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Development of the brain

The division of the forebrain vesicle into the Telencephalon & the diencephalon
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  • Lateral diverticulum appears from each side of the forebrain called optic vesicle
  • Optic vesicle & the stalk will form the retina & the optic nerve
  • cavities are formed inside the cerebral hemispheres on either side & they are called lateral ventricles
  • The 3rd ventricle is formed by Telencephalon (Anterior part) & diencephalon (Posterior part) and its openings to the lateral ventricles form the interventricular foramen
  • On each side of the lateral ventricle, alar plate forms:


    1. A thickening to form the thalamus
    2. Medial & lateral geniculate bodies
  • Lower part of the alar plate on each side will differentiate into hypothalamic nuclei
Development of the cerebral hemispheres

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prosencephalon in a 7-week embryo
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Medial surface of the right half of the telencephalon and diencephalon in an 8-week embryo
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Medial surface of the right half of the telencephalon and diencephalon in a 10-week embryo.

  • Cerebral hemispheres grow & expand,


    1. Anteriorly : Form frontal lobes (1st)
    2. Laterally & superiorly : Parietal lobes (2nd)
    3. Posteriorly & inferiorly : occipital & temporal lobes (Last)
  • The matrix cells lining the floor of the forebrain proliferate to produce neuroblasts, they collectively forms corpus striatum; Later corpus striatum differentiate into caudate & lentiform nuclei
  • Longitudinal thickening of the wall of the forebrain which protrudes into the lateral ventricle forms the hippocampus
  • Many axons leave & arrive the differentiating cerebral hemispheres (Ascending & descending tracts)
  • They pass between the thalamus & caudate nucleus medially and lentiform nucleus laterally, the compact bundle of ascending & descending tracts is called the internal capsule
  • The differentiation matrix cells


    1. The matrix cells lining the cavity of the cerebral hemispheres produce large number of neuroblasts & neuro glial cells (They migrate to the marginal zone)
    2. The remaining matrix cells will form the ependyma (Lines the lateral ventricle)
  • At the 12th week, cortex become very cellular due to migrating neuroblasts; different areas of the cortex will show specific cell types due to the influence of the ascending & descending tracts


    1. Motor cortex : Pyramidal cells
    2. Sensory areas : Granular cells
Development of the commisures
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Medial surface of the right half of the brain in a 4-month embryo showing the various commissures. Broken line, future site of the corpus callosum. The hippocampal commissure is not indicated

  • Lamina terminalis (The cephalic end of the neural tube) forms a bridge between the 2 cerebral hemispheres & enables nerve fibers to move from one to the other
  • Anterior commisures


    1. 1st commisure to develop
    2. Runs in the lamina terminalis
    3. Connects olfactory bulbs of the temporal lobes on either side with each other
  • The fornix


    1. 2nd commisure to develop
    2. Connects hippocampus in each hemisphere
  • Corpus callosum


    1. 3rd commisure to develop
    2. It’s the largest & most important
    3. It connects frontal lobes (Initially) & parietal lobes (Later) of both sides
    4. As it develops, it arches over the roof of the developing 3rd ventricle
  • The remaining of the lamina terminalis (Lies between the corpus callosum & the fornix) form the septum pellucidum
  • Optic chiasma is formed by the inferior part of the lamina terminalis

Myelination of the CNS
  • The myelin sheath of the CNS is formed & maintained by oligodendrocytes (PNS axons are myelinated by Schwann cells)
  • Myelination of the spinal cord begins initially in the cervical region in the 4th month & extends downwards
  • Sensory fibers myelinate first & descending motor fibers at last
  • Myelination of the brain begins in the 6th month & confined to the fibers of the basal ganglia
  • At birth the brain is largely unmyelinated (In new born there’s very little cerebral function as most of the action are reflexes)
  • After birth, descending tracts starts to myelinate




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Development of the brain Development of the brain Reviewed by Ucfm on September 01, 2010 Rating: 5
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