Neurogenesis

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Neural development neurogenesis

• Dr. Suman Pd. Adhikari

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Early development

• Zygote to 8 cells
• Morula (16-64 cell stage)
• Blastula / Blastocyst
• Gastrula

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Early Differentiation

• During early development-3 weeks after
  conception, the human embryo has divided into
  three germ layers
• Ectoderm
• Mesoderm
• Endoderm
• Inducing factors differentiate the ectoderm layer
  into skin and nervous tissue

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Overview of nervous system development

• Nervous system starts forming immediately after
  the primitive gut invaginates the embryonic ball
  of cells known as blastula
• Several principles guide the neural development
• 1st Different brain regions and neuron
  populations are generated at distinct times of
  development and exhibit specific temporal
  schedules
• 2nd Sequence of cellular process comprising
  ontogeny predicts that abnormalities in early
  events leads to differences in subsequent stages
• 3rd Specific molecular signals , extracellular
  growth factors and cognate receptors or
  transcription factors play roles at multiple
  stages of development
• ILGF
• BDNF

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neurogenesis

• Neurogenesis is the process by which neurons are
  generated from neural stem and progenitor cells.
• Neurogenesis is responsible for populating the
  growing brain with neurons.
• Recently neurogenesis was shown to continue in
  several small parts of the brain of mammals
  the hippocampus and the sub ventricular zone

Stem cells
Neural Progenitors
Neurons
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Neurogenesis contd.

• The neural plate forms after gastrulation is
  completed.
• During the third week of gestation the notochord
  sends signals to the overlying ectoderm, inducing
  it to become neuroectoderm.
• This results in formation of neural plate
• Prior to induction cells are undifferentiated
  (able to be transplanted to a new site)---stem
  cells
• After induction, cells are destined to become a
  neuron

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Organizing centers for neurogenesis

• Spemanns organizer (dorsoblastopore lip)
• Hensens node (similar to Spemanns org)
• Roofplate and notochord become organizers
• Secondary organizers
• Isthmic organizer (IsO)
• Anterior neural ridge (ANR)
• Cortical hem

Spemann
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Neural plate appears
Primitive streak appear
Edges of Neural plate elevate
Edges of Neural plate fuse
Neural tube formed
NEURULATION (3-4 WEEKS)
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Neural plate
Neural groove
Neural tube
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• Neural crest cells derive from the edges of the
  neural plate and dorsal neural tube
• Cells migrate dorso-laterally to form melanocytes
  and ventro-medially to form dorsal root sensory
  ganglia and sympathetic chains of the peripheral
  nervous system and ganglia of the enteric nervous
  system
• Also gives rise to diverse tissues including
  cells of neuroendocrine, cardiac, mesenchymal,
  and skeletal systems, forming the basis of many
  congenital syndromes involving brain and other
  organs

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• Another non neuronal structure of mesodermal
  origin formed during neurulation is the notochord
  found on the ventral side of the neural tube
• Notochord plays a critical role during neural
  tube differentiation
• It is a signaling source of soluble growth
  factors, such as sonic hedgehog (Shh), which
  impact gene patterning and cell determination

Notochord
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Neural Proliferation

• After the neural tube is formed, the developing
  nervous system cells rapidly increase in number
• Cell division occurs in the ventricular zone of
  the neural tube when they leave the cell
  division cycle, cells migrate into other layers

Ventricular Zone
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Neural tube
Pial surface of neural tube
Marginal zone
Ventricular Zone
Cell division occur here
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Regional Differentiation

• After closure, neural tube expands differentially
  to form major morphological subdivisions
• Proliferation depends on soluble growth factors
  made by proliferating cells themselves or
  released from regional signaling centers
• The neural tube can be described in 3 dimensions
  
• Longitudinal
• Circumferential
• Radial

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• Longitudinal dimension reflects the rostrocaudal
  (anteriorposterior) organization --consists of
  brain and spinal cord.
• Circumferential dimension, tangential to the
  surface, represents two major axes dorso-ventral
  axis (cell groups are uniquely positioned from
  top to bottom) medial to lateral axis
• Finally, radial dimension represents organization
  from innermost cell layer adjacent to the
  ventricles to outermost surface

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4 weeks
5 weeks
Five-vesicle state
Three-vesicle state
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• In spinal cord, the majority of tissue comprises
  lateral plates, which later divide into dorsal or
  alar plates-composed of sensory interneurons and
  motor or basal plates-consisting of ventral motor
  neurons.
• Floor plate, in response to Shh from the
  ventrally located notochord, produces its own
  Shh, which in turn induces neighboring cells in
  to express region-specific transcription factors
  that specify cell phenotype and function.

