Neurogenesis and Neurodegeneration

Introduction

Neurogenesis And Neurodegeneration is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

Adult neurogenesis — the generation of new functional neurons from neural stem cells (NSCs) in the mature brain — was first demonstrated ^{1Platelet concentrate-derived extracellular vesicles promote adult hippocampal neurogenesisPMID 41218272Open reference} conclusively in humans by Eriksson et al. in 1998 using bromodeoxyuridine (BrdU) incorporation to label dividing progenitor cells in the ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference} hippocampal dentate gyrus.^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference} This landmark finding overturned ^{4Lifestyle strategies and mechanistic implications for slowing neurodegenerationPMID 41772140Open reference} the long-held dogma that the adult mammalian brain is incapable of generating new neurons. Since then, it has become clear that impaired ^{5'Small-molecule-based activation of Wnt/β-catenin signaling: An underexplored yet promising strategy for neuroprotection'PMID 41604971Open reference} neurogenesis is a common hallmark across neurodegenerative diseases, contributing not only to neuronal loss but also to the [^6] failure of endogenous repair mechanisms that might otherwise slow disease progression ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}. [^7]

Adult neurogenesis occurs in two principal neurogenic niches: the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) lining the lateral ventricles. The progressive impairment of neurogenesis in diseases such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease has implications for cognitive decline, mood disorders, and olfactory dysfunction that accompany these conditions ^{1Platelet concentrate-derived extracellular vesicles promote adult hippocampal neurogenesisPMID 41218272Open reference}. [^8]

Neurogenic Niches in the Adult Brain

Subgranular Zone (SGZ)

The subgranular zone lies at the inner border of the granule cell layer in the dentate gyrus of the hippocampus. Here, radial glia-like Type 1 neural stem cells (expressing GFAP, Sox2, and Nestin) give rise to transit-amplifying Type 2 progenitors, which differentiate into Type 3 neuroblasts (expressing doublecortin/DCX and PSA-NCAM), and ultimately mature into glutamatergic dentate granule neurons that integrate into existing hippocampal circuits.^{1Platelet concentrate-derived extracellular vesicles promote adult hippocampal neurogenesisPMID 41218272Open reference} [^9]

Approximately 700 new neurons are estimated to be added to each human hippocampus per day under normal conditions, based on carbon-14 birth-dating studies by Spalding et al. (2013). These new neurons are critical for pattern separation, spatial memory formation, and contextual fear conditioning — functions central to hippocampal processing and vulnerable in early Alzheimer’s disease ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}. ^{6Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults, Nature, 20182018 · PMID 29513649Open reference}

Subventricular Zone (SVZ)

The subventricular zone lines the lateral walls of the lateral ventricles and is the largest neurogenic niche in the adult mammalian brain. ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}0 It contains Type B neural stem cells and Type C transit-amplifying cells that proliferate and differentiate continuously. In rodents, ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}1 SVZ-derived neuroblasts (Type A cells) migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}2 GABAergic and dopaminergic interneurons ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}3. ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}4

In humans, SVZ neurogenesis is prominent in early life but declines substantially with age, with the RMS becoming largely vestigial by adulthood. However, SVZ-derived progenitors may retain capacity for reactive neurogenesis following injury, making this niche therapeutically relevant ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}5. ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}6

Neurogenesis Impairment in Neurodegenerative Diseases

A 2021 study by Terreros-Roncal et al. in Science systematically examined postmortem human samples from patients with ALS, Huntington’s disease, Parkinson’s disease, dementia with Lewy bodies, and frontotemporal dementia. The study found that adult-born dentate granule cells showed abnormal morphological development, altered expression of differentiation markers, and that the homeostasis of the neurogenic niche was disrupted across all conditions examined. Aging and neurodegeneration reduced the phagocytic capacity of microglia demonstrated that while thousands of immature neurons (DCX-positive) are identifiable in the dentate gyrus of neurologically healthy subjects up to the ninth decade of life, the number and maturation of these neurons progressively declined as AD advanced.^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}7

The decline is attributable to multiple pathological factors:

• Amyloid-Beta disrupts Wnt/beta-catenin signaling, a critical pathway for NSC proliferation and differentiation

• Tau(/proteins/tau) hyperphosphorylation impairs cytoskeletal integrity in neural progenitor cells

• Chronic neuroinflammation creates a hostile microenvironment that suppresses neurogenesis

• Loss of cholinergic innervation from the basal forebrain reduces trophic support

The decline in neurogenesis may contribute directly to hippocampus-dependent cognitive impairments including episodic memory deficits, among the earliest clinical manifestations of the disease [^6].

