Brain Development
Gastrulation, disjunction, and neurulation
During the 3rd week of gestation gastrulation occurs and a trilaminar disc is formed: ectoderm / mesoderm / endoderm
The notochord derived from mesoderm forms down the middle.
This induces overlying ectoderm to form neuroectoderm and neurulation occurs.
A) Neurulation and neuropore closure to form the neural tube and B) disjunction of surface ectoderm to form the neuroectoderm occur between days 21 and 27 of gestation. Problems at this point cause dysraphism and inclusion cyst disorders.
Neuroepithelium vs neural crest
Neural crest cells pinch off during same time frame to form cranial ganglia and postganglionic ANS and PNS / melanocytes / adrenal medulla / mid face / forebrain meninges.
Neural tube neuroepithelium gives rise to microglia and neurons whilst microglia and cerebral vasculature are mesodermal in origin.
Vesicles and flexures
Forebrain meninges neural crest derived whilst midbrain and hindbrain meninges are are mesoderm derived.
3 primary vesicles (4th week) followed by 5 secondary vesicles (5th week) form from the anterior neural tube: prosencephalon (giving rise to telencephalon and diencephalon) / mesencephalon / and rhombencephalon (giving rise to metencephalon and myelencephalon)
Telencephalon forms the cerebral hemispheres and lateral ventricles.
Diencephalon forms the thalamus / metathalamus (geniculate bodies) / epithalamus (habenular and habenular commissure and stria medullaris thalami and pineal gland) / and hypothalamus (supraoptic and tuberal and and mamillary regions) / the optic vesicles (retina and optic nerves) / and the 3rd ventricle.
Mesencephalon forms the midbrain and aqueduct.
Metencephalon forms the pons / cerebellum / and superior half 4th ventricle.
Myelencephalon forms the medulla and inferior 4th ventricle.
Anterior cephalic and cervical flexures from at the prosencephalon and myelencephalon. Followed by a posterior pontine flexure.
the pontine flexure between pons and medulla spreads the alar plate sensory structures lateral whilst keeping the motor basal structures medial. This also thins out the pia roof forming the tela choroidea which forms the choroid plexus. The lateral and medial apertures then involute to allow CSF egress from the 4th ventricle. Failure of this involution causes a Blake's Pouch cyst.
The metencephalon dorsal structures thicken and merge (rhombic lips) to form the cerebellum.
C shaped development
5th week the cerebral hemispheres start to grow with the Insula staying central and C shape folding of the overlying lobes around it leaving the insular at the bottom of the sylvian fissure and overlying the central core structures of the dorsal and ventral striatum and capsular structures over the diencephalon.
The commissures form from the lamina terminalis in the 7th week (the cap over the anterior neuropore which forms the anterior wall of the 3rd ventricle after development). The lamina terminalis gives rise to the optic chiasm / anterior commissure (connecting medial temporal lobes and amygdala) / the fornix (hippocampus to mamillary bodies) / and corpus callosum (the vestigial septum pellucidum representing the connecting origin from the lamina terminalis).
Corpus callosum forms from anterior to posterior with exception of rostrum which forms last.
15 weeks Telencephalic folding starts forming the sulci and gyri.
Germinal matrix
Germinal matrix is a Subependymal zone lateral to lateral ventricle. Contains neuronal and glial progenitor cells that migrate outwards during neurogenesis via radial glial cells. GM regresses back to caudothalamic groove. By 32 weeks only present at groove and completely regressed by 36 weeks. Friable and responsible for germinal matrix hemorrhages in premature neonates.