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• Shh activity in the ventral neural tube (blue
  dots) is distributed in a ventral-high,
  dorsal-low profile within the ventral neural
  epithelium.

T.M. Jessell, 2000
Shh activity
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3 vesicle stage
5 vesicle stage
Neural tube
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The Ventricular and Sub ventricular Proliferative
Zones

• Precursor proliferation occurs primarily in two
  densely packed regions during development.
• Primary site is the VZ lining the walls of the
  entire ventricular system
• For cerebral cortex, hippocampus and cerebellar
  cortex, precursors from the VZ migrate out to
  secondary zones
• In the early embryo, neural tube VZ progenitors
  are arranged as a one-cell layer thick,
  pseudostratified neuroepithelium

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• The bipolar VZ precursors have cytoplasmic
  processes that span from the ventricular to the
  pial surface
• During the cell cycle, the VZ appears
  multilayered, or stratified, because cell nuclei
  undergo movements, called interkinetic nuclear
  migration
• New cells are produced through the cell cycle,
  which comprises four stages
• Mitosis (M) when nuclei and cells divide
• G1 when cells grow in size before dividing again
• S phase when cells synthesize deoxyribonucleic
  acid (DNA) and replicate chromosomes
• G2 period followed by M phase.

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• Precursor cell division (M phase) occurs at the
  ventricular margin, producing two new cells
• The progeny then reenter G1 as they move outwards
  towards the pia.
• Under influence of extracellular signals these
  cells enter into S phase, which occurs near the
  upper VZ margin.
• After DNA replication nuclei move back down
  during G2 to the ventricular surface where they
  undergo mitosis and divide.

IZ, intermediate zone VZ, ventricular zone V,
ventricle
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Radial and Tangential Patterns of Neurogenesis
and Migration

• There are 3 well-recognized spatio-temporal
  patterns of neurogenesis that underlie regional
  brain formation.
• There are 2 radial patterns of cell migration
  from the VZ, referred to as inside-to-outside and
  outside-to-inside.
• The third involves non radial or tangential
  migration of cells, some of which originate in
  secondary proliferative zones.

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Radial and Tangential Patterns of Neurogenesis
and Migration contd
Schematic drawing of radial and tangential
migration during cerebral cortex development
MGE medial ganglionic eminence LGE lateral
ganglionic eminence PP preplate SP sub
plate CP cortical plate IZ intermediate
zone VZ ventricular zone
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Cortical development/ corticogenesis

• In early stages VZ cells divide giving rise to
  both a postmitotic neuron and another dividing
  precursor.
• At the end of neurogenesis, precursor division
  gives rise to two postmitotic neurons only
• The newly born neurons do not remain in the VZ
  but instead migrate out to their final
  destinations, such as the cerebral cortical
  plate, traveling along the processes of radial
  glial cells

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VZ ventricular zone IZ intermediate zone PP
preplate SVG sub ventricular zone SP
subplate CP cortical plate MZ marginal zone
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Developmental cell death

• Cell elimination is required to coordinate the
  proportions of interacting neural cells.
• Three types of developmental cell death have been
  described
• Phylogenetic cell death removes structures in
  one species that served evolutionarily earlier
  ones, such as the tail or the vomeronasal nerves
• Morphogenetic cell death required to form the
  optic vesicles, as well as the caudal neural tube
• Histogenetic cell death process that allows the
  removal of selected cells during development of
  specific brain regions.

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• On the basis of morphological criteria, three
  types of programmed cell death have been
  described.
• Apoptotic cell death Most common and is
  characterized by chromatin condensation and
  membrane blebbing, followed by nuclear
  fragmentation and cell shrinkage.
• Autophagic degeneration Involves contiguous
  groups of degenerating cells and features
  autophagic vacuoles and pyknotic nuclei.
• Non lysosomal disintegration and
  cytoplasmic-type cell death, forms that exhibit
  similarities to necrosis

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Concept of Neural Patterning

• In 3-dimensional system of the embryo, initial
  establishment of A/P polarity is signaled by the
  organizer
• During gastrulation, organizer tissues come to
  underlie neural plate and differentiate into the
  notochord
• The chordal mesoderm, which underlies the future
  midbrain, hindbrain, and spinal cord, apparently
  sends out distance signals from prechordal
  mesoderm
• Neural inducers like chordin, noggin, and
  follistatin induce primitive neural tissue
• Possible candidate posteriorizers
  (transforming signals) include bFGF
• and retinoic acid.

