Download Early Stages of brain development

M&P1988,2-21
2-Zell Stadium einer Zygote in vitro
A
Brain development:
An introduction
Brain development: An introduction
[email protected]
www.charite.de/ch/physio/sep/research-rg
(with password 'rgmns0405')
A
Dec04
- Induction of neuroectoderm & early
neural morphogenesis
- Regional specification & factors that determine
identity of individual neurons and glial cells
B
March05
- Axon outgrowth, target innervation,
synaptogenesis
- Regressive events, competitive interactions,
lesion and repair in the CNS
Recommended reading:
1) Nicholls JG, Martin AR, Wallace BG, Fuchs PA (2001) From
Neuron to Brain, Sinauer, Sunderland, MA (580 pp.)
2) Sanes DH, Reh TA, Harris WA (2000) Development of the
Nervous System, Academic press, San Diego (500 pp.)
3) Christ B, Wachtler F (1998) Medizinische Embryologie, Ullstein
Medical, Wiesbaden (348 pp.) (German)
4) Moore, KL, Persaud TVN (1998) The Developing Human,
Saunders, Philadelphia (563 pp.)
5) Gilbert SF (2003) Developmental Biology, Sinauer, Sunderland,
MA (838 pp.)
6) Squire LR, Bloom FE, McConell SK, Roberts JL, Spitzer NC,
Zigmond MJ (2003) Fundamental Neuroscience, Academic Press,
Amsterdam, Boston etc. (1426 pp.)
Q1:
Why and when do developmental
abnormalities arise?
3% of newborns - significant anomalies (such as meromelia, microcephaly)
14% of newborns - smaller single defects
(such as color blindness, abnormal pigmentation, lighter
forms of mental retardation etc. )
About 25% of anomalies are due to a combination of genetic and
environmental factors
Developmental anomalies
Frequency and causes of human congenital anomalies
50-60% of all cases of spontaneous abortion chromosomal anomalies
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Developmental anomalies
3th-5th week: Highest risk during the period of
organogenesis
Birth
Risk of a developmental disturbance
Age-dependent risk of congenital anomalies
Weeks of pregnancy
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Amelia
Developmental anomalies
Example: Severe anomaly
of limb development
after the influence of
thalidomide (= Contergan)
Meromelia
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Developmental
Teratologie anomalies
Example: Additional fingers/toes
C, D - polydaktyly, E, F - partial duplication of foot or numb
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Meroanencephaly, spina bifida
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Developmental
Teratologie anomalies
A bit of terminology...
Embryonic period: 1st-8th week
zygote
morula
blastocyst
gastrula (w 3 germ layers)
At the end of the embryonic period the heart and circulation are functioning
Fetal period: 9th week till birth
Prenatal, postnatal period
Development is not over at birth: the brain triples its weight between birth and
16 years of age
1. Why and when do developmental
abnormalities arise?
1st summary:
- About 15% of the developmental anomalies are due to gene defects.
Another 10% are due to environmental factors (viral infections, alcohol,
poisons), the remaining 75% are of multifactorial or unclear origin.
- The development of a new human being comprises the embryonic period
(1st till 8th week), the fetal period (9th till 28 week) and the postnatal
period. During the 4-5th period of gestation the risk of a developmental
anomaly is highest.
- Both the lack as well as the excessive or premature influence of
a developmental signal can cause a developmental anomaly.
Q2:
How is the CNS 'anlage' formed?
M&P1988,2-21
2-Zell Stadium einer Zygote in vitro
Does the genome encode all the properties of the organism/brain
to be built?
No, we have a so called generative, not a descriptive program
of development. This means: Each of the actually active genes
only generates the signals for the following stage of
development.
Note: 1010-1012 cells in the brain, but only about 105 genes
Developmental programs
MP-1998-2-21
The entire information for the development of a
multicellular orgnizm is contained in the fertilized egg...
1) Cell division (w or wo cell growth)
2) Pattern formation (evolvement of the body building plan,
establishment of rostro-caudal and dorso-ventral axes, limb axes)
3) Cell differentiation (as a consequence of induction)
4) Growth & changes in body/organ shape
5) Activity-dependent adjustment of structure and function,
compensation of small developmental anomalies
Developmental
Entwicklungsprogramme
programs
The development of a multicellular organism
comprises the following processes
that may occur in parallel
Developmental
Entwicklungsprogramme
programs
1st week:
Formation of the blastocyst
M&P98
The daughter cells (blastomeres) become smaller with each division, 1 cycle requires
about 20-24 h. The generation of new mRNA (due to translation) still occurs under the
control of maternal genes ('maternal factors').
