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Synaptogenesis

Synaptogenesis is the process by which synapses—the specialized junctions facilitating chemical or electrical communication between neurons—are formed and maintained, serving as the foundational units for neural circuit assembly and information processing in the brain.^[1] This dynamic process begins early in embryonic development, around 8–12 weeks of gestation in humans, and involves the precise alignment of presynaptic and postsynaptic elements to establish functional connectivity.^[2] Synaptogenesis peaks during early infancy, with synaptic density reaching maximum levels by about 2 years of age in many brain regions, followed by a period of synaptic pruning that refines circuits by eliminating approximately 50% of synapses by adolescence.^[3] At the molecular level, synaptogenesis is orchestrated by a cascade of interactions involving cell adhesion molecules, such as neurexins and neuroligins, which bridge pre- and postsynaptic membranes to initiate contact and stabilize nascent synapses.^[4] Scaffold proteins like PSD-95 and liprin-α then recruit additional components to organize the postsynaptic density and presynaptic active zone, while cytoskeletal elements, particularly actin, provide structural support for synaptic maturation.^[4] Neural activity plays a pivotal role in refining these connections through calcium-dependent signaling pathways, including those mediated by CaMKII and neurotrophic factors like BDNF, which promote synapse strengthening and elimination based on use-dependent principles.^[4] Although most prolific during development, synaptogenesis persists into adulthood, contributing to neural plasticity in response to experience or injury.^[1] The timing and extent of synaptogenesis vary across brain regions, with peaks occurring later in areas associated with higher cognitive functions, such as the prefrontal cortex at around 3.5 years, compared to sensory regions like the visual cortex at 8–12 months.^[3] Disruptions in this process, often linked to genetic mutations in adhesion or scaffold proteins, are implicated in neurodevelopmental disorders including autism spectrum disorder and schizophrenia, underscoring its critical role in cognitive and behavioral development.^[4] By enabling the brain's intricate wiring, synaptogenesis not only supports learning and memory but also underpins the adaptability that allows organisms to respond to environmental demands throughout life.^[1]

Overview

Definition and Process

Synaptogenesis is the process by which synapses form between neurons or between neurons and target cells, creating specialized junctions that facilitate communication through the release of neurotransmitters from the presynaptic terminal across a narrow synaptic cleft to receptors in the postsynaptic density.^[5] These synapses serve as the fundamental units of neural circuits, with the presynaptic terminal housing synaptic vesicles containing neurotransmitters, the synaptic cleft measuring approximately 20-40 nm in width, and the postsynaptic density comprising a protein-rich scaffold that anchors receptors and signaling molecules.^[6] This formation is essential for establishing functional connectivity in the nervous system during development.^[4] The process unfolds in distinct stages, beginning with axon growth, where extending axons tipped by growth cones navigate toward potential synaptic partners at rates of about 1 mm per day, guided by extracellular cues.^[6] This is followed by target recognition, in which growth cones identify appropriate postsynaptic partners through specific interactions that halt extension and initiate contact.^[7] Subsequent differentiation involves the coordinated assembly of pre- and postsynaptic specializations, where the presynaptic site develops active zones for vesicle docking and release, while the postsynaptic site forms densities with clustered receptors.^[6] Finally, stabilization occurs as nascent synapses mature and persist, influenced by activity-dependent mechanisms that strengthen functional connections.^[7] Synaptogenesis peaks during embryonic and early postnatal periods, with timelines varying by species and brain region. In rodents, such as rats and mice, intense synaptogenesis begins around embryonic day 18 (E18) and continues robustly into the postnatal period, reaching a peak density around postnatal days 24-30 before subsequent refinement.^[8] In humans, the process initiates in the third trimester of gestation and accelerates postnatally, with synaptic density peaking around ages 2-5 years in cortical areas, followed by extension and selective pruning into adolescence and early adulthood.^[9]^[8]

Biological Significance

Synaptogenesis plays a pivotal role in the assembly of neural circuits, enabling the precise wiring of neurons to form complex networks essential for sensory processing, motor control, and higher cognitive functions. During development, this process establishes the foundational connectivity that allows for the integration of sensory inputs, coordination of motor outputs, and emergence of cognitive capabilities, such as pattern recognition and decision-making. Disruptions in circuit assembly due to aberrant synaptogenesis can lead to widespread functional deficits across these domains.^[10] The biological importance of synaptogenesis extends to synaptic plasticity, serving as the structural basis for learning and memory formation. By facilitating the addition, strengthening, and remodeling of synapses in response to activity, it underpins mechanisms like long-term potentiation, which are critical for encoding experiences and adapting neural responses over time. This developmental groundwork ensures that mature circuits retain the capacity for dynamic changes, supporting behavioral flexibility throughout life. Dysregulation of synaptogenesis has profound pathological implications, contributing to both neurodevelopmental disorders and neurodegeneration. In conditions like autism spectrum disorder, mutations in synaptic adhesion molecules such as neurexins impair synapse formation and stability, leading to altered excitatory-inhibitory balance and social-cognitive deficits. Similarly, in Alzheimer's disease, progressive synapse loss—often exceeding neuronal death—correlates strongly with cognitive decline, highlighting synaptogenesis failure as a key driver of disease progression.^[11]^[12] Mechanisms of synaptogenesis exhibit strong evolutionary conservation across vertebrates, with core molecular pathways shared from fish to mammals, though increasing circuit complexity in higher species reflects adaptations in regulatory elements. This conservation underscores the fundamental role of synaptogenesis in nervous system evolution, allowing for scalable neural architectures that support diverse behavioral repertoires.^[13]

Developmental Mechanisms

Exuberant Synaptogenesis and Pruning

During early brain development, neurons generate an excess of synaptic connections, a phase known as exuberant synaptogenesis, which provides a surplus of potential circuits for refinement. This overproduction typically results in 2 times more synapses than ultimately required in adulthood, allowing for flexibility in establishing functional neural networks. In humans, synaptic density in the visual cortex peaks during critical periods from birth to around 8 months postnatal, with rapid formation followed by selective stabilization.^[14]^[15] Synaptic pruning then eliminates superfluous connections, reducing density by 40-50% to achieve mature circuitry. This process is driven by activity-dependent competition, where more active synapses are strengthened via mechanisms like long-term potentiation, while underused ones weaken and are retracted. Microglia play a key role in this elimination, phagocytosing weak synaptic elements through complement-dependent signaling, such as the C1q-C3-CR3 pathway, ensuring precise refinement based on neural activity patterns.^[16]^[17] A prominent example occurs in the retinogeniculate projections of the visual system, where retinal ganglion cell axons initially form widespread, overlapping connections in the lateral geniculate nucleus. Spontaneous retinal waves drive early segregation into eye-specific territories before eye opening, followed by visual experience-dependent pruning that refines dendritic spines on relay neurons, eliminating inappropriate synapses to sharpen visual processing. In mammals like macaques, overall synapse density in the primary visual cortex increases approximately 4-fold postnatally to a peak of about 90 synapses per 100 μm³ of neuropil around the third postnatal month, then declines to adult levels of 40-50 synapses per 100 μm³ by puberty, primarily through loss of asymmetric spine synapses.^[16]^[17]^[18]

Guidance Cues and Signaling Molecules

Growth cones, the motile tips of extending axons, play a central role in sensing environmental cues to guide pathfinding during synaptogenesis. These dynamic structures feature filopodia and lamellipodia that detect attractive and repulsive signals, enabling precise navigation toward target regions in the developing nervous system. Key guidance cues include netrins, which can mediate both attraction and repulsion depending on the cellular context and co-receptor expression. For instance, netrin-1 promotes axon attraction via DCC receptors while inducing repulsion through UNC-5 receptors, facilitating commissural axon crossing at the midline. Semaphorins primarily exert repulsive effects, with semaphorin 3A signaling through neuropilin-1 and plexin-A receptors to steer axons away from inhibitory zones, such as in the optic nerve's avoidance of the midline. Slit proteins, acting via Robo receptors, provide midline repulsion in vertebrates and invertebrates, ensuring proper axonal trajectories across the central nervous system. Transcription factors like UNC-4 contribute to specifying motor neuron connectivity by regulating gene expression that influences guidance cue responsiveness. In C. elegans, UNC-4 directs ventral nerve cord motor neurons to form appropriate synapses by repressing genes that would otherwise misroute axons. This function is conserved in vertebrates, where homologs such as UNCX contribute to patterning spinal motor neuron columns for limb innervation.^[19] Downstream signaling cascades, particularly involving Rho GTPases, transduce these cues to regulate cytoskeletal dynamics within growth cones. RhoA activation promotes actomyosin contraction leading to cone collapse in response to repellents like semaphorins, while Rac1 and Cdc42 drive actin polymerization for protrusion and advance toward attractants such as netrins. These GTPases integrate signals via effectors like ROCK and PAK, ensuring adaptive responses that refine initial synaptic targeting before later pruning eliminates transient projections.