Parkinson’s Disease

In Parkinson’s disease, both SGZ and SVZ neurogenesis are affected, though the SVZ is particularly impaired due to the loss of dopaminergic innervation from the substantia nigra. Dopamine directly modulates SVZ progenitor proliferation through the D3 receptor, and degeneration of the nigrostriatal pathway leads to decreased Type C cell proliferation [^7].

alpha-synuclein aggregation disrupts normal neural stem cell function. Postmortem studies of PD brains have shown reduced numbers of proliferating cells in the SVZ. Reduced hippocampal neurogenesis may contribute to the non-motor symptoms of PD, including cognitive decline, depression, and anosmia (loss of smell, related to reduced olfactory bulb neurogenesis) [^8].

Huntington’s Disease

In Huntington’s disease, the mutant huntingtin protein causes complex disruptions to neurogenesis. In knock-in mouse models, striatal progenitors display delayed cell cycle exit and expansion of the intermediate progenitor pool with overexpression of the pluripotency factor Sox2. [In the adult brain, while basal SVZ neurogenesis may remain relatively intact, it cannot be upregulated in response to injury, and striatal neuroblasts fail to mature into functional neurons due to a hostile microenvironment. Dentate gyrus neurogenesis is impaired in R6/2 mouse models, paralleling the cognitive decline observed in HD patients [^9].

Key Molecular Regulators of Neurogenesis

BDNF (Brain-Derived Neurotrophic Factor)

BDNF is one of the most potent regulators of adult neurogenesis (see neurotrophic factors. It is highly expressed in the hippocampus and acts through its high-affinity receptor TrkB. Upon BDNF-TrkB binding, three major intracellular signaling cascades are activated: PI3K/Akt (cell survival), MAPK/ERK (neurogenesis and synaptogenesis), and PLC-gamma (long-term potentiation). Physical exercise can increase BDNF synthesis by approximately threefold in the human brain.^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}8

BDNF expression is regulated downstream of the Sox2-Wnt pathway: SOX2 binds to bivalently marked promoters of neurogenic genes including BDNF to maintain the bivalent chromatin state, and BDNF expression is reduced in SOX2-deficient neural progenitors ^{2Dual roles of basal NLRP3 expression in cognitive and neurogenic agingPMID 41779054Open reference}9.

Wnt Signaling

The canonical Wnt/beta-catenin pathway plays a central role in hippocampal neurogenesis (see Wnt signaling. Wnt ligands (particularly Wnt3a, secreted by local astrocytes in the SGZ) activate beta-catenin, which in complex with TCF/LEF transcription factors displaces Sox2 from the NeuroD1 promoter, triggering neuronal differentiation. Downregulation of Wnt signaling is associated with the pathophysiology of AD, and pharmacological Wnt activators such as andrographolide (which inhibits GSK-3beta have shown promise in restoring neurogenesis in AD mouse models.[^6]

Notch Signaling

Notch signaling is a master regulator of NSC maintenance and quiescence. Activation of Notch by its ligands (Delta-like 1/Dll1, Jagged1) triggers release of the Notch intracellular domain (NICD), which translocates to the nucleus and forms a complex with Rbpj. The NICD-Rbpj complex induces expression of transcriptional repressors Hes1 and Hes5, which repress proneural genes (Ascl1/Mash1, Neurogenin2) and maintain cells in an undifferentiated, stem-like state. When Rbpj is genetically deleted in the adult mouse brain, all neural stem cells differentiate, demonstrating that Notch signaling is absolutely required for NSC maintenance.[^7]

DCX (Doublecortin)

DCX is a microtubule-associated protein expressed by neuronal precursor cells and immature neurons. Neural precursor cells begin to express DCX while actively dividing, and their neuronal daughter cells continue to express DCX for 2-3 weeks as the cells mature. DCX has been validated as a reliable and specific marker that reflects levels of adult neurogenesis. However, a 2024 study using digital droplet PCR revealed broader-than-expected expression of DCX transcript in both hippocampal and cortical cell populations, suggesting that DCX expression alone may not exclusively indicate neurogenesis in all brain regions ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}0.

