Dendritic Regrowth Research Articles

Insulin restores retinal ganglion cell functional connectivity and promotes visual recovery in glaucoma.

Dendrite pathology and synaptic loss result in neural circuit dysfunction, a common feature of neurodegenerative diseases. There is a lack of strategies that target dendritic and synaptic regeneration to promote neurorecovery. We show that daily human recombinant insulin eye drops stimulate retinal ganglion cell (RGC) dendrite and synapse regeneration during ocular hypertension, a risk factor to develop glaucoma. We demonstrate that the ribosomal protein p70S6 kinase (S6K) is essential for insulin-dependent dendritic regrowth. Furthermore, S6K phosphorylation of the stress-activated protein kinase-interacting protein 1 (SIN1), a link between the mammalian target of rapamycin complexes 1 and 2 (mTORC1/2), is required for insulin-induced dendritic regeneration. Using two-photon microscopy live retinal imaging, we show that insulin rescues single-RGC light-evoked calcium (Ca2+) dynamics. We further demonstrate that insulin enhances neuronal survival and retina-brain connectivity leading to improved optomotor reflex-elicited behaviors. Our data support that insulin is a compelling pro-regenerative strategy with potential clinical implications for the treatment and management of glaucoma.

A biologically inspired repair mechanism for neuronal reconstructions with a focus on human dendrites.

Investigating and modelling the functionality of human neurons remains challenging due to the technical limitations, resulting in scarce and incomplete 3D anatomical reconstructions. Here we used a morphological modelling approach based on optimal wiring to repair the parts of a dendritic morphology that were lost due to incomplete tissue samples. In Drosophila, where dendritic regrowth has been studied experimentally using laser ablation, we found that modelling the regrowth reproduced a bimodal distribution between regeneration of cut branches and invasion by neighbouring branches. Interestingly, our repair model followed growth rules similar to those for the generation of a new dendritic tree. To generalise the repair algorithm from Drosophila to mammalian neurons, we artificially sectioned reconstructed dendrites from mouse and human hippocampal pyramidal cell morphologies, and showed that the regrown dendrites were morphologically similar to the original ones. Furthermore, we were able to restore their electrophysiological functionality, as evidenced by the recovery of their firing behaviour. Importantly, we show that such repairs also apply to other neuron types including hippocampal granule cells and cerebellar Purkinje cells. We then extrapolated the repair to incomplete human CA1 pyramidal neurons, where the anatomical boundaries of the particular brain areas innervated by the neurons in question were known. Interestingly, the repair of incomplete human dendrites helped to simulate the recently observed increased synaptic thresholds for dendritic NMDA spikes in human versus mouse dendrites. To make the repair tool available to the neuroscience community, we have developed an intuitive and simple graphical user interface (GUI), which is available in the TREES toolbox (www.treestoolbox.org).

Optic nerve injury-induced regeneration in the adult zebrafish is accompanied by spatiotemporal changes in mitochondrial dynamics

Axonal regeneration in the central nervous system is an energy-intensive process. In contrast to mammals, adult zebrafish can functionally recover from neuronal injury. This raises the question of how zebrafish can cope with this high energy demand. We previously showed that in adult zebrafish, subjected to an optic nerve crush, an antagonistic axon-dendrite interplay exists wherein the retraction of retinal ganglion cell dendrites is a prerequisite for effective axonal repair. We postulate a 'dendrites for regeneration' paradigm that might be linked to intraneuronal mitochondrial reshuffling, as ganglion cells likely have insufficient resources to maintain dendrites and restore axons simultaneously. Here, we characterized both mitochondrial distribution and mitochondrial dynamics within the different ganglion cell compartments (dendrites, somas, and axons) during the regenerative process. Optic nerve crush resulted in a reduction of mitochondria in the dendrites during dendritic retraction, whereafter enlarged mitochondria appeared in the optic nerve/tract during axonal regrowth. Upon dendritic regrowth in the retina, mitochondrial density inside the retinal dendrites returned to baseline levels. Moreover, a transient increase in mitochondrial fission and biogenesis was observed in retinal ganglion cell somas after optic nerve damage. Taken together, these findings suggest that during optic nerve injury-induced regeneration, mitochondria shift from the dendrites to the axons and back again and that temporary changes in mitochondrial dynamics support axonal and dendritic regrowth after optic nerve crush.

Poststroke dendritic arbor regrowth requires the actin nucleator Cobl.

