Neurulation (Days 18 - 28)

Neurulation is the term given for the early development of the nervous system. The central nervous system develops from a neural plate which is derived from the ectoderm. The development of this plate is induced by chemicals released from the developing notochord, which triggers the overlying ectoderm to thicken and form the plate. The plate extends from the primitive node to the oropharyