Neural Tube - Development

Development

There are 2 ways in which the neural tube develops: Primary neurulation and Secondary neurulation.

1. Primary neurulation divides the ectoderm into three cell types, the neural tube, which is internally located, the epidermis, which is externally located, and the neural crest cells, which develop in the region between the neural tube and epidermis but then migrate to new locations. Primary neurulation begins after the neural plate has formed. The edges of the neural plate start to thicken and lift upward forming the neural folds. The center of the neural plate remains grounded allowing a U-shaped neural groove to form. This neural groove sets the boundary between the right and left sides of the embryo. The neural folds pinch in towards the midline of the embryo and fuse together to form the neural tube.
2. 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 posterior 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.

The manner in which the neural tube closes in mammals in the head is inverted in respect to the manner of closure in the trunk:

• In the head:

1. Neural crest cells migrate
2. Neural tube closes
3. Overlying ectoderm closes

• In the trunk:

1. Overlying ectoderm closes
2. Neural tube closes
3. Neural crest cells migrate