Ischemic stroke is a major cause of death and long-term disability. We demonstrate that middle cerebral artery occlusion (MCAO) in mice leads to a strong decline in dendritic arborization of penumbral neurons. These defects were subsequently repaired by an ipsilateral recovery process requiring the actin nucleator Cobl. Ischemic stroke and excitotoxicity, caused by calpain-mediated proteolysis, significantly reduced Cobl levels. In an apparently unique manner among excitotoxicity-affected proteins, this Cobl decline was rapidly restored by increased mRNA expression and Cobl then played a pivotal role in poststroke dendritic arbor repair in peri-infarct areas. In Cobl knockout (KO) mice, the dendritic repair window determined to span day 2 to 4 poststroke in wild-type (WT) strikingly passed without any dendritic regrowth. Instead, Cobl KO penumbral neurons of the primary motor cortex continued to show the dendritic impairments caused by stroke. Our results thereby highlight a powerful poststroke recovery process and identified causal molecular mechanisms critical during poststroke repair.

Descending motor circuitry required for NT-3 mediated locomotor recovery after spinal cord injury in mice

Locomotor function, mediated by lumbar neural circuitry, is modulated by descending spinal pathways. Spinal cord injury (SCI) interrupts descending projections and denervates lumbar motor neurons (MNs). We previously reported that retrogradely transported neurotrophin-3 (NT-3) to lumbar MNs attenuated SCI-induced lumbar MN dendritic atrophy and enabled functional recovery after a rostral thoracic contusion. Here we functionally dissected the role of descending neural pathways in response to NT-3-mediated recovery after a T9 contusive SCI in mice. We find that residual projections to lumbar MNs are required to produce leg movements after SCI. Next, we show that the spared descending propriospinal pathway, rather than other pathways (including the corticospinal, rubrospinal, serotonergic, and dopaminergic pathways), accounts for NT-3-enhanced recovery. Lastly, we show that NT-3 induced propriospino-MN circuit reorganization after the T9 contusion via promotion of dendritic regrowth rather than prevention of dendritic atrophy.

Striatonigral neurons divide into two distinct morphological-physiological phenotypes after chronic L-DOPA treatment in parkinsonian rats

Dendritic regression of striatal spiny projection neurons (SPNs) is a pathological hallmark of Parkinson’s disease (PD). Here we investigate how chronic dopamine denervation and dopamine replacement with L-DOPA affect the morphology and physiology of direct pathway SPNs (dSPNS) in the rat striatum. We used a lentiviral vector optimized for retrograde labeling (FuG-B-GFP) to identify dSPNs in rats with 6-hydroxydopamine (6-OHDA) lesions. Changes in morphology and physiology of dSPNs were assessed through a combination of patch-clamp recordings and two photon microscopy. The 6-OHDA lesion caused a significant reduction in dSPN dendritic complexity. Following chronic L-DOPA treatment, dSPNs segregated into two equal-sized clusters. One group (here called “cluster-1”), showed sustained dendritic atrophy and a partially normalized electrophysiological phenotype. The other one (“cluster-2”) exhibited dendritic regrowth and a strong reduction of intrinsic excitability. Interestingly, FosB/∆FosB induction by L-DOPA treatment occurred preferentially in cluster-2 dSPNs. Our study demonstrates the feasibility of retrograde FuG-B-GFP labeling to study dSPNs in the rat and reveals, for the first time, that a subgroup of dSPNs shows dendritic sprouting in response to chronic L-DOPA treatment. Investigating the mechanisms and significance of this response will greatly improve our understanding of the adaptations induced by dopamine replacement therapy in PD.

Mammalian dendritic regrowth: a new perspective on neural repair.

Mammalian dendritic regrowth: a new perspective on neural repair.

Successful survival in culture of spinal cord (SC) dorsal root ganglia (DRG) neurons and glia cells after automatic vitrification

