Transcript CNS DEVELOPMENT

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- 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 lining of:
1-The alimentary,
2- The respiratory
3-The 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 and 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.
Prosencephalon or (forebrain),
2.
Mesencephalon or (midbrain),
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Rhombencephalon or (hindbrain).
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The longitudinal axis of the developing CNS (neuraxis) does not remain straight
but is bent by :
1- Cephalic, or Midbrain flexure occurring at the junction of the forebrain and
midbrain
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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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
(medulla).
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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 & 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
withered, paralysed and
anaesthetic lower limbs
together with
incontinence of the
bowel and bladder.