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Head organizer
Tail organizer

• BMP Inhibitors
• Cordin and Noggin
• Wnt inhibitors
• Cerberus, Dickkopf and frzb1

• FGF
• WNT
• RA
• BMP inhibitors

Anteriorize" neural tube
Posteriorize" neural tube
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Patterning of the brain and spinal cord through
compartmentalization
Melton, Iulianella, Trainor, 2004
Hox genes play important roles in establishing
regional cell identity. This is achieved via
opposing gradients of RA and FGF signaling.
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Specific Inductive Signals and Patterning Genes
in Development

• Induction of CNS begins at the neural plate stage
  when notochord, underlying mesenchyme, and
  surrounding epidermal ectoderm produce signaling
  molecules that affect the identity of neighboring
  cells
• Ectoderm produces BMPs that promote and maintain
  epidermal differentiation
• BMP's epidermis-inducing activity is blocked by
  inhibitory proteins, such as noggin, follistatin,
  and chordin, that are secreted by Hensen's node ,
  signaling center at the rostral end of the
  primitive streak

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• Pax6 gene expression
• Emx2 expression
• The signalling factor fibroblast growth factor 8
  (FGF8
• regulates Pax6 and Emx2 expression.
• Bone morphogenetic proteins (BMPs)
• Wingless-Int proteins (Wnts)

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New mechanism for regulating gene expression

• miRNAs contribute to normal development and brain
  function
• miRNAs can affect the regulation of RNA
  transcription, alternative splicing, molecular
  modifications, or RNA translation.
• miRNAs are 21 to 23 nucleotide long single-strand
  RNA molecules.
• Unlike mRNAs, miRNAs are noncoding RNAs that are
  not translated but are instead processed to form
  loop structures.
• miRNAs exhibit a sequence that is partially
  complementary to one or several other cellular
  mRNAs.

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Processing and function of miRNA. After
transcription, the primary miRNA forms a hairpin
conformation. This structure allows the enzyme
Drosha to cleave the transcript, producing a
pre-miRNA that then exits the nucleus through
nuclear pores.
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Regulation by extracellular factors

• Although defined initially in cell culture, a
  number of mitogenic growth factors are now
  well-characterized in vivo, including those
  stimulating proliferation, such as
• basic FGF (bFGF)
• EGF
• IGF-I
• Shh
• and signals inhibiting cell division, such as
• pituitary adenylate-cyclase-activating
  polypeptide (PACAP)
• GABA
• Glutamate
• members of the TGF-ß superfamily

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Cell Migration and Aggregation

• Cells migrate away from the VZ along a temporary
  network of radial glial cells
• The cells of the neocortex migrate in an
  inside-out pattern the deepest layers form first
  so that the cells of the superficial layers must
  migrate through them (like lava out of a volcano)
• Migration of the cells of the neural crest-- form
  the PNS, and thus may have a long way to migrate
• Neural crest cells transplanted to a new part of
  the neural crest migrate to the destination that
  is appropriate for cells in the new location

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• The radial patterns of neurogenesis reflect
  whether a structure is phylogenetically older
  (Eg dentate gyrus) or more recently evolved,
  Eg cortex.
• Early generated cells are positioned on the
  outside, with later born cells residing inside,
  closer to the VZ.
• It suggests that as more cells are generated,
  they passively move earlier born cells farther
  away.
• Of interest to psychiatry, the cerebral cortex is
  the paradigmatic model of inside-to-outside
  neurogenesis.

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• Derived from the embryonic forebrain
  telencephalic vesicles, the characteristic
  six-cell layers represent a common
  cytoarchitectural and physiological basis for
  neocortical function.
• Within each layer, neurons exhibit related
  axodendritic morphologies, use common
  neurotransmitters, and establish similar afferent
  and efferent connections.
• In general, pyramidal neurons in layer 3
  establish synapses within and between cortical
  hemispheres whereas deeper layer 5/6 neurons
  project primarily to subcortical nuclei,
  including thalamus, brainstem, and spinal cord.
• The majority of cortical neurons originate from
  the forebrain VZ.