Molecular Components

Synaptic Adhesion Molecules

Synaptic adhesion molecules (SAMs) are trans-synaptic cell surface proteins that physically bridge the presynaptic active zone and the postsynaptic density, facilitating the alignment and stabilization of synaptic junctions during synaptogenesis. These molecules are highly enriched at synaptic specializations, with presynaptic components such as neurexins localized to active zones where they interact with the synaptic vesicle release machinery, and postsynaptic counterparts like neuroligins concentrated in the postsynaptic density (PSD) to recruit scaffolding proteins and receptors. Beyond mechanical adhesion, SAMs initiate bidirectional signaling cascades that coordinate pre- and postsynaptic differentiation, ensuring the formation of functional synapses.^[20] SAMs are classified into several families based on their extracellular domains and binding properties. The neurexin-neuroligin complex represents a key heterophilic adhesion system, where presynaptic neurexins (encoded by three genes, Nrxn1-3, producing α and β isoforms) bind to postsynaptic neuroligins (Nlgn1-4) in a calcium-dependent manner; this interaction promotes the recruitment of presynaptic vesicle proteins and postsynaptic PSD scaffolds like PSD-95.^[21] Another prominent class involves Eph receptor tyrosine kinases and their ephrin ligands, particularly EphBs and ephrin-Bs, which form bidirectional signaling pairs across the synaptic cleft; EphBs on the postsynaptic side cluster NMDA receptors and drive actin cytoskeletal rearrangements via Rho GTPases, while reverse signaling through ephrin-Bs modulates presynaptic organization.^[22] Immunoglobulin superfamily members, such as synapse adhesion-like molecules (SALMs), contribute to specificity by regulating excitatory synapse assembly; for instance, SALM2 interacts with presynaptic leucine-rich repeat proteins to cluster PSD-95 and AMPA receptors postsynaptically.^[23] Cadherins, including N-cadherin, provide homophilic adhesion through calcium-dependent extracellular cadherin repeats linked to the cytoskeleton via catenins, stabilizing nascent synapses and influencing spine morphology without directly inducing presynaptic differentiation.^[24] These adhesion molecules exhibit remarkable evolutionary conservation across bilaterian species, underscoring their fundamental role in synapse formation from invertebrates like C. elegans to mammals. Neurexins, in particular, generate extensive isoform diversity through alternative splicing at multiple sites (e.g., six canonical sites, SS#1-6), yielding over 2,000 potential combinations from the three genes, which enables fine-tuned synaptic specificity and connectivity patterns in the vertebrate brain.^[25] This splicing variability is conserved evolutionarily, with similar mechanisms observed in rodents and primates, highlighting how SAMs adapt to the complexity of neural circuits.^[26]

Contributions of Wnt Proteins

Wnt proteins are a family of secreted glycoproteins that play crucial roles in synaptogenesis by binding to Frizzled (Fzd) receptors and co-receptors such as low-density lipoprotein receptor-related protein 5/6 (LRP5/6), thereby activating either the canonical pathway involving β-catenin stabilization and transcriptional regulation or non-canonical pathways, including planar cell polarity (PCP) and Wnt/Ca²⁺ signaling.^[27] In the canonical pathway, Wnt binding inhibits the destruction complex (comprising Axin, APC, GSK3β, and CK1), allowing β-catenin to accumulate, translocate to the nucleus, and co-activate transcription factors like TCF/LEF to promote gene expression essential for synaptic differentiation.^[28] Non-canonical pathways, in contrast, modulate cytoskeletal dynamics and calcium release independently of β-catenin, influencing cell polarity and local signaling at synaptic sites.^[29] In the central nervous system (CNS), Wnt proteins drive presynaptic and postsynaptic assembly. Specifically, Wnt7a, expressed by granule cells in the hippocampus and cerebellum, induces presynaptic clustering of active zone proteins such as Bassoon and Piccolo, facilitating the organization of synaptic vesicle release machinery. This process occurs through activation of the canonical pathway via Fzd5 and Dishevelled, leading to enhanced accumulation of synaptic components at nascent synapses without requiring transcriptional changes.^[30] Postsynaptically, Frizzled receptors mediate dendrite morphogenesis; for instance, Wnt5a signaling through Fzd4 promotes dendrite branching and growth in cortical neurons by engaging the PCP pathway to regulate actin cytoskeleton dynamics via the distal PDZ motif of Fzd4.^[31] Similarly, Wnt7b-Fzd7 signaling co-activates CaMKII and JNK pathways to increase dendritic arbor complexity in hippocampal neurons.^[32] At the neuromuscular junction (NMJ), Wnt proteins collaborate with agrin to stabilize postsynaptic acetylcholine receptor (AChR) clusters. Wnt3, secreted by motor neurons, enhances agrin-induced AChR aggregation in cultured myotubes by increasing cluster size and number through a non-canonical pathway involving PKC and CamKII, independent of agrin but synergistic with it.^[33] This interaction stabilizes nascent AChR clusters during early NMJ formation. Additionally, β-catenin contributes to postsynaptic maturation by regulating AChR clustering and dispersion; conditional knockout of β-catenin in skeletal muscle reduces AChR cluster density and impairs NMJ maturation, while stabilization promotes proper postsynaptic differentiation.^[34] In muscle-specific β-catenin gain-of-function models, excessive signaling disrupts AChR organization, highlighting the need for precise β-catenin levels in NMJ stability.^[35] Experimental evidence from knockout studies underscores these roles. In Wnt7a-deficient mice, presynaptic differentiation is delayed, with reduced clustering of synaptic proteins and immature glomerular rosettes in the cerebellar mossy fiber pathway.^[36] Similarly, Fzd5 knockdown in hippocampal cultures diminishes presynaptic assembly, with approximately 23% fewer Bassoon-positive puncta and impaired synaptic function.^[30] These findings demonstrate that Wnt signaling is indispensable for achieving normal synapse numbers during development.