Amyloid-Beta and Tau Effects on Neural Progenitor Cells

Amyloid-Beta

Amyloid-Beta has a complex, dose-dependent relationship with neurogenesis:

• At low concentrations: A-beta promotes survival of primary neurons and exerts a neurogenic effect on neural progenitor cells, consistent with its proposed physiological role in synaptic function

• At pathological concentrations: A-beta interferes with critical signaling pathways, particularly the Wnt/beta-catenin pathway, directly inhibits expression of neurogenic transcription factors like NeuroD1, induces oxidative stress and mitochondrial dysfunction in neural progenitors, and activates proinflammatory microglia/cell-types/microglia that create a hostile neurogenic niche

• Paradoxical early stimulation: A-beta-1-42 stimulates neurogenesis of SVZ precursors in vivo, raising the hypothesis that early overstimulation may result in premature depletion of the stem cell pool — explaining why some studies detect increased proliferative markers in early AD but decreased neurogenesis in advanced disease

Tau

tau protein/proteins/tau-protein) effects on neurogenesis are multifaceted:

• Tau knockdown delays neuronal maturation in primary neurons and induces transcriptional repression of neuronal genes

• Hyperphosphorylated tau disrupts cytoskeletal integrity, hindering neural progenitor cell differentiation and migration

• Tau pathology] propagates through neurogenic niches via prion-like spreading, potentially corrupting the neurogenic microenvironment

Dopamine’s Role in SVZ Neurogenesis

Dopaminergic innervation from the substantia nigra and ventral tegmental area directly modulates SVZ neurogenesis. Type C transit-amplifying cells express dopamine receptors, particularly the D3 receptor, which is the only dopamine receptor specifically expressed in neurogenic areas in both embryonic and postnatal brain ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}1.

Dopamine increases proliferation of SVZ-derived cells by releasing EGF in a PKC-dependent manner. In Parkinson’s disease, degeneration of the nigrostriatal pathway reduces dopaminergic inputs to SVZ Type C cells, resulting in decreased progenitor cell proliferation. This may contribute to the anosmia seen in early PD and potentially to broader cognitive deficits.[^8]

Experimentally, reduced progenitor cell proliferation in PD models can be restored by administering the D3 receptor agonist pramipexole, which augments SVZ proliferation, enhances neuronal differentiation, and increases new neurons in the olfactory bulb ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}2.

Neural Stem Cell Exhaustion in Aging and Disease

Neural stem cells undergo progressive functional decline with aging, characterized by metabolic abnormalities, disrupted protein quality control, mitochondrial dysfunction, reduced genetic stability, and diminished capacity for proliferation and differentiation ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}3.

Quiescence as a protective mechanism: NSC quiescence restricts the number of stem cell divisions and is essential for long-term stem cell pool maintenance. Loss of quiescence results in an imbalance in progenitor cell populations, ultimately leading to premature stem cell depletion.

FOXO transcription factors: Loss of FOXO transcription factors, the downstream effectors of the insulin/IGF1 pathway, leads to premature exhaustion of the NSC pool. Deregulated nutrient signaling leads to abnormal and wasteful NSC activation followed by premature exhaustion, which is a major component of brain aging.

Glucose metabolism (2024 breakthrough): Ruetz et al. (2024, Nature) conducted CRISPR-Cas9 knockout screens in young and old mouse NSCs and identified over 300 genes whose deletion reactivated aged NSCs. Most notably, knockout of Slc2a4 (encoding the glucose transporter GLUT4) improved activation of old but not young quiescent NSCs, revealing that glucose uptake increases in NSCs during aging and that transient glucose starvation restores old NSC activation capacity.[^9]

The Hippocampal Neurogenesis Controversy

The Conflicting 2018 Studies

In March 2018, two studies published within weeks of each other reached diametrically opposing conclusions, igniting one of the most intense debates in modern neuroscience ^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}4.

Sorrells et al. (Nature, 2018) examined postmortem hippocampal tissue and reported that proliferating progenitors and young neurons in the dentate gyrus sharply decline in the first year of life, with no young neurons detected in adult samples (18-77 years). Their conclusion: dentate gyrus neurogenesis does not continue, or is extremely rare, in the adult human.^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}5

Boldrini et al. (Cell Stem Cell, 2018) assessed whole autopsy hippocampi from healthy individuals aged 14-79 years and found similar numbers of intermediate neural progenitors and thousands of immature neurons across ages. Their conclusion: healthy older subjects display preserved neurogenesis throughout aging.^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}6

Key Methodological Differences

The divergent findings were traced to critical methodological differences:

1. Sample selection: Most tissue in the Sorrells study came from individuals with chronic epilepsy, which may independently influence neuroblast numbers