To assess the survival and re-growth of neuronal cells after cryopreservation with a new automatic vitrification system. Experimental research with SC and DRG isolated from rat fetuses. Stationary organotypic DRG and SC cultures were prepared from rat fetuses (gestational day 15, Lewis inbred, Harlan, Israel). mmediately after dissection, the DRG and SC were cut into small slices (400 micron) and seeded in 12 well culture plates containing 1 mL of NVR-Gel. Cultures were enriched once in gel with GDNF conjugated iron oxide nanoparticles (10 ng/mL) and subsequently with nutrient medium at each consecutive feeding. Monitoring of the DRG-SC growth pattern was done by daily phase contrast microscopic observations 24 hours after setting the cultures onward and by IF staining. The various neuronal samples (SC, DRG, glia cells) were cryopreserved with a new device for automatic vitrification (Sarah®, Fertilesafe, IL), consisting of a special capsule containing EM gold grids attached to 0.25ml straws (IMV,France). The straws loaded with neuronal tissue were placed into the mixing chamber of the device attached to a syringe pump dispensing timed and gradual increasing concentrations of equilibrium solution for 12-25 min. and vitrification solutions for 3-5 min., before plunginginto liquid nitrogen slush (VitMaster, IMT IL). After 1 week the samples were rewarmed according to the Origio’s kit instructions. To assess survival, the cells recovered were fixed (4% paraformaldehyde), permeabilized with 0.1% Triton X-100 in PBS and then immunoblocked with a 1% BSA in PBS for 1 h at RT prior to double staining with mouse anti S-100 antibodies (1:80, Acris Antibodies, glial cell marker) and rabbit anti neurofilament antibodies (NF, Novus Biologicals, 1:500, neuronal cell marker). The primary antibodies were diluted in 0.1% BSA and 0.05% Tween-20 in PBS and incubated with the specimens overnight at 4 °C. After rinsing, the DRG specimens were incubated for 1 h at RT with fluorescent secondary antibodies. Organotypic cultures of SC, DRG neurons and glia cells demonstrated a remarkable survival as well as nerve fiber regeneration after cryopreservation/rewarming. The SC neurons maintained their multipolar shape with re-growth of dendrites and axons. The round shaped DRG neurons exhibited euchromatic nuclei with prominent nucleoli and an active regeneration of nerve processes. This is the first report on successful vitrification of SC cells, DRG neurons and glia cells using an automatic freezing device. The remarkable resumption of growth for neuronal cells will greatly expand regenerative research opportunities.

Normal Spastin Gene Dosage Is Specifically Required for Axon Regeneration

Axon regeneration allows neurons to repair circuits after trauma; however, most of the molecular players in this process remain to be identified. Given that microtubule rearrangements have been observed in injured neurons, we tested whether microtubule-severing proteins might play a role in axon regeneration. We found that axon regeneration is extremely sensitive to levels of the microtubule-severing protein spastin. Although microtubule behavior in uninjured neurons was not perturbed in animals heterozygous for a spastin null allele, axon regeneration was severely disrupted in this background. Two types of axon regeneration-regeneration of an axon from a dendrite after proximal axotomy and regeneration of an axon from the stump after distal axotomy-were defective in Drosophila with one mutant copy of the spastin gene. Other types of axon and dendrite outgrowth, including regrowth of dendrites after pruning, were normal in heterozygotes. We conclude that regenerative axon growth is uniquely sensitive to spastin gene dosage.

Use-dependent dendritic regrowth is limited after unilateral controlled cortical impact to the forelimb sensorimotor cortex.

Compensatory neural plasticity occurs in both hemispheres following unilateral cortical damage incurred by seizures, stroke, and focal lesions. Plasticity is thought to play a role in recovery of function, and is important for the utility of rehabilitation strategies. Such effects have not been well described in models of traumatic brain injury (TBI). We examined changes in immunoreactivity for neural structural and plasticity-relevant proteins in the area surrounding a controlled cortical impact (CCI) to the forelimb sensorimotor cortex (FL-SMC), and in the contralateral homotopic cortex over time (3-28 days). CCI resulted in considerable motor deficits in the forelimb contralateral to injury, and increased reliance on the ipsilateral forelimb. The density of dendritic processes, visualized with immunostaining for microtubule-associated protein-2 (MAP-2), were bilaterally decreased at all time points. Synaptophysin (SYN) immunoreactivity increased transiently in the injured hemisphere, but this reflected an atypical labeling pattern, and it was unchanged in the contralateral hemisphere compared to uninjured controls. The lack of compensatory neuronal structural plasticity in the contralateral homotopic cortex, despite behavioral asymmetries, is in contrast to previous findings in stroke models. In the cortex surrounding the injury (but not the contralateral cortex), decreases in dendrites were accompanied by neurodegeneration, as indicated by Fluoro-Jade B (FJB) staining, and increased expression of the growth-inhibitory protein Nogo-A. These studies indicate that, following unilateral CCI, the cortex undergoes neuronal structural degradation in both hemispheres out to 28 days post-injury, which may be indicative of compromised compensatory plasticity. This is likely to be an important consideration in designing therapeutic strategies aimed at enhancing plasticity following TBI.

