents

Dendrites and spines are postsynaptic structures that develop in association with presynaptic fibers. Recent studies have shown that granule cells of the fascia dentata survive in slice cultures and differentiate in a manner known from in situ studies. However, all extrinsic afferent fibers are absent under culture conditions. In the present study, we study whether dendrites and spines of granule cells in slice cultures differentiate normally, although they are not contacted by their normal layer-specific afferents. Slices of hippocampus were prepared from rat pups at the day of birth. After 5, 10, 15, and 20 days of incubation, granule cells in these cultures were Golgi impregnated. For comparison, perfusion-fixed hippocampal sections of 5-, 10-, 15-, and 20-day-old rats were impregnated the same way. Our results show that the total density of spines on granule cell dendrites in culture increased as in perfusion-fixed animals. However, after 20 days of incubation, the absolute number of dendritic spine on cultured neurons was reduced because of a reduction of peripheral dendrites. This reduction was accompanied by an increase in the number of stem dendrites originating from the perikaryon. The density of spines on these proximal dendrites was larger in cultured granule cells than in controls. Our results suggest that the lack of major extrinsic (entorhinal) afferents that normally terminate on peripheral granule cell dendrites causes retraction of these dendrites. At the same time, there is growth of paroximal dendritic portions. Proximal dendrites are targets of associational fibers, which are known to sprout under these culture conditions. © 1994 Wiley-Liss, Inc.