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• At the earliest stages, the first postmitotic
  cells migrate outward from the VZ to establish a
  superficial layer termed the preplate.
• 2 cell types comprise the preplate, Cajal-Retzius
  cells, which form outermost layer 1 or marginal
  zone, and subplate neurons, which lay beneath
  future layer 6.
• These distinct regions form when later born
  cortical plate neurons migrate within and divide
  the preplate in two

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Differentiation and neuronal process outgrowth

• After newly produced neurons and glial cells
  reach their final destinations, they
  differentiate into mature cells.
• For neurons, this involves outgrowth of dendrites
  and extension of axonal processes, formation of
  synapses, and production of neurotransmitter
  systems, including receptors and selective
  reuptake sites.
• Most axons will become insulated by myelin
  sheaths produced by oligodendroglial cells.
• Occur with a peak period from 5 months of
  gestation onward.
• During first several years of life, many
  neuronal systems exhibit exuberant process growth
  and branching, which is later decreased by
  selective pruning of axons and synapses
  dependent on experience, while myelination
  continues for several years after birth and into
  adulthood.

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Synaptogenesis

• Once aggregation is complete, axons and dendrites
  grow out from the neurons
• Axon growth is directed by a growth cone at the
  growing axon tip
• Three hypotheses have been proposed to explain
  how growth cones make their way to correct
  destination
• Chemoaffinity hypothesis
• Blueprint hypothesis
• Topographic-gradient hypothesis

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Growth Cone
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Neurodevelopmental basis of psychiatric diseases
Disease Basis Comment
Schizophrenia Smaller prefrontal cortex and hippocampus and enlarged ventricles Differences in the levels of transcription factors, such as NeuroD, Math1, or Lhx, Wnt3a Lef1, or bFGF
Autism spectrum disorders (Autistic disorder, Asperger's syndrome, and pervasive developmental disorder) Diminished gray matter, disorganized laminar patterns, misoriented pyramidal neurons, ectopic neurons Dysregulation of many processes, including neuron proliferation, migration, survival, organization, and programmed cell death

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Neurodevelopmental basis of psychiatric diseases
Condition diseases Comment
Abnormal level of reelin protein mRNA BPAD Schizophrenia Autism Reelin is glycoprotein- important signal for neuronal migration produced by Cajal-Retzius cells
Reelin mutation Lissencephaly/ Smooth brain Cerebellar hypoplasia Abnormal hippocampal formation Gyral patterning malformation with loss of gyri and sulci

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• Normally there is balance between neurogenesis,
  death of unwanted cells (tumoral cells), and
  adaptive synaptogenesis.
• Dysregulation of these mechanisms can lead to
• Neurodegeneration
• Brain tumors
• Brain dysfunctions

Proliferation
Synaptogenesis
Cell death
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Neurodegeneration

• Impaired neurogenesis
• Synaptic dysfunction
• Massive cell death
• Alzheimers, Parkinson's

Normal

-
Brain dysfunctions

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• Nerve cell dysfunction
• Cell degeneration
• Depression, Schizophrenia

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Brain tumors

• Abnormal proliferation
• Insufficient cell elimination
• Neuroblastoma, Glioma

Proliferation
Synaptogenesis
-
Cell death
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holoprosencephaly

• Condition in which prosencephalon fail to develop
  into cerebral hemispheres
• Mutations in Shh ---arhinencephaly
• Defect in development of face
• Closely spaced eyes, small head size, and
  sometimes clefts of the lip and roof of the
  mouth, as well as other birth defects
• In most cases of holoprosencephaly, the
  malformations are so severe that babies die
  before birth

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Neural tube defects

• Abnormalities of neural tube closure may lead to
  spina bifida and anencephaly.
• Genetic disorders affecting development include
  trisomy 21, or Down syndrome, fragile X and
  phenylketonuria.
• Environmental toxins, including alcohol can
  interfere with the normal course of development.
  
• Neurulation defects are well-known following
  exposure to retinoic acid in dermatological
  preparations and anticonvulsants, especially
  Valproic acid, as well as diets deficient in
  folic acid

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Spina bifida
Incomplete closure of the embryonic neural tube
results in an incompletely formed spinal cord
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SUMMARY
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• REFERENCES
• Kaplan and Sadocks Comprehensive Textbook of
  Psychiatry, 9th edition
• Lange Embryology
• Inderbir Singh Embryology
• Color Atlas of Neuroscience Greenstein, 2000
• Google scholar
• Google images
• Wikipedia, Scholarpedia

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THANK YOU