The steroid hormone-regulated geneBroad Complex is required for dendritic growth of motoneurons during metamorphosis ofDrosophila

Dendrites are subject to subtle modifications as well as extensive remodeling during the assembly and maturation of neural circuits in a wide variety of organisms. During metamorphosis, Drosophila flight motoneurons MN1-MN4 undergo dendritic regression, followed by regrowth, whereas MN5 differentiates de novo (Consoulas et al. [2002] J. Neurosci. 22:4906-4917). Many cellular changes during metamorphosis are triggered and orchestrated by the steroid hormone 20-hydroxyecdysone, which initiates a cascade of coordinated gene expression. Broad Complex (BRC), a primary response gene in the ecdysone cascade, encodes a family of transcription factors (BRC-Z1-Z4) that are essential for metamorphic reorganization of the central nervous system (CNS). Using neuron-filling techniques that reveal cellular morphology with very high resolution, we tested the hypothesis that BRC is required for metamorphic development of MN1-MN5. Through a combination of loss-of-function mutant analyses, genetic mapping, and transgenic rescue experiments, we found that 2Bc function, mediated by BRC-Z3, is required selectively for motoneuron dendritic regrowth (MN1-MN4) and de novo outgrowth (MN5), as well as for soma expansion of MN5. In contrast, larval development and dendritic regression of MN1-MN4 are BRC-independent. Surprisingly, BRC proteins are not expressed in the motoneurons, suggesting that BRC-Z3 exerts its effect in a non-cell-autonomous manner. The 2Bc mutants display no gross defects in overall thoracic CNS structure, or in peripheral structures such as target muscles or sensory neurons. Candidates for mediating the effect of BRC-Z3 on dendritic growth of MN1-MN5 include their synaptic inputs and non-neuronal CNS cells that interact with them through direct contact or diffusible factors.

Catecholamine enzymes and neuropeptides are expressed in fibres and somata in the intermediate gray matter in chronic spinal rats

Spinal cord injury disrupts control of sympathetic preganglionic neurons because bulbospinal input has been lost and the remaining regulation is accomplished by spinal circuits consisting of dorsal root afferent and spinal neurons. Moreover, an initial retraction and regrowth of dendrites of preganglionic neurons in response to deafferentation creates the potential for remodelling of spinal circuits that control them. Although catecholamines and neuropeptide Y are found in descending inputs to the preganglionic neurons, their presence in spinal circuits has not been established. Spinal circuits controlling preganglionic neurons contain substance P but participation of these peptidergic neurons in remodelling responses has not been examined. Therefore, we compared immunoreactivity for the catecholamine-synthesizing enzyme dopamine β-hydroxylase, for neuropeptide Y and for substance P in the intermediate gray matter of the spinal cord in control rats and in rats seven or fourteen days after transection at the fourth thoracic cord segment. Sympathetic preganglionic neurons were retrogradely labelled by intraperitoneal injection of the tracer FluoroGold. These experiments yielded three original findings. 1) At one and two weeks after cord transection, fibres and terminals immunoreactive for dopamine β-hydroxylase and neuropeptide Y were consistently found in the intermediolateral cell column in segments caudal to the transection. The area of fibres and terminals containing these immunoreactivities was markedly reduced compared to control rats or to segments rostral to the transection in the spinal rats. 2) Immunoreactivity for substance P was increased after cord transection and the distribution of fibres immunoreactive for this peptide in segments caudal to the transection extended more widely through the intermediate gray matter. These reactions demonstrated a plastic reaction to cord transection by spinal neurons expressing substance P. 3) Dopamine β-hydroxylase expression was up-regulated in somata within the intermediate gray matter of spinal segments caudal to the transection. The numbers of somata immunoreactive for this enzyme increased six-fold by fourteen days after cord transection, compared to the few somata counted in control rats.In conclusion, the presence of a catecholamine synthesizing enzyme and neuropeptides in fibres surrounding sympathetic preganglionic neurons caudal to a cord transection suggests a source of catecholamines and these peptides within spinal circuits in the chronic spinal rat. The presence of dopamine β-hydroxylase in a markedly greater number of neuronal somata after cord transection reflects significant up-regulation of gene expression and may indicate a switch by these neurons to an adrenergic phenotype, revealing a plastic response to injury within the spinal cord.

