Development of the brain

The division of the forebrain vesicle into the Telencephalon & the diencephalon Lateral diverticulum appears from each side of...

The division of the forebrain vesicle into the Telencephalon & the diencephalon

• 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 3^rd 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

prosencephalon in a 7-week embryo

Medial surface of the right half of the telencephalon and diencephalon in an 8-week embryo

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 (1^st)
2. Laterally & superiorly : Parietal lobes (2^nd)
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 12^th 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

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. 1^st 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. 2^nd commisure to develop
2. Connects hippocampus in each hemisphere

• Corpus callosum

1. 3^rd 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 3^rd 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 4^th month & extends downwards
• Sensory fibers myelinate first & descending motor fibers at last
• Myelination of the brain begins in the 6^th 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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