Early Stages:
At the end of week two, a structure called the primitive streak appears as a groove in the epiblast layer of the bilaminar disk.
Cells within the epiblast migrate downward through the primitive streak, giving rise to three layers from the initial two. These three germinal layers form the trilaminar embryonic disk:
- Endoderm – innermost layer
- Mesoderm – middle layer
- Ectoderm – outermost layer
The nervous system is derived from the ectoderm, which is the outermost layer of the embryonic disc. For
Neurulation:
In the third week of development, the notochord appears in the mesoderm. The notochord secretes growth factors which stimulate the differentiation of the overlying ectoderm into neuroectoderm – forming a thickened structure known as the neural plate.
The lateral edges of the neural plate then rise to form neural folds. The neural folds move towards each other and meet in the midline, fusing to form the neural tube (precusor to the brain and spinal cord).
During fusion of the neural folds, some cells within the folds migrate to form a distinct cell population – known as the neural crest. They give rise to a diverse cell lineage – including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia
The formation of neural tube is known as neurulation, and is achieved by the end of the fourth week of development.