Post on 16-Jul-2016
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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.
flexures: 1) Cephalic flexure (midbrain region) 2) Cervical flexure (hindbrain-SC)
primary brain vesicles 1) Prosencephalon (forebrain) 2) Mesencephalon (midbrain) 3) Rhombencephalon (hindbrain)
• 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 zone-
nucleus 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.
• 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.
• 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
Pons
• 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
• 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.
• Rostral part- telencephalon- primordia of the cerebral hemispheres • Caudal part- diencephalon
Forebrain
• 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
• Epithalamus- pineal gland
• The growing thalami meet across the third ventricle, form the interthalamic adhesion.
• Hypothalamus
Pituitary gland
• 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).
Telencephalon
• 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. • 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.
• 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.
• 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
1. 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
2. Langman’s Medical Embryology by T.W. Sadler (2012). 12th ed. Lippincott Williams & Wilkins, Philadelphia. ISBN: 978-1-4511-4461-1
3. Human Embryology by Larsen WJ (2001). 3rd ed. Churchill Livingstone, Philadelphia. ISBN: 978-0-443-06583-5
4. Netter’s Atlas of Human Embryology by Larry R. Cochard (2002). 1st ed. Icon Learning Systems, New Jersey. ISBN: 0-914168-99-1
5. Human Embryology and Developmental Biology by Bruce M. Carlson (2009). 4th ed. Mosby, Elsevier, Philadelphia. ISBN: 978-0-323-05385-3