Peripheral Synapse Formation

Neuromuscular Junction Development

The neuromuscular junction (NMJ) forms through coordinated interactions among motor neuron axons originating from the ventral spinal cord, Schwann cells derived from neural crest progenitors, and skeletal muscle fibers arising from somitic mesoderm.^[37]^[38]^[39] Motor neurons emerge from progenitor domains in the ventral neural tube of the developing spinal cord, extending axons peripherally to innervate target muscles, while Schwann cells migrate along these axons from their neural crest origins to support guidance and ensheathment.^[37]^[38] Skeletal muscle fibers develop from myogenic precursors in the somites, which segment along the embryonic axis and differentiate into multinucleated myofibers capable of contraction.^[39] Motor neuron axons navigate to limb and body wall muscles via guidance cues, including attractant netrins and repellent semaphorins, which direct pathfinding from the spinal cord to precise target regions.^[40] Upon reaching the muscle, axons initially form multiple contacts, resulting in polyinnervation where several axons converge on a single endplate, ensuring robust early innervation before refinement.00290-1) Schwann cells accompany the axons, extending processes to stabilize initial contacts and promote synapse formation at the muscle midbelly.^[41] Postsynaptic specialization begins with clustering of acetylcholine receptors (AChRs) at the endplate, driven by agrin secreted from the motor neuron axon, which binds to low-density lipoprotein receptor-related protein 4 (LRP4) on the muscle surface.^[42] This interaction activates muscle-specific kinase (MuSK), triggering downstream signaling that recruits rapsyn, an intracellular scaffold protein essential for stabilizing and aggregating AChRs into high-density clusters.^[42] The agrin-MuSK-LRP4 pathway ensures precise alignment of postsynaptic sites with incoming axons, forming the characteristic pretzel-shaped endplate.^[42] Presynaptic differentiation involves the assembly of active zones for neurotransmitter release, regulated retrogradely by muscle β-catenin, which influences motor neuron terminal branching and active zone organization.^[43] Synaptotagmin, a calcium-binding protein on synaptic vesicles, serves as the primary sensor for calcium influx during action potentials, enabling rapid and synchronized acetylcholine release to activate postsynaptic AChRs. These components establish efficient transmission, with terminal Schwann cells further modulating presynaptic maturation by covering axon terminals.^[41] During NMJ maturation, the switch from gamma to epsilon subunits in AChRs contributes to repressing extrasynaptic receptor expression, refining the junction to its mature form.^[44]

Specificity and Maturation Processes

During the postnatal period, the neuromuscular junction (NMJ) undergoes synapse elimination, where excess motor axons withdraw from each endplate in an activity-dependent manner. In mice, each muscle endplate is initially innervated by 3-5 axons at birth, which compete for postsynaptic territory; by the end of the second to third postnatal week, this polyaxonal innervation is refined to a single axon per endplate, ensuring precise motor control.^[45] This process is driven by differential activity patterns among competing axons, with more active inputs strengthening their connections while less active ones retract, often involving terminal Schwann cells that phagocytose weaker terminals.^[45] Synaptic specificity at the NMJ is achieved through molecular cues that repel inappropriate axons and promote competition among suitable ones. Ephrin-A ligands, expressed in a rostrocaudal gradient on developing limb muscles, activate EphA receptors on motor axons to mediate forward repulsive signaling, preventing mismatched innervation; for instance, caudal axons are more sensitive to ephrin-A5 repulsion, ensuring topographic mapping.^[46] Additionally, brain-derived neurotrophic factor (BDNF) and its receptor TrkB facilitate competitive elimination by enhancing synaptic strength in active axons while promoting retraction of weaker competitors through retrograde signaling from muscle to presynaptic terminals.^[47] Functional maturation of the NMJ involves structural and molecular refinements that enhance transmission efficiency. Postnatally, the endplate develops deep postjunctional folds in the sarcolemma, increasing the postsynaptic surface area and concentrating acetylcholine receptors (AChRs) at fold crests while sodium channels localize to depths, thereby amplifying the safety factor for action potential generation.^[48] Concurrently, there is a switch in AChR subunit composition from the fetal γ-containing form (α₂βγδ) to the adult ε-containing form (α₂βεδ) around postnatal days 5-9 in mice, which alters channel kinetics for faster desensitization and more reliable synaptic responses.^[49] Wnt proteins contribute to this maturation by stabilizing AChR clusters and promoting presynaptic differentiation.^[50] Overall synaptic strength increases approximately 10-fold during this period, primarily due to a rise in quantal content—the number of acetylcholine quanta released per action potential—compensating for initial low release probability and supporting robust neuromuscular transmission.^[51]