2. Cell counting methodology: Boldrini et al. used stereology (unbiased systematic sampling)

3. Tissue fixation: This emerged as the single most critical variable. Moreno-Jimenez et al. (2019) demonstrated that none of nine commercially available antibodies could detect DCX-positive cells in human hippocampal samples fixed for 6 months in formalin — a fixation artifact affecting tissue from subjects of all ages^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}7

Resolution

Tobin et al. (2019, Cell Stem Cell) further demonstrated that hippocampal neurogenesis persists through the tenth decade of life and is detectable even in patients with mild cognitive impairment and Alzheimer’s disease, studying 18 participants with a mean age of 90.6 years.^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}8

The controversy has been substantially resolved by a 2026 study in Nature using multiomic single-cell sequencing to analyze 355,997 nuclei from human postmortem hippocampi across young adults, aged adults, SuperAgers, and individuals with AD. The study identified neural stem cells, neuroblasts, and immature granule neurons, confirming adult human hippocampal neurogenesis at the transcriptomic level. Dysregulated neurogenesis was largely associated with changes in chromatin accessibility, with early alterations evident in preclinical AD.^{3Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseasesPMID 40618260Open reference}9

Therapeutic Approaches

Exercise-Induced Neurogenesis

Voluntary physical exercise is the most robustly validated non-pharmacological enhancer of adult hippocampal neurogenesis. The seminal work of van Praag et al. (1999, Nature Neuroscience) demonstrated that mice with access to a running wheel showed enhanced progenitor proliferation, neuronal differentiation, and survival in the dentate gyrus.^{4Lifestyle strategies and mechanistic implications for slowing neurodegenerationPMID 41772140Open reference}0

In humans, moderate-intensity aerobic exercise (60-70% of maximum heart rate, 30-40 minutes, 3-4 times per week) optimally stimulates BDNF production and hippocampal neurogenesis. Critically, Choi et al. (2018, Science) demonstrated that in the 5xFAD Alzheimer’s mouse model, neither stimulation of neurogenesis alone nor exercise without neurogenesis ameliorated cognition — only the combination of neurogenesis plus BDNF (mimicking exercise) provided cognitive benefit.^{4Lifestyle strategies and mechanistic implications for slowing neurodegenerationPMID 41772140Open reference}1

Pharmacological Enhancement

Several pharmacological agents show promise:

• Andrographolide: Activates Wnt/beta-catenin signaling by inhibiting GSK-3beta; treatment of APP/PS1 AD mice increased proliferation, neural progenitors, neuroblasts, and newborn immature neurons in the dentate gyrus[^6]

• Allopregnanolone: A neurosteroid that promotes neural progenitor proliferation; has entered clinical trials for AD

• SSRIs: Fluoxetine and other SSRIs increase hippocampal neurogenesis in rodents, with some antidepressant effects potentially mediated through neurogenic mechanisms

• Lithium: Inhibits GSK-3beta and enhances Wnt signaling-dependent neurogenesis

Stem Cell Therapies

Approaches to leveraging NSCs therapeutically include:

• Endogenous NSC stimulation: Pharmacological or genetic activation of resident NSC populations

• Exogenous NSC transplantation: Transplantation of neural stem cells derived from iPSCs, which can promote tissue regeneration and functional recovery

• Direct reprogramming: In vivo conversion of resident glial cells into neurons using transcription factors such as NeuroD1 and Ascl1, bypassing the need for stem cell transplantation

• Engineered exosomes: Delivery of pro-neurogenic factors via engineered exosomes that can cross the Blood-Brain Barrier

Biomarkers of Neurogenesis

Immunohistochemical Markers (Postmortem)

• DCX (Doublecortin): Gold-standard marker for immature neurons; expressed for 2-3 weeks during maturation

• PSA-NCAM: Expressed on migrating neuroblasts and immature neurons

• Ki67: Proliferation marker for dividing cells

• NeuN: Mature neuronal marker confirming neuronal identity

CSF and Blood Biomarkers

• DCX in cerebrospinal fluid: A highly sensitive immunoassay has been developed; DCX is detectable in CSF and correlates with brain neurogenesis levels

• BDNF serum levels: Peripheral BDNF levels correlate with hippocampal BDNF and may serve as an indirect proxy for neurogenic activity

Neuroimaging Biomarkers

• Cerebral blood volume (CBV) MRI: Dentate gyrus CBV correlates with neurogenesis in rodents and is being adapted for human studies