A completely new offspring is present when the translation is completely under the
control of the embryonic genome (end of first week?) >ethical considerations
Embryonic disc
Primitive node
(Hensen's node)
Primitive streak
Connecting stalk
The equivalent to Hensen's node (in chick and mammalian embryos)
is the Spemann organizer (in amphibians)
Formation of the body axis
After the 2nd week:
Stage of 2-laminar embryonic disc
Induction of an additional
body axis in the host
embryo
Polarity: Differential morphological properties along an axis
Induction: A small group of cells determines the properties of
the neighboring tissue
Primary induction: Induction of the entire body axis by a group of cells
(organizer cells)
Formation of the body axis
Transfer of Hensen's node from a quail embryo
to a chicken embryo
Nobel price for medicine 1935 to Spemann
Formation of the body axis
The concept of the organizer was introduced by
Hilde Mangold und Hans Spemann on the
basis of transplantation studies in amphibians
Prechordal mesoerm
CW-1998-3-5
Secreted
proteins
Embryonic disc
Primitive node
(Hensen's node)
Primitive streak
Connecting stalk
Transcription
factors
Head process
Formation of the body axis
The genes of the organizer encode secreted proteins,
receptor protein kinases and transcription factors
that drive the further fate of the neighboring tissue
Regulation of transcription
Transcription factors bind to the regulatory regions
of one or more genes and influence
the rate at which these genes are transcribed
Sequential and hierarchical activation of genes encoding
secreted factors and transcription factors
Master Gene: sonic hedgehog (e.g. shh)
Transcription factors (e.g. homeobox-containing genes, HOX)
Growth factors (e.g. FGF)
The development of the body axis is a remarkable example of the homology
between genes of invertebrates (drosophila) and vertebrates (amphibians,
birds, mammals)
Formation of the body axis
What happens during the primary induction?
Endovaginal sonogramm ca. 3 weeks after conception:
Now starts a particularly critical period of the embryonic development,
when defects in organogenesis may produce profound anomalies
A - Amnionhöhle, YS - Dottersack, E - Endometrium
Aus: M&P1998
Size: 3 mm
Early neural morphogenesis, neurulation
End of third week:
At this stage we have a neural tube, with a rostral and a caudal
neuropore, and a (still incomplete) number of somites
Somites are compact
aggregates of
mesenchymal cells from
which cells migrate to
give rise to the
vertebrae, ribs and the
axial muscles
MP1998_18.22
~23 d
Early neural morphogenesis, neurulation
NMWF2001_23-02 (from chicken embryos)
The precursor cells for the CNS (neural tube) and the
PNS (neural crest) are now separate from each other
A 21 day old human embryo has 3 germ layers
Part of the ectoderm gives rise to the nervous system >neuroectoderm
Mesenchym (mesoblast): a loose network of embryonic connective tissue.
Some mesenchym forms a layer called mesoderm. The mesenchym may influence the
fate of cells emerging from the ectoderm.
2. How is the CNS 'anlage' formed?
2nd summary:
- The human development follows a generative developmental program.
- A new offspring is present when the translation is completely under
the control of the embryonic genome.
- In mammals the cells of Hensen's node contain genes that are
sufficient to produce a complete body building plan. This area
corresponds to Spemann's (body) organizer.
- A group of hierarchically organized genes controls the formation of
the body axis prducing secreted molecules (nodal, shh) or transcription
factors (Lim-1). The master genes are at the top of the hierarchy and
influence many other genes.
- In humans the body axis is defined at the end of the 3rd week of
gestation (with the rostral and caudal neuropores present, the somites
and three germ layers.
- The ectoderm has organized into the neural tube and the neural crest.
Q3:
How is the regional specification of
neural tissue achieved?
The embryo has a length of 4.5 mm
Early stages of human brain development
End of the 4th week:
The rostral and caudal neuropores are fused,
pit (primorium of the
the full set of somites is present, the upper limbOtic
bud
inner
ear) emerges.
The brain gets organized into several vesicles that
reflect its increasing regional differences
Midbrain-hindbrain organizer (WNT1, engrailed-1, FGF-8)
Hindbrain
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(shh)
The notochord is a rod of cells of mesenchymal origin
that will drive the differention of the neural tube cells.
NMWF2001_23-06 chicken 3 d
The 'activator-transformer hypothesis': Each segment (rhombomere)
exhibits the same general pattern of neuron differentiation (default), but
then in each segment the pattern is modified in specific ways.
Several genes have been identified whose pattern of expression correlates with
the segmentary boundaries of the developing hindbrain
Hindbrain segmentation
In contrast to the rest of the vertebrate brain,
the embryonic hindbrain (rhombencephalon)
has a conspicuously segmented structure
Fate of
each
segment
Homeotic genes are master genes that encode the expression
of many other genes
Hindbrain segmentation
Overall architecture
of repeating units
The identity of each rhombomere is determined by the
differential expression of HOX genes
Positional identity
The positional identity along the
body axis is also based on a HOX code
I
II
III
IV
Gene I
Gene II
Gene III
Gene IV
Tissue
Positional identity
How is a cranio-caudal positional signal generated ?