Central Synapse Formation

Regulatory Pathways

Transcriptional regulators play a pivotal role in controlling the expression of genes essential for central synapse assembly. The transcription factor CREB (cAMP response element-binding protein) is activated through phosphorylation downstream of NMDA receptor signaling, which triggers calcium influx and initiates the expression of synapse-specific genes such as those encoding synaptic proteins and structural components.^[52] This activity-induced pathway ensures that synaptogenesis aligns with neuronal maturation and environmental cues during central nervous system development. Similarly, MeCP2 (methyl-CpG-binding protein 2) acts as a transcriptional modulator by binding to methylated DNA regions, repressing or activating genes involved in synaptic connectivity, including BDNF, thereby fine-tuning synapse number and function in excitatory circuits.^[53] Mutations in MeCP2, as seen in Rett syndrome, disrupt this regulation, leading to impaired synaptic gene expression and reduced synapse density.^[54] Glial cells, particularly astrocytes, provide extrinsic signals that orchestrate excitatory synapse formation in the central nervous system. Astrocytes secrete thrombospondins (TSPs), a family of extracellular matrix glycoproteins, which bind to neuronal α2δ-1 subunits on the presynaptic membrane to promote the clustering of synaptic vesicles and active zone proteins.^[55] This interaction specifically induces the formation of functional excitatory synapses without affecting inhibitory ones, highlighting the role of astrocytic factors in establishing excitatory-inhibitory balance during synaptogenesis.^[56] In thrombospondin-deficient models, synapse density is markedly reduced, underscoring their necessity for baseline excitatory synaptogenesis in cortical and hippocampal regions.^[57] Morphogen gradients further regulate the spatial and temporal aspects of central synapse assembly by influencing dendritic and presynaptic differentiation. Bone morphogenetic proteins (BMPs), such as BMP-7, act through Smad signaling to induce dendritic growth and maturation in hippocampal neurons, enhancing postsynaptic sites for excitatory inputs.^[58] This process involves BMP-mediated activation of pathways that promote actin cytoskeleton reorganization, directly linking morphogen exposure to spine morphogenesis and synaptogenic potential. In parallel, fibroblast growth factors (FGFs), particularly FGF22, serve as target-derived signals that drive presynaptic differentiation by binding to FGFR1b and FGFR2b receptors on axons, triggering the assembly of presynaptic terminals at specific postsynaptic sites.^[59] Distinct FGF-receptor combinations ensure selective organization of excitatory versus inhibitory presynapses, contributing to circuit specificity.^[60] Feedback loops involving retrograde signaling maintain bidirectional communication during synapse maturation. Postsynaptic neurons release brain-derived neurotrophic factor (BDNF), which travels retrogradely to activate TrkB receptors on presynaptic terminals, stabilizing active zones and enhancing neurotransmitter release probability to support synapse strengthening.^[61] This BDNF-TrkB pathway forms a positive feedback mechanism that refines synaptic efficacy, with disruptions leading to immature or unstable connections in central circuits.^[62]

Activity-Dependent Mechanisms

Activity-dependent mechanisms play a crucial role in refining and stabilizing central synapses during development, where neural activity serves as a signal to select functionally appropriate connections. The foundational concept underlying this process is the Hebbian principle, which states that synapses between neurons that are active simultaneously are strengthened, while those with uncorrelated activity weaken or are eliminated—a maxim often phrased as "cells that fire together wire together." This principle, first articulated by Donald Hebb in 1949, has been substantiated through studies showing that correlated presynaptic and postsynaptic firing patterns drive synaptic stabilization in cortical circuits. For instance, in the developing neocortex, spontaneous waves of activity ensure that converging inputs from the same source are preferentially maintained, illustrating how Hebbian plasticity contributes to the wiring of sensory maps. Central to Hebbian strengthening is the role of calcium dynamics at glutamatergic synapses. Correlated activity leads to the activation of postsynaptic NMDA receptors, allowing calcium influx that initiates intracellular signaling pathways, such as those involving CaMKII and Ras signaling, which promote the trafficking and insertion of AMPA receptors into the synaptic membrane. This insertion increases synaptic efficacy, mimicking the expression of long-term potentiation (LTP) observed in mature circuits but adapted for developmental synapse maturation. Experimental evidence from cultured hippocampal neurons demonstrates that NMDA receptor activation directly induces rapid AMPA receptor exocytosis, resulting in functional synapse enhancement without altering presynaptic release.^[63] These mechanisms operate with heightened sensitivity during critical periods, discrete developmental windows when sensory experience exerts outsized influence on synaptic organization. In the primary visual cortex, for example, monocular lid suture during this period causes a profound shift in ocular dominance, with cortical neurons becoming responsive predominantly to the non-deprived eye due to activity-dependent competition among thalamocortical inputs. This plasticity, peaking around postnatal weeks 4-7 in kittens, underscores how patterned sensory activity refines binocular circuits; deprivation disrupts this balance, leading to weakened synapses from the deprived eye. Seminal experiments by Hubel and Wiesel established that such experience-driven changes are time-limited, closing after the critical period and highlighting the interplay between activity and synaptic specificity. Empirical studies further quantify the impact of activity deprivation on synapse formation. Dark-rearing, which eliminates visual input, significantly reduces dendritic spine density—a proxy for excitatory synapses—in the developing cortex, with one study reporting a approximately 15% decrease in layer 2/3 pyramidal neurons of the visual cortex at postnatal day 30 compared to light-reared controls.^[64] Similar sensory deprivation in the barrel cortex, analogous to whisker trimming, has been shown to diminish synapse numbers by up to 30% in layer IV, emphasizing that ongoing activity is essential for achieving mature synaptic density and preventing excessive pruning of immature connections. These findings, primarily from pre-2020 research, reinforce that activity not only strengthens select synapses but also scales overall synaptic architecture to match environmental demands.^[64]

