Revision 3 for 'Central nervous system embryology'

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Central Nervous System Embryology

The early central nervous system begins as a simple neural plate that folds to form a groove then tube, open initially at each end. Within the neural tube stem cells generate the 2 major classes of cells that make the majority of the nervous system - neurons and glia. These cells subsequently differentiate into many different types generated with highly specialized functions and shapes. Following is a summary of the embryology of the central nervous system. 

3 weeks

  • By the 3rd week of pregnancy the embryo thickens along the dorsal midline axis to form the neural plate. 
  • The plate then invaginates to form a groove and is flanked by neural folds. 
  • As the groove deepens the neural folds fuse to form the neural tube. 

4 weeks

  • The neural tube forms by the 4th week and detaches from the surface of the ectoderm to assume a deeper position. 
  • The tube begins to rapidly differentiate, the rostral end expands into the brain and the caudal end develops into the spinal cord. 
  • Small group of neural fold cells migrate from between the ectoderm and neural tube to form the neural crest. 
  • The neural crest subsequently gives rise to the sensory and autonomic neurons. 
  • 3 primary brain vesicles appear rostrally: prosencephalon (forebrain), mesencephalon (midbrain), rhombencephalon (hindbrain). 
  • The rest forms the spinal cord. 

5 weeks

By the 5th week secondary brain vesicles are evident:

  • Prosencephalon gives rise to the telencephalon (end-brain) & diencephalon (inter-brain).
  • Mesencephalon does not divide. 
  • Rhombencephalon gives rise to the metencephalon (after-brain), myelencephalon (spinal-brain).

After 5 weeks

All vesicles develop rapidly into the major structures and cavities of the adult brain:

  • Telencephalon - cerebral hemispheres (grey matter), white matter and basal ganglia. Cavity: lateral ventricles. 
  • Diencephalon - hypothalamus, thalamus and epithalamus. Cavity: third ventricle. 
  • Mesencephalon - midbrain. Cavity: aqueduct of Silvyus. 
  • Metencephalon - pons, cerebellum. Cavity: fourth ventricle. 
  • Myelencephalon - medulla oblangata. Cavity: central canal. 

Due to restricted space within the cranial cavity the following occurs:

  • The midbrain and cervical flexures develop which bend the forebrain towards the brainstem. 
  • The cerebral hemispheres are forced to take a 'horse-shoe' shaped course backwards and laterally. They grow over and cover most of the diencephalon and midbrain. 
  • Gyri and sulci develop at the end of the third fetal month which increase the surface area of the cerebral cortex. 

 

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