Download Development of brain stem and cerebellum

Development of Brain Stem,
Cerebellum and Cerebrum
• The neural tube cranial to the 4th pair of somites develop into the brain.
• 3 dilatations and 2 flexures form at the cephalic end of the neural tube
during the 4th week.
primary brain vesicles
1) Prosencephalon (forebrain)
2) Mesencephalon (midbrain)
3) Rhombencephalon (hindbrain)
flexures:
1) Cephalic flexure (midbrain region)
2) Cervical flexure (hindbrain-SC)
• Secondary brain vesicles (5th week) and their derivatives…
Brain flexures
• Midbrain flexure-Cervical flexure.
• Unequal growth of the brain between flexures produces
the pontine flexure in the opposite direction (5th week).
This flexure results in thinning of the roof of the hindbrain.
• Initially, the primordial brain has the same basic structure as the
developing spinal cord….
• However, the brain flexures produce variations in the position of
white and gray matter.
Hindbrain
• Spinal cord-servical flexure-hindbrain
Myelencephalon
• The caudal part of the myelencephalon (future closed part of medulla)
resembles the spinal cord.
• Central canal
• Neuroblasts from the alar plates migrate into the marginal zonenucleus gracilis, nucleus cuneatus
• Pyramids
• The rostral part of the myelencephalon (future open part of medulla) is
wide and rather flat.
• Roof plate is streched and thinned.
• Neural canal (future 4th ventricle) becomes rhomboidal-diamond shape.
• As the alar plates come to lie lateral, the sensory nuclei develop lateral to
the motor nuclei.
• Neuroblasts in the basal plates develop into motor nuclei.
• Neuroblasts in the alar plates develop into sensory nuclei.
Metencephalon
• Walls form the pons and cerebellum.
• Cavity forms the superior part of the 4th ventricle.
Pons
• Pontine flexure causes divergence of the lateral walls.
• Neuroblasts of each basal plate develop into motor nuclei.
• Some neuroblasts from the alar plate….pontine nuclei
Cerebellum
• Develops from thickenings of dorsal parts of the alar plates (cerebellar swellings).
• The swellings enlarge and fuse in the median plane, cerebellar vermis and
hemispheres forms.
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The primary fissure forms and divides the cerebellum into anterior and
middle lobes.
Continued fissuration subdivides the expanding cerebellum into further
lobes and then, starting in the 3rd month, into lobules and folia.
• The structure of cerebellum reflects its evolutionary development;
1. Archicerebellum (flocculonodular lobe)
2. Paleocerebellum (anterior lobe and vermis)
3. Neocerebellum (posterior lobe)
• Neuroblasts of the mantle zone of the alar plate migrate to the marginal zone,
differentiate into the neurons of the cerebellar cortex.
• Other neuroblasts of the alar plates give rise to central nuclei (e.g. dentate
nucleus)
Choroid plexuses
• Thin ependymal roof is covered by piamater with numerous blood vessels. This
vascular membrane together with the ependymal cells forms the tela choroidea
of the 4th ventricle.
• Pia mater proliferates and tela choroidea invaginates the 4th ventricle and
differentiates into choroid plexus.
• Similar plexuses develop in the roof of the third ventricle and the medial walls
of the lateral ventricles.
Midbrain
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Neural canal….. Cerebral aquaduct
Neuroblast from the alar plates migrate to form sup/inf colliculi at tectum.
Neuroblast from the basal plates give rise to groups of neurons in tegmentum.
Substantia nigra; gray matter
Fibers growing from the cerebrum form the crus cerebri.
Forebrain
• Rostral part- telencephalon- primordia of the cerebral hemispheres
• Caudal part- diencephalon
• Alar and basal plates and sulcus limitans are recognizable until the
junction of midbrain and forebrain!!!
Diencephalon
• In the lateral walls of the 3rd ventricle 3 swellings develop
– Epithalamus
– Thalamus
– Hypothalamus
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Epithalamus- pineal gland
The growing thalami meet across the third ventricle, form the interthalamic adhesion.
Hypothalamus
Pituitary gland
Telencephalon
• Consists of a median part and 2 lateral cerebral vesicles.
• The lateral ventricle in each hemisphere communicates with the third
ventricle through an interventricular foramen (of Monro).
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Developing cerebral hemispheres expand in all directions until they
cover the diencephalon.
The rostral wall of the forebrain, the lamina terminalis, is very thin.
• The mesenchyme trapped in the longitudinal fissure between the
cerebral hemispheres; falx cerebri
• The corpus striatum apppears during the 6th week as a swelling in the
future temporal lobe.
• The floor of each hemisphere expands slowly than the lateral walls;
cerebral hemispheres become C-shaped.
• Corpus striatum divides into the caudate and lentiform nuclei
by the internal capsule (fiber pathway).
• Caudal end of each hemisphere turns ventrally and then rostrally
forming the temporal lobe; it carries the lateral ventricle.
• The expansions of the neural canal in the brain vesicles and cerebral
hemispheres give rise to the cerebral ventricles.
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lateral ventricles in the cerebral hemispheres, the
3rd ventricle in the diencephalon,
the narrow cerebral aqueduct (of Sylvius) in the mesencephalon
4th ventricle in the rhombencephalon.
• The telencephalon gives rise to
commissural tracts that
connect corresponding areas
of the left and right cerebral
hemispheres. These include
the anterior and hippocampal
commissures and the corpus
callosum.
• The small posterior and
habenular commissures arise
from the epithalamus.
• Initially the surface of
the cerebral
hemispheres is smooth
• As growth proceeds
sulci and gyri develop
• The walls of the developing cerebral hemispheres initially show the
3 typical zones of the neural tube (ventricular, mantle, marginal).
• Cells of the mantle zone migrate to the marginal zone and give rise
to the cortical layers.
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Morphogens and transcription factors play role in the development of
nervous system.
Dorsal to ventral gradient of sonic hedgehog (Shh) and bone
morphogenetic proteins (BMPs) determine dorsal-ventral cell fates.
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Shh ventralizes the neural tube, induces the floor and basal plates.
BMPs increase the expression of dorsalizing genes; PAX3, 7 in the
alar and roof plates.
References
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The Developing Human: Clinically Oriented Embryology by Keith L. Moore, T.
V. N. Persaud and Mark G. Torchia (2013). 9th ed. Elsevier Saunders,
Philadelphia. ISBN: 978-0-8089-2444-9
Langman’s Medical Embryology by T.W. Sadler (2012). 12th ed. Lippincott
Williams & Wilkins, Philadelphia. ISBN: 978-1-4511-4461-1
Human Embryology by Larsen WJ (2001). 3rd ed. Churchill Livingstone,
Philadelphia. ISBN: 978-0-443-06583-5
Netter’s Atlas of Human Embryology by Larry R. Cochard (2002). 1st ed. Icon
Learning Systems, New Jersey. ISBN: 0-914168-99-1
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