Adult Synaptogenesis

Neurogenesis in Dentate Gyrus

Adult neurogenesis in the dentate gyrus of the hippocampus involves the continuous generation of new granule cells from neural precursors in the subgranular zone (SGZ), which subsequently migrate into the granule cell layer and extend axons to form mossy fiber synapses onto pyramidal neurons in the CA3 region.^[65] These mossy fiber synapses develop functional properties, including strong excitatory transmission, allowing the new neurons to integrate into existing hippocampal circuits.^[66] The process begins with the birth of progenitor cells in the SGZ, followed by differentiation into immature granule cells that extend dendrites into the molecular layer to receive inputs and axons that target CA3 strata lucidum and pyramidale.^[67] The rate of neurogenesis in the dentate gyrus peaks during young adulthood and progressively declines with age, dropping to approximately 17% of peak levels by two years in rodents.^[68] Physical exercise significantly enhances this process, increasing precursor cell proliferation and newborn neuron survival by up to twofold in young adults and partially reversing age-related declines to about 50% of youthful levels.^[69] This modulation highlights environmental factors' role in sustaining synaptogenesis throughout adulthood. Functionally, the integration of new granule cells via mossy fiber synapses supports pattern separation, a computational process essential for distinguishing similar experiences to form discrete memories in the hippocampal network.^[70] Additionally, the refinement of excitatory inputs from the perforant path onto these new granule cells is activity-dependent, with synaptic strengthening occurring through mechanisms like long-term potentiation during the critical 2- to 4-week maturation window.^[71] Evidence from BrdU labeling studies demonstrates that 50-70% of newly generated neurons in the dentate gyrus survive the initial postnatal period and establish functional mossy fiber synapses within 4 weeks, enabling their contribution to circuit activity.^[72] This survival rate underscores the efficiency of adult synaptogenesis in adding computational capacity to the dentate gyrus.^[65]

Plasticity in Olfactory Bulb

The olfactory bulb exhibits remarkable synaptic plasticity in adulthood, driven by continuous neurogenesis from the subventricular zone (SVZ), which supplies new interneurons that integrate into local circuits and support adaptive odor processing.^[73] Among these, periglomerular neurons, primarily GABAergic interneurons, migrate from the SVZ and differentiate to form inhibitory synapses onto the primary dendrites of mitral and tufted cells within individual glomeruli.^[73] These synapses enable precise lateral inhibition, refining glomerular output and enhancing odor discrimination; initially, young periglomerular neurons may connect to multiple glomeruli, but sensory experience promotes refinement to uniglomerular specificity.^[73] Granule neurons, comprising the majority (~95%) of adult-born interneurons in the olfactory bulb, establish reciprocal dendrodendritic synapses with the lateral dendrites of mitral cells in the external plexiform layer, facilitating feedback inhibition and lateral connectivity across the bulb.^[73] This population undergoes high turnover, with a median survival of approximately two months for new arrivals, corresponding to a substantial replacement rate that aligns with dynamic sensory demands.^[73] The dendrodendritic synapses exhibit structural plasticity, including spine remodeling, which modulates mitral cell excitability and contributes to pattern separation in odor representations.^[73] Olfactory experience, particularly odor learning, drives input-specific enhancements in synaptic density among these interneurons. For instance, associative odor-reward learning increases spine density on adult-born granule cell dendrites by 22-37% in proximal, distal, and basal domains, strengthening both excitatory and inhibitory connections without altering somatic or apical regions.^[74] This plasticity improves odor memory and discrimination of similar scents, with enriched odor exposure further stabilizing spines and reducing turnover.^[73] Underlying these changes, GABAergic contacts on periglomerular and granule neurons refine through sensory-driven activity, where olfactory input guides synapse stabilization and elimination.^[73] Non-integrated or weakly connected new neurons are selectively eliminated via apoptosis, a process regulated by factors like noradrenergic signaling from the locus coeruleus, ensuring circuit efficiency and adaptability. This turnover mechanism parallels adult neurogenesis in the dentate gyrus, supporting sensory plasticity in both systems.^[73]

