Transcript 02 Development of the CNS

Prof. Saeed makarem
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EARLY DEVELOPMENT
By the beginning of the second week, three germ cell layers
become established:
Ectoderm, Mesoderm and Endoderm.
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Each germ
layer gives rise
to particular
tissues and
organs in the
adult.
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The Ectoderm
gives rise to:
1- Epidermis of the Skin
2- Nervous system.
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The Mesoderm
forms:
1-Skeleton,
2-Muscular tissues
3-Connective tissues.
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The Endoderm gives
rise:
to the:
1-Alimentary,
2- Respiratory
3-Genitourinary tracts.
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• During the 3rd week, the dorsal
midline ectoderm undergoes
thickening to form the neural
plate.
• The lateral margins of the
neural plate become elevated,
forming neural folds.
• In between the two neural folds
a longitudinal, midline
depression, will form the neural
groove.
• The neural folds then become
apposed and fuse together.
• Now the neural groove is
transformed into the neural
tube.
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• Some cells from the
apices of the neural
folds become
detached to form
groups of cells lying
dorsolateral to the
neural tube.
• These are known as
the neural crests.
• By the middle of the
fourth week the
formation of the neural
tube is completed.
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DEVELOPMENT OF THE BRAIN & SPINAL CORD
• Enormous growth,
distortion & cellular
differentiation occur
in the neural tube.
• This is maximal in
the rostral part,
which develops
into the brain,
while the caudal
part will forms the
spinal cord.
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The central cavity
within the neural
tube becomes:
The ventricles of
the brain &
The central canal
of the spinal cord.
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The neural crests
cells form the
sensory ganglia of
the spinal & cranial
nerves, and also the
autonomic ganglia.
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• As development continues, a longitudinal groove, called the sulcus
limitans, appears on the inner surface of the lateral walls of the
embryonic spinal cord and caudal part of the brain.
• The dorsal and ventral cell groupings thus forming the alar plate and
the basal plate, respectively.
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Nerve cells that develop within the Alar plate have
predominantly sensory functions, while those in
the basal plate are predominantly motor.
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• Further development
also brings about
the differentiation
of grey and white
matter.
• The grey matter is
located centrally
around the central
canal,
• While the white
matter forming an
outer coat.
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This basic developmental pattern can still
easily be recognized in the adult spinal cord.
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DEVELOPMENT OF THE BRAIN
• During development, the rostral portion of the neural tube
undergoes massive differentiation and growth to form the brain.
By the 5th week, three primary brain vesicles can be identified:
1.
2.
3.
Prosencephalon or (forebrain),
Mesencephalon or (midbrain),
Rhombencephalon or (hindbrain).
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The longitudinal axis of the developing CNS does not remain straight but is
bent by :
1- Cephalic, or Midbrain flexure occurring at the junction of the forebrain
and midbrain
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2-Cervical flexure between the brain and the spinal cord.
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By the 7th week further differentiation distinguishes five secondary
brain vesicles produced by:
• Division of the prosencephalon into 2 lateral vescicle the
telencephalon and a median part the diencephalon.
• Also, division of the rhombencephalon into the metencephalon
and myelencephalon.
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The junction between the metencephalon and the
myelencephalon is marked by an additional bend in
the neuraxis, called the pontine flexure.
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cavity
Lateral V
3rd.V
Aqueduct
of Midbrain
4th Vent.
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FURTHER DEVELOPMENT OF THE BRAIN
• Of the three basic
divisions of the
brain, the
Prosencephalon or
forebrain is by far
the largest.
• It is also referred to
as the cerebrum.
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The telencephalon
undergoes the greatest
further development
and gives rise to the
two cerebral
hemispheres.
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The cerebral
hemispheres
consist of:
• Outer layer of
grey matter
(the cerebral
cortex)
• Inner mass of
white matter,
within which
various groups
of grey matter
nuclei are
buried
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The Mesencephalon,
or midbrain, is
relatively
undifferentiated,
(it still retains a
central tube-like cavity
surrounded by grey
matter).
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The
metencephalon
develops into the
Pons anteriorly
& cerebellum
posteriorly.
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The
myelencephalon
forms the medulla
oblongata.
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As the brain
develops, its
central cavity
also undergoes
considerable
changes in size
and shape
forming a
system of
ventricles,
which contain
cerebrospinal
fluid (CSF).
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DEVELOPMENTAL ANOMALIES
Disorders of development disrupt the normal
growth and structural organization of the
spinal cord and brain.
Because the nervous system is derived from
embryonic ectoderm, these developmental
anomalies also involve the coverings of the
nervous system (skin and bone).
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ANENCEPHALY
In anencephaly,
the brain and skull
are minute and the
infant does not
usually survive.
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SPINA BIFIDA
• In spina bifida, the
lower part of the
spinal cord and
nerve roots are
underdeveloped
and may lie
uncovered by skin
or the bony spine
on the infant's
back.
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MENINGIOMYELOCELE
• It is Spina Bifida with
• the meninges of the
spinal cord appear on
the back of the infant.
• Such infants are left
with flail, paralyzed
and anaesthetic lower
limbs together with
incontinence of the
bowel and bladder.