Protein kinase C may be involved in synaptic repair of auditory neuron dendrites after AMPA injury in the cochlea

A suitable model of sudden deafness occurring after acoustic trauma or ischemia, is obtained in guinea pigs by an acute intracochlear perfusion of 200 μM α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), a glutamate analog. By overloading the AMPA/kainate receptors, located post-synaptically to inner hair cells (IHCs), it induces a massive swelling of primary auditory neuron dendrites, which disconnects the IHCs. This synaptic uncoupling and the resulting hearing loss are followed by a progressive regrowth of dendrites, which make new synapses with IHCs, leading to a functional recovery of auditory responses that is completed after 5 days. Knowing the role of protein kinase C in neuroplstic events, we studied the expression of its isoforms α, β 1.11 and γ, respectively pre- and post-synaptic, in auditory neurons at various times after AMPA administration. In untreated cochleas, we observed an expression of PKC α, β 1.11 and γ in cell bodies of primary auditory neurons. After the intracochlear administration of AMPA, both isozymes were transiently overexpressed, with a peak at 3–6 h, followed by a decrease after about 24 h. At this point in time immuno-electron microscopy revealed some regrowing dendrites immunoreactive for PKCγ. Five days after AMPA, when the auditory responses were restored, PKCγ levels were still elevated in ganglion cell bodies.

Number and dendritic morphology of retinal ganglion cells that survived after axotomy in adult cats

Retinal ganglion cells (RGCs) of adult cats were labeled by injection of diI into the proximal stump of completely transected optic nerves. Approximately 2% to 5% of the RGC population appeared viable 2 months after these axotomies, based on diI retention. The morphological type and dendritic arbor of these surviving RGCs were examined after intracellular injections of Lucifer Yellow into diI-labeled RGCs. Postaxotomy survival rate was much higher for alpha-like cells than for beta-like cells. However, in one of four retinas examined, a large number of RGCs seemed to survive axotomy, and among these, beta cells survived at an unusually high rate. Dendritic arbors of surviving RGCs were also examined after intracellular injection of horseradish peroxidase. Some dendrites of these RGCs lacked branches and were thin in caliber. Other dendrites displayed many spiny processes and bulbous swellings. Essentially, these results confirm the previous suggestion that alpha cells survive axotomy longer than beta cells. The ability of alpha cells to regenerate axons may thus be attributable to their relatively high resistance to axotomy. The atypical dendritic profiles seen after optic nerve transection may reflect either degeneration or regrowth of dendrites.

Alterations in synaptic connectivity in cerebral cortex in neuronal storage disorders

AbstractNeuronal storage diseases are inborn errors of metabolism characterized by widespread intraneuronal storage and progressive neurological dysfunction, including mental retardation, movement disorders, and seizures. Most of these disorders are associated with specific defects in particular lysosomal enzymes. Moreover, a wide array of genetic defects has been documented. The immense progress made during the past three decades in understanding the primary molecular and enzymatic defects in this family of diseases stands in sharp contrast to the modest advances made in delineating pathogenetic mechanisms and treatment strategies. Although these disorders result from highly selective defects in metabolic pathways, they ultimately are complex conditions characterized by a plethora of seemingly unrelated secondary molecular and cellular abnormalities. Studies of children and animals affected by these diseases have revealed that significant changes in neuronal connectivity occur in the cerebral cortex. These include not only degenerative changes in axons and synapses of inhibitory neurons, but also a regrowth of dendrites and new synapse formation involving pyramidal neurons. The latter finding indicates that neurons in the cerebral cortex undergo significant “rewiring” during the height of the disease process. This regrowth of primary dendrites and formation of new synapses as part of a neurological disease is unprecedented. Understanding the causes and consequences of these changes in cortical synaptic connectivity in neuronal storage diseases may provide insight not only into the precise mechanisms underlying mental retardation in these diseases and their reversibility, but also into processes governing dendritic elaboration and synapse formation in the normal developing nervous system. © 1995 Wiley‐Liss, Inc.