Recent Advances in Regeneration

Recent advances in synaptogenesis regeneration have leveraged advanced imaging techniques to achieve unprecedented resolution in visualizing synaptic dynamics across the brain. Expansion microscopy (ExM), refined in 2025 protocols, enables brain-wide tracking of synaptic proteins at nanoscale resolution, revealing individual synapse formation rules during learning processes by physically expanding tissue samples up to fourfold while preserving ultrastructure. For instance, light-microscopy-based connectomics (LICONN) integrates ExM with deep-learning reconstruction to map synaptic connectivity in mammalian brains, identifying previously undetected synaptic proteins and their roles in regenerative plasticity. These methods surpass traditional electron microscopy by allowing multiplexed imaging of multiprotein complexes at synapses, facilitating the study of regenerative responses in adult neural circuits, such as those in the dentate gyrus. High-throughput CRISPR screens have identified novel regulators of excitatory synapse formation, advancing therapeutic strategies for regeneration. A 2025 pooled CRISPR knockout screen using high-content imaging uncovered 644 synaptic genes influencing synapse assembly, highlighting PTEN as an inhibitor of synaptogenesis via its suppression of the PI3K pathway, which limits dendritic spine growth. Similarly, the screen pinpointed DAG1 as a key adhesion molecule promoting synapse stability through dystroglycan-mediated extracellular matrix interactions, with knockouts reducing excitatory postsynaptic densities by up to 40%. These findings establish scalable platforms for dissecting cell-cell interactions in synaptogenesis, prioritizing targets for enhancing regeneration in neurodegenerative conditions. Stem cell-based approaches have shown promise in boosting synaptogenesis for disease therapies. In 2025 studies, human induced pluripotent stem cell (iPSC)-derived neurons overexpressing brain-derived neurotrophic factor (BDNF) exhibited enhanced axonal regeneration and synaptic connectivity, with BDNF amplifying neuron-intrinsic programs to increase synapse density in culture models. This overexpression restored activity-dependent plasticity in models of Parkinson's and ALS, where transplanted iPSC-neurons formed functional synapses with host circuits, improving motor recovery in preclinical trials. Such progress underscores BDNF's role in bridging developmental and regenerative synaptogenesis, offering scalable cell therapies for synaptic loss in neurodegeneration. Activity patterns critically influence regenerative synaptogenesis, with specific firing codes dictating branching and synapse numbers. Research from 2025 demonstrates that burst firing patterns in developing neurons promote neurite branching and elevate synapse counts by approximately 25% compared to tonic activity, mediated by calcium-dependent signaling cascades. Unique activity regimens, such as patterned electrical stimulation, drive synapse formation in regenerating axons by upregulating presynaptic vesicle release machinery, as observed in hippocampal cultures. These mechanisms highlight how activity-dependent refinement can be harnessed to optimize synaptic regeneration post-injury. The protein synaptophysin plays a pivotal role in facilitating vesicle fusion during synaptogenesis by modulating membrane curvature. A 2025 study revealed that synaptophysin acts as a curvature-promoting factor on synaptic vesicle (SV) membranes, enabling lateral expansion during exocytosis and accelerating fusion kinetics by 2-3 fold in vitro assays. This function ensures efficient neurotransmitter release in nascent synapses, with synaptophysin-deficient models showing impaired regenerative synapse maturation. Such insights into synaptophysin's biophysical properties inform strategies to enhance vesicle trafficking in regenerative contexts. Cell-type selectivity in synaptogenesis relies on subtype-specific cues guiding inhibitory neuron targeting. Developmental studies in 2025 using connectomic analyses of mouse visual cortex identified precise inhibitory synapse specificity, where parvalbumin-positive interneurons preferentially form synapses on distinct pyramidal cell subtypes via molecular cues like neurexin-neuroligin interactions. These cues dictate target choice, with disruptions altering inhibitory-excitatory balance and impairing regenerative plasticity. This selectivity ensures circuit-specific regeneration, as seen in inhibitory neuron grafts restoring balance in injury models.