• Hippocampal volumetry: Dentate gyrus volume on high-resolution MRI can serve as a structural surrogate, though influenced by many factors beyond neurogenesis

Recent Research Findings

Multiomic resolution of the neurogenesis debate (2026): Single-cell sequencing of 355,997 nuclei confirmed adult hippocampal neurogenesis at the transcriptomic level and identified chromatin accessibility changes as early drivers of neurogenic decline in preclinical AD.^{4Lifestyle strategies and mechanistic implications for slowing neurodegenerationPMID 41772140Open reference}2

CRISPR screens reveal aging regulators in NSCs (2024): Ruetz et al. identified GLUT4 (Slc2a4) as a key regulator, demonstrating that glucose uptake increases in aging NSCs and that GLUT4 knockout restores activation capacity.[^9]

Cross-species neurogenesis atlas (2025): Two Nature Neuroscience studies revealed that immature dentate granule cells across species converge onto common biological processes for neuronal development but employ largely species-specific gene expression programs, cautioning against direct translation of rodent findings to humans.

Impact of neurodegenerative diseases across the neurogenic cascade (2021): Terreros-Roncal et al. systematically characterized how five different neurodegenerative diseases disrupt adult hippocampal neurogenesis at multiple levels.^{4Lifestyle strategies and mechanistic implications for slowing neurodegenerationPMID 41772140Open reference}3

Cross-Disease Neurogenesis Comparison Matrix

Neurotrophic Factor Comparison

Cell Cycle Dysregulation in Neurogenesis

Neurogenesis requires proper progression through the cell cycle. In neurodegenerative diseases, multiple mechanisms disrupt cell cycle control:

• p53 activation: DNA damage and oxidative stress activate p53, leading to cell cycle arrest in NSCs

• Cyclin D1 reduction: Key cell cycle protein reduced in AD and HD models

• Retinoblastoma (Rb) pathway: Altered phosphorylation of Rb affects NSC proliferation

• Epigenetic modifications: Histone methylation and acetylation changes alter chromatin accessibility at neurogenesis gene loci

• Metabolic dysfunction: Reduced mitochondrial function in NSCs limits ATP availability for cell division

External Links

• Allen Brain Atlas - Adult Neurogenesis

• BrainSpan - Developmental Transcriptome

See Also

• Amyloid-Beta

• basal forebrain

• alpha-synuclein

• huntingtin

• Alzheimer’s Disease

• Amyloid-Beta Aggregation

• Tau Pathology

• Parkinson’s Disease

• ALS

• Huntington’s Disease

Background

The study of Neurogenesis And Neurodegeneration has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

Recent Research Updates (2024-2026)

This section highlights recent publications relevant to this mechanism.

• Neuromodulatory role and therapeutic potential of N 6 -methyladenosine RNA methylation in neurodegenerative diseases. (2026 Jun 1) - Neural regeneration research

• Platelet concentrate-derived extracellular vesicles promote adult hippocampal neurogenesis. (2026 May) - Biomaterials

• Dual roles of basal NLRP3 expression in cognitive and neurogenic aging. (2026 Mar 4) - Biogerontology

• Lifestyle strategies and mechanistic implications for slowing neurodegeneration. (2026 Mar 2) - npj metabolic health and disease

• Small-molecule-based activation of Wnt/β-catenin signaling: An underexplored yet promising strategy for neuroprotection. (2026 Mar) - Bioorganic chemistry

Pathway Diagram

flowchart TD
    A["Neurogenesis"]  -->  B["Neural Stem Cells"]
    A  -->  C["Proliferation"]
    A  -->  D["Differentiation"]
    A  -->  E["Migration"]
    A  -->  F["Integration"]

    B  -->  G["Subventricular Zone"]
    B  -->  H["Hippocampus SGZ"]

    G  -->  I["Olfactory Bulb"]
    H  -->  J["Cortex"]

    I  -->  K["Olfactory Function"]
    J  -->  K

    C  -->  L["Growth Factors"]
    D  -->  L
    E  -->  L
    F  -->  L

    L  -->  M["BDNF"]
    L  -->  N["EGF"]
    L  -->  O["FGF"]

    M  -->  P["neuronal Survival"]
    N  -->  P
    O  -->  P

    P  -->  Q["Normal Function"]
    P  -->  R["Neurodegeneration"]

    Q  -->  S["Cognitive Reserve"]
    R  -->  S

    style A fill:#0a1929
    style Q fill:#0e2e10
    style R fill:#3b1114
    style S fill:#0e2e10