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Central nervous system Development of the brain
Prof. Dr. Malak A. Al-yawer
Department of Anatomy/ Embryology Section
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Learning objectives
At the end of this lecture, the medical student will be able to
Define the primary brain vesicles and state their fates
Name the flexures that present in the cephalic end of neural tube
State the role of basal and alar plates in the formation of different parts of the brain
Describe the embryonic origin of olfactory system
State the embryonic origin of different types of commissures
Define the embryonic origin of cranial nerves and state the embryonic origin of their motor nuclei, sensory ganglion and parasympathetic ganglion
State some clinical correlates
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Primary brain vesicles Three dilations( primary brain vesicles):
(a) Prosencephalon (forebrain)
(b) Mesencephalon (midbrain)
(c) Rhombencephalon (hindbrain)
Two flexures:
A. Cervical flexure hindbrain - spinal cord
(b) Cephalic flexure midbrain
Rhombencephalic isthmus mesencephalon -rhombencephalon
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Rhombencephalon (Hindbrain)
consists of the
(a) Metencephalon
pontine flexure -rhombencephalic isthmus
later forms the pons and cerebellum
(b) Myelencephalon
most caudal of the brain vesicles
forms the medulla oblongata
Pontine flexure
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Myelencephalon differs from the spinal cord in that its lateral walls are everted
the sulcus limitans can be clearly distinguished.
The basal plate contains motor nuclei
The alar plate contains three groups of sensory relay nuclei
The roof plate
Tela choroidea
Choroid plexus
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Metencephalon 1. pons Basal plate / 3 groups of
motor neurons
Alar plate / 3 groups of sensory nuclei
Marginal layer of its basal plates expands & makes a bridge for nerve fibers
Pontine nuclei originate in the alar plates of the metencephalon and myelencephalon
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Metencephalon 2. Cerebellum
rhombic lips cerebellar plate shows vermis and 2 hemispheres (12
week embryo) A transverse fissure separates the
nodule from the vermis and the lateral flocculus from the hemispheres .
Flocculonodular lobe is phylogenetically the most primitive part of the cerebellum.
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Mesencephalon (Midbrain )
Basal plate / 2 groups of motor nuclei
Marginal layer of its basal plate enlarges and forms the crus cerebri
Alar plates Initially / 2 longitudinal
elevations separated by a shallow midline depression
With further development, a transverse groove divides each elevation into superior & inferior colliculi
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Prosencephalon
1. Diencephalon develops from the median portion of the
prosencephalon
characterized by outgrowth of the optic vesicles and forms the
optic cup and stalk
pituitary
thalamus
hypothalamus
epiphysis
Roof plate
choroid plexus of 3rd ventricle
Its most caudal part develops into the pineal body ( epiphysis)
Alar plates / lateral walls of the diencephalon.
Hypothalamic sulcus
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Prosencephalon 2. Telencephalon
consists of 2 lateral outpocketings ( cerebral hemispheres) a median portion(lamina
terminalis) / used by commissures
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• Insula: the depressed area between the frontal and temporal lobes. At the time of birth it is completely covered.
• During the final part of fetal life, the surface of the cerebral hemispheres grows so rapidly that a great many convolutions (gyri) separated by fissures and sulci appear on its surface
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Olfactory system epithelial– mesenchymal
interactions between 1. neural crest cells and
ectoderm of the frontonasal prominence - olfactory placodes / primary sensory neurons
2. same crest cells and the floor of the telencephalon - olfactory bulbs/ secondary neurons
By the 7th week, these contacts are well established.
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Commissures • Anterior commissure first to appear connecting the olfactory bulb and related
brain areas of one hemisphere to those of the opposite side
• Hippocampal commissure ( fornix commissure )
second to appear arise in the hippocampus and converge on
the lamina terminalis to the mamillary body and the hypothalamus
• Corpus callosum appears by the 10th week of development connects the non olfactory areas of the
right and the left cerebral cortex • Posterior and habenular commissures /
below and rostral to the stalk of the pineal gland.
• Optic chiasma rostral wall of the diencephalon contains fibers from the medial halves of
the retinae.
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Cranial Defects Holoprosencephaly (HPE)
• Loss of midline tissue has resulted in a midline cleft lip, lack of nasal tissue, and eyes that are too close together
• The loss of midline tissue causes the lateral ventricles to merge into a single chamber.
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Drawings illustrating various types of skull defects
meningocele (B)
Meningoencephalocele(C)
Meningohydroencephalocele(D)
usually occur in the occipital region, but may occur in the frontonasal region.
Origin of these defects is due to abnormal neural tube closure/ can be prevented by maternal use of folic acid
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Hydrocephalus
• is due to an obstruction of the aqueduct of Sylvius (aqueductal stenosis).
• This prevents the CSF of the lateral and third ventricles from passing into the 4th ventricle and from there into the subarachnoid space, where it would be resorbed.
• As a result, fluid accumulates in the lateral ventricles and presses on the brain and bones of the skull.
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Microcephaly
• a cranial vault is smaller than normal
• due to poor growth of the brain
• frequently associated with mental retardation.
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Cranial nerves • 4th week of development/ nuclei
for all 12 cranial nerves are present.
• All except the olfactory (I) and optic (II) nerves arise from the brainstem, and of these only the oculomotor (III) arises outside the region of the hindbrain.
• The hindbrain is divided into eight rhombomeres (r1–r8) - give rise to motor nuclei of cranial nerves IV, V, VI, VII, IX, X, XI, and XII.
• Motor neurons for cranial nuclei are within the brainstem, while sensory ganglia are outside of the brain.
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Cranial nerve sensory ganglia
originate from 1. ectodermal placodes and 2. neural crest cells • Ectodermal placodes include 1. Nasal placode 2. otic placode 3. four Epibranchial placodes
dorsal to the pharyngeal (branchial) arches & contribute to ganglia for nerves of the pharyngeal arches (V, VII, IX, and X).
Parasympathetic (visceral efferent) ganglia
• derived from neural crest cells
• their fibers are carried by cranial nerves III, VII, IX, and X
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Summary The cephalic end of the neural tube shows three dilations (primary
brain vesicles) The cephalic end of the neural tube forms two flexures: cervical and
cephalic flexures Rhombencephalic isthmus separates the mesencephalon from the
rhombencephalon Distinct basal and alar plates, representing motor and sensory
areas, respectively, are found on each side of the midline in the rhombencephalon and mesencephalon
In the prosencephalon, however, the alar plates are accentuated and the basal plates regress
Differentiation of the olfactory system is dependent on epithelial– mesenchymal interactions
All cranial nerves except olfactory (I) and optic (II) nerves arise from the brainstem
Cranial nerve sensory ganglia originate from ectodermal placodes and neural crest cells
Parasympathetic (visceral efferent) ganglia are derived from neural crest cells