(retinoic acid is also produced by Hensen's node, the Spemann
organizer)
Retinoic acid activates
the transcription of HOX genes,
and different HOX genes have different sensitivity for retinoic acid
Positional identity
A gradient of retinoic acid
controls the differential rostro-caudal pattern of
HOX gene expression
Notochord
Positional identity
The characteristics of the vertebrate nervous system
vary not only along cranio-caudal axis
but also along the dorso-ventral axis
NMWF2001_23-08, chicken 3 d, immunostaining
If the notochord is missing, floor plate cells and motoneurons fail to form
Positional identity
In the spinal cord, a band of specialized glial cells,
called the floor plate, drives the further development
of the neurons in the ventral area (e.g. motoneurons);
the floor plate is, in its turn, induced by the notochord
shh
shh always specifies the ventral phenotype, but the latter is different
Positional identity
As a rule of thumb, the fate of pluripotent precursor cells
is first determined along the rostro-caudal axis
and then along the dorso-ventral axis
in dependence on what HOX genes are turned on (from caudal to rostral: spinal motoneurons,
dopaminergic neurons in the posterior hindbrain, serotoninergic neurons in rostral hindbrain,
ocular motoneurons in the midbrain)
3rd summary:
3. How is the regional specification of the neural tissue
achieved?
- By the end of the 4th week the rostral and caudal neuropores are
closed. The regional differentiation of the brain vesicles and caudal
neural tube proceeds.
- In the brain the now operant organizer is the midbrain-hindbrain
organizer (transplantation leads to ectopic cerebellum and tectum;
WNT1, en-1, FGF-8).
-The embryonic hindbrain has a segmentl structure (7 rhombomeres).
Each rhombomere exhibits a characteristic pattern of neuron
differentiation (default pattern), but then it is modified in a
specific way uner the influence of particular, segment-specific
gene expression patterns.
- The identity of each rhombomere is determined by the differential
rostrocaudal pattern of HOX gene expression. The expression of
the HOX genes is controlled by retinoic acid (higher conc. caudally).
- The dorso-ventral polarity is controlled by shh (the notochord
induces floor plate cells, the floor plate cells induce the basal plate
cells, like motoneurons).
Q3:
How do single cells of the CNS
acquire their phenotype?
- stem cells (ominipotent, pluripotent)
- precursor cells (= already determined but not yet
differentiated X-blasts)
- differentiated cells (= postmitotic cells, with clear
phenotype)
Cell Regulationspotential
differentiation
The cells of the embryo/fetus are
subdivided according to their developmental
stage into
Embryonic stem cells
Lab case 1
Embryonic stem cells also divide
and differentiate under in vitro.
They can be transpanted back
into the maternal uterus .
The supernumerary zygotes are
a potential source for gaining
defined cells for transplantation
(blood, skin, neurons?)
Embryonic
Regulationspotential
stem cells
Lab case 2
By injecting embryonic
stem cells with a
targetted mutation into
a blastocyst one can
generate transgenic
animals with defined
properties .
Cells made to express a
fluorescent protein
under a specific
promoter can be traced
towards their final
destination.
Wetal99
NMWF2001_23-04
During cell division the cell is near the
ventricular surface, and the pial
connection is temporarily lost. DNA
synthesis occurs while the nucleus is
near the pial cell surface
Cell differentiation
Cells divide near the ventricular surface and then
migrate away
Following cell division, one or both of the
daughter cells may migrate away. This is
the point when they become neurons
or glial cells.
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MP1998_18.05
In cortical structures radial glial cells serve as a scaffolf for migration.
Isoforms for the integrin family of receptors mediate migration.
Cells that later
produce the GnRH
migrate > 2 mm,
from the olfactory
placode into the
thalamus
Cell Regulationspotential
differentiation
Once neurons migrate away from the ventricular zone, they become
postmitotic, i.e. they will never divide again. In contrast, glial cell
precursors can even divide after they have reached their final
destination
4. How do single cells of the CNS acquire their
phenotype?
4th summary:
- Depending on the degree of determination, immature cells are
subdivided into stemm cells (omnipotent, pluripotent), (partially)
determined precursor cells, differentiated cells.
- Cells divide near the ventricular surface and then migrate away.
- The neuroepithelium (neuroectoderm) gives rise to neuroblasts >
neurons, glioblasts > astrocytes, oligodendrocytes. Microglial cells are
derived from the mesenchym.
- Once a neuron migrates away from the ventricular surface it will
never divide again. In contrast, glial cell precursors can even divide
after they have reached their final destination.