Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube or neural canal (which strictly speaking is the center of the neural tube), the ectodermal wall of which forms the rudiment of the nervous system.

Development
There are two ways in which the neural tube develops: Primary neurulation and Secondary neurulation.


 * In primary neurulation, the cells of the neural plate invaginate and pinch off to form the tube.


 * In secondary neurulation, the cells of the neural plate form a cord-like structure that migrates inside the embryo and hollows to form the tube.

Each organism uses primary and secondary neurulation to varying degrees.


 * Neurulation in fish proceeds only via the secondary form.


 * In avian species the caudal regions of the tube develop using secondary neurulation and the anterior regions develop by primary neurulation.


 * In mammals, a similar pattern is observed where secondary neurulation begins around the 35th somite.

Structure
There are four subdivisions of the neural tube that will each eventually develop into distinct regions of the central nervous system: The prosencephalon, the mesencephalon, the rhombencephalon and the spinal cord.
 * The prosencephalon further goes on to develop into the telencephalon (the forebrain or cerebrum) and the diencephalon (the optic vesicles and hypothalamus).
 * The mesencephalon develops into the midbrain.
 * The rhombencephalon develops into the metencephalon (the pons and cerebellum) and the myelencephalon (the medulla oblongata).

The neural tube, for a short time is open both cranially and caudally. These openings, called neuropores, close during the fourth week in the human. Improper closure of the neuropores can result in neural tube defects such as anencephaly or spina bifida.

The dorsal part of the neural tube contains the alar plate, which is primarily associated with sensation. The ventral part of the neural tube contains the basal plate, which is primarily associated with motor (ie, muscle) control.