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.
- 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.
- 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.
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.