Effects of the steroid hormone, 20-hydroxyecdysone, on the growth of neurites by identified insect motoneurons in vitro

During metamorphosis in the hawkmoth, Manduca sexta, identified larval leg motoneurons survive the degeneration of their larval targets to innervate new muscles of the adult legs. The dendrites and axon terminals of these motoneurons regress at the end of the larval stage and then regrow during adult development. Previous studies have implicated the insect steroid, 20-hydroxyecdysone (20-HE), in similar examples of dendritic reorganization during metamorphosis. The present studies were undertaken to test whether 20-HE acts directly on the leg motoneurons to regulate dendritic growth. Larval leg motoneurons were labeled with a fluorescent dye to permit their identification in culture following the dissociation of thoracic ganglia at later stages of development. Leg motoneurons isolated from early pupal stage animals (just before the normal onset of dendritic regrowth) survived in vitro and grew processes regardless of whether 20-HE was added to the culture medium. The extent of process outgrowth, however, as measured by the total length of all processes and the number of branches, was significantly greater for motoneurons maintained in the presence of 20-HE. The enhancement could be blocked by the addition of a juvenile hormone analog. By contrast, larval leg motoneurons that were isolated just before the normal period of dendritic regression did not show enhanced growth of neurities in the presence of 20-HE. The results suggest that 20-HE acts directly on the leg motoneurons to regulate the growth of processes during metamorphosis.

Dendritic regrowth of retinal ganglion cells in adult rats.

IN adult rat retinal ganglion cells, we investigated how their dendrites are maintained during axonal regeneration induced by a peripheral nerve (PN) graft. Intracellular Lucifer Yellow injection revealed that rapid dendritic retraction occurred immediately after axotomy and grafting. However, dendrites that probably sprouted after axotomy were often found in the regenerating ganglion cells, and their dendritic field tended to recover in area by 3 months after grafting. Although such dendrites were also found in an axotomized cell receiving no PN graft, they are much more frequently observed in the regenerating cells. Therefore, a PN graft can facilitate the rat ganglion cells to regrow their dendrites.

Evidence for nonrandom regression of dendrites of Purkinje neurons during aging

This study examined dendritic networks of Purkinje neurons for randomness of age-related changes in cerebella from Fischer 344 rats at three ages. Terminal dendritic segments were studied in relation to their distance from the neuronal soma (path length). The data indicated that the terminal dendritic segments furthest from the cell body were preferentially affected during aging and there was a redistribution of the remaining terminal segments in cells from rats of the intermediate age group. The data also suggested that dendritic regrowth occurred in proximal regions of networks from rats in the oldest group.

Granule cell loss and dendritic regrowth in the hippocampal dentate gyrus of the rat after chronic alcohol consumption

The effects of chronic alcohol consumption (CAC) on the relative number of dentate gyrus granule cells and their dendritic trees, were studied in animals fed alcohol for 6, 12 and 18 months and in their respective controls. The granule cell density was estimated with the unbiased disector method. Following 6 months of alcohol consumption, the thickness of the dentate gyrus granular layer and the relative number of dentate granule cells were significantly decreased when compared with controls. The granule cell dendritic arborizations showed an increase of their dendritic extent in alcohol-treated rats. No significant differences were found in the density of dendritic spines between alcohol-fed and control animals. These results indicate the existence of hippocampal granule cell dendritic re-growth in alcohol-fed rats, probably occurring as a compensatory response to the granule cell deficit which follows the alcohol-induced granule cell degeneration. These degenerative and regenerative changes might have functional implications for the organization of the synaptic hippocampal circuitry after long periods of alcohol consumption.

Morphometric and dendritic analysis of fascia dentata granule cells in human aging and senile dementia

In this study the cellular morphology in the human fascia dentata of 5 very old demented cases (4 Alzheimer's disease and 1 multi-infarct dementia patients) was compared with 5 (very) old control cases. The postmortem delay in fixation was for all cases within 3.5 h. In the demented group, a significant reduction in thickness of the molecular layer, density of dendritic spines in the middle third of the molecular layer and total dendritic length (±30%) was found. The number of dendritic segments, indicative of the branching frequency showed no difference. In both the control and the demented group, three-quarters of all dendritic bifurcations of granule cells occurred in the inner third of the molecular layer in which the commissural and associational fibers terminate. The size of the dendrites in the demented group could be the result of at least 3 independent processes: (a) a regressive change due to partial denervation of the outer two-thirds of the molecular layer as axons from the perforant pathway are lost; (b) a dendritic regrowth in response to sprouting of the commissural-associational fiber systems and septal afferents, which is presumed to occur in response to degeneration of perforant path axons; (c) a dendritic regrowth in response to the loss of the dendrites of neighboring cells which have died. Analysis of our material suggests that dendritic degeneration is the predominant factor in the demented group.