Neurulation

Another event taking place during the third week of development is the process of neurulation. The process of neurulation gives rise to the following derivatives:

  1. Brain

  2. Spinal cord

  3. Retina

  4. Neural crest cells

The primitive node extends as a tube of mesenchymal cells in the middle of the embryo to give rise to the notochordal process. Eventually, the notochordal process will fuse with the endoderm inferior to it to give rise to the notochordal plate. This notochordal plate will then form a solid rod that spans the entire length of the embryo as the notochord.

This sets us up for the rest of the process of neurulation as the notochord is the signaling center that instructs the ectoderm layer of cells.

With the formation of the notochord, a thickening occurs in the midline of the embryo around the 17th day of development, and this gives rise to the neural plates. This begins the formation of neuroectoderm from the cranial end towards the caudal end. The neural plate is broader in the caudal region and will give rise to the brain while its remaining parts will develop into the future spinal cord.

Following the formation of the neural plate, it will begin to fold inwards in the midline to give rise to a neural groove.

The walls or sides making up the neural groove are known as the neural folds.

At this juncture, parts of the neuroectoderm are brought together and these are known as neural crest cells.

Eventually, the neural folds meet each other to fuse and form a tube of neuroectoderm that spans the length of the developing embryo and this is known as the neural tube.

The neural tube will separate from the ectoderm which itself will reform over the neural tube in order to give rise to its derivatives.

Through the process of neurulation, the central nervous system will develop at the cranial region of the neural tube while the remaining regions of the neural tube will give rise to the spinal cord.

Neural Crest Cells

As the neural crest cells leave the neuroectoderm, they become more mesenchymal in origin. As these neural crest cells are pluripotent, they are able to differentiate into many different cell types in the developing embryo.

These neural crest cells will migrate away from the neural tube and give rise to many structures in the body including the following examples amongst others:

  1. Chromaffin cells

  2. Craniofacial cartilage and bones

  3. Enteric nervous system

  4. Schwann cells and satellite cells

  5. Peripheral nervous system

  6. Melanocytes of the skin

  7. Dermis of the skin

  8. Carotid bodies

  9. Endocardial cushions

  10. Meninges of the brain (pia and arachnoid mater)

Clinical Notes:

  1. Spina Bifida - this occurs when the vertebrae do not form entirely

    • Spina bifida occulta - small gaps in vertebrae of L5-S1 region

    • Spinal bifida miningocele - protrusion of meninges of the spinal cord through the gap in vertebrae

  2. Cleft lip or palate - this occurs when there is a deficiency in the number of neural crest cells that have migrated to the region of the developing face.