Neurulation

Neurulation is the formation of the neural tube from the ectoderm of the embryo. It follows gastrulation in all vertebrates. During gastrulation cells migrate to the interior of the embryo, forming the three germ layers: the endoderm (the deepest layer), the mesoderm (the middle layer), and the ectoderm (the surface layer) from which all tissues and organs will arise. 

In a simplified way, it can be said that the ectoderm gives rise to skin and the nervous system, the endoderm to the intestinal organs, and the mesoderm to the rest of the organs.

After gastrulation, the notochord—a flexible, rod-shaped body that runs along the back of the embryo—is formed from the mesoderm. During the third week of gestation the notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm. 

This results in a strip of neuronal stem cells that runs along the back of the fetus. This strip is called the neural plate, and it is the origin of the entire nervous system. 

The neural plate folds outwards to form the neural groove. Beginning in the future neck region, the neural folds of this groove close to create the neural tube (this form of neurulation is called primary neurulation). 

The anterior (ventral or front) part of the neural tube is called the basal plate; the posterior (dorsal or rear) part is called the alar plate. The hollow interior is called the neural canal. By the end of the fourth week of gestation, the open ends of the neural tube (the neuropores) close off.

Neurulation

Transverse sections that show the progression of the neural plate into the neural tube. 

This is a series of illustrations of transverse sections that show the progression of the neural plate into the neural tube. The first illustration shows the neural plate as flat, laying atop the mesoderm and notochord. The second shows the neural plate bending down, with the tow tends joining at the neural plate borders, which are now referred to as the neural crest. The third shows the closure of the neural tube and how this disconnects the neural crest from the epidermis. The neural crest cells differentiate to form most of the peripheral nervous system. Finally, the notochord degenerates and other mesoderm cells differentiate into the somites.

Secondary neurulation of vertebrates occurs when primary neurulation terminates. It is the process by which the neural tube at the lower levels and the caudal to the mid-sacral region is formed. 

In general, it entails the cells of the neural plate forming 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 (except fish, which use only secondary neurulation).

Clinical Example: Spina bifida is a developmental congenital disorder caused by the incomplete closing of the neural tube during neurulation.

Spina bifida

An illustration of a child with spina bifida

This is an illustration of a child with spina bifida. An open defect is seen at the base of the child's spine.