Central nervous system embryology

Central nervous system (CNS) embryology is complex, and below is a brief summary of its development.

The early CNS 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 two 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.

Development

By the 3 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 4 week and detaches from the surface of the ectoderm to assume a deeper position. The tube begins to rapidly differentiate, its rostral end expands into the brain and the caudal end develops into the spinal cord. A small group of neural fold cells migrate from between the ectoderm and neural tube to form the neural crest. The neural crest gives rise to the sensory and autonomic neurons. Three primary brain vesicles appear rostrally:

• prosencephalon (forebrain)
• mesencephalon (midbrain)
• rhombencephalon (hindbrain)

The rest forms the spinal cord.

By the 5 week, secondary brain vesicles are evident. The prosencephalon gives rise to the telencephalon (end-brain) and diencephalon (inter-brain). The mesencephalon does not divide and the rhombencephalon gives rise to the metencephalon (after-brain) and the myelencephalon (spinal-brain).

After 5 weeks all vesicles develop rapidly into the major structures and cavities of the adult brain:

• telencephalon forms the grey matter of the cerebral hemispheres, white matter, basal ganglia and lateral ventricles
• diencephalon forms the hypothalamus, thalamus, epithalamus and the third ventricle
• mesencephalon forms the midbrain and cerebral aqueduct (of Sylvius)
• metencephalon forms the pons, cerebellum and fourth ventricle
• myelencephalon form the medulla oblongata and central canal

Due to continued growth of the CNS within the restricted space of the cranial cavity, developing structures fold in characteristic ways. Midbrain and cervical flexures develop which bend the forebrain towards the brainstem. The cerebral hemispheres are forced to take a 'horse-shoe' shaped course backward 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.