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https://pmc.ncbi.nlm.nih.gov/articles/PMC2962766/
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https://pmc.ncbi.nlm.nih.gov/articles/PMC4756920/
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https://onlinelibrary.wiley.com/doi/10.1002/hipo.20872
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Cartography of neurexin alternative splicing mapped by single ... - NIH
Neurexins are evolutionarily conserved presynaptic cell-adhesion molecules that are essential for normal synapse formation and synaptic transmission.
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Wnt/β-catenin signalling: function, biological mechanisms, and ...
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WNT signaling in neuronal maturation and synaptogenesis - PMC
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WNT signaling in neuronal maturation and synaptogenesis - Frontiers
In the cerebellum, Wnt7a is expressed in granular cells at the same time as the mossy fiber axon, which is the presynaptic contact (Hall et al., 2000). Several ...
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Frizzled-5, a receptor for the synaptic organizer Wnt7a, regulates ...
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A novel Wnt5a-Frizzled4 signaling pathway mediates ... - PubMed
We report that Frizzled4 (Fzd4), a member of the Fzd family of Wnt receptors, specifically signals downstream of Wnt5a to promote dendrite branching and growth.
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Wnt7b signalling through Frizzled-7 receptor promotes dendrite ...
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Axonal remodeling and synaptic differentiation in the cerebellum is ...
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Wnt signaling during synaptic development and plasticity - PMC
New studies demonstrate that Wnt do not only regulate synapse formation but also they promote the remodeling of mature terminals.Missing: seminal | Show results with:seminal
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Neuroanatomy, Motor Neuron - StatPearls - NCBI Bookshelf
Jul 24, 2023 · Motor neurons include upper and lower types. Upper neurons originate in the cerebral cortex, while lower neurons in the spinal cord innervate ...
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Schwann cell precursors: where they come from and where they go
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The formation of skeletal muscle: from somite to limb - PMC - NIH
During embryogenesis, skeletal muscle forms in the vertebrate limb from progenitor cells originating in the somites.
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Motor Axon Pathfinding - PMC - PubMed Central - NIH
In fact, the midline is the source of many known guidance molecules implicated in motor axon growth, including Netrins, Semaphorins, Ephrins, and Slits ( ...
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Schwann Cells in Neuromuscular Junction Formation and ...
Sep 21, 2016 · The neuromuscular junction (NMJ) is a tripartite synapse that is formed by motor nerve terminals, postjunctional muscle membranes, and terminal ...
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https://pmc.ncbi.nlm.nih.gov/articles/PMC5030347/
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https://doi.org/10.1016/j.cell.2008.10.002
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https://doi.org/10.1038/nn2053
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Developmental neuromuscular synapse elimination - PubMed Central
This review discusses how the motor neurons, synaptic glia and muscle fibers each contributes to the developmental phenomenon.
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Roles for Ephrins in Positionally Selective Synaptogenesis between ...
First, developing muscles express all five of the ephrin-A genes. Second, rostrally and caudally derived motor axons differ in sensitivity to outgrowth ...
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Journal of Neuroscience Research - Wiley Online Library
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The Neuromuscular Junction in Health and Disease - PubMed Central
Dec 3, 2020 · The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting ...
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Neuromuscular junction maturation defects precede impaired lower ...
... NMJ undergoes a process known as AChR subunit switching. AChRs found at the NMJ consist of five subunits: two α, one β, one δ, and either γ or ɛ (41,42).
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Wnt proteins contribute to neuromuscular junction formation through ...
Both Wnt4 and Wnt11 stimulate AChR mRNA levels and AChR clustering downstream of activation of the β-catenin pathway. Strikingly, Wnt4 and Wnt11 co- ...
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Presynaptic active zones of mammalian neuromuscular junctions
The associated reduction in postsynaptic sensitivity results in a compensatory increase in quantal content to restore the amplitude of excitatory postsynaptic ...
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CREB-mediated synaptogenesis and neurogenesis is crucial for the ...
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The Impact of MeCP2 Loss- or Gain-of-Function on Synaptic Plasticity
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How do TSP1 and TSP2 induce synaptogenesis? The increase in synapse number by TSPs could be caused by an increase in formation of new synapses, stabilization of ...
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Molecular Mechanisms of Astrocyte-Induced Synaptogenesis - PMC
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Control of excitatory CNS synaptogenesis by astrocyte-secreted ...
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Article FGF22 and Its Close Relatives Are Presynaptic Organizing ...
Thus, FGFs could be involved not only in synaptic differentiation per se but also in the specificity with which particular inputs synapse on particular targets.
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Distinct sets of FGF receptors sculpt excitatory and inhibitory ... - NIH
Our results reveal the specific receptors and signaling pathways that mediate FGF-dependent presynaptic differentiation, and thereby provide a mechanistic ...
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Brain-derived neurotrophic factor-mediated retrograde signaling ...
These results suggest that BDNF released from postsynaptic cells activates presynaptic TrkB, leading to LTP.
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Retrograde signaling at central synapses - PNAS
Transcellular retrograde signaling from the postsynaptic target cell to the presynaptic neuron plays critical roles in the formation, maturation, and ...
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A Morphological Correlate of Synaptic Scaling in Visual Cortex
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Neurogenesis in the Adult Hippocampus - PMC - PubMed Central
Adult hippocampal neurogenesis generates new excitatory granule cells in the dentate gyrus, whose axons form the mossy fiber tract that links the dentate gyrus ...
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Development of hippocampal mossy fiber synaptic outputs by new ...
Sep 16, 2008 · We report that mossy fiber en passant boutons of adult-born dentate granule cells form initial synaptic contacts with CA3 pyramidal cells within 2 weeks after ...
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Neurons born in the adult dentate gyrus form functional synapses ...
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Physical exercise prevents age-related decline in precursor cell ...
Increasing age reduced adult neurogenesis at 9 months to 50% of the value at 6 weeks and to 17% at the age of 2 years. At both 1 and 2 years, precursor cell ...
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Exercise Enhances Learning and Hippocampal Neurogenesis in ...
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Pattern Separation in the Dentate Gyrus and CA3 of the Hippocampus
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Maturation and Functional Integration of New Granule Cells into the ...
The major excitatory/glutamatergic input to the dentate gyrus enters through the perforant pathway from the entorhinal cortex. Distinct areas of the ...
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Monosynaptic inputs to new neurons in the dentate gyrus - Nature
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https://www.science.org/doi/10.1126/science.1135801
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https://pmc.ncbi.nlm.nih.gov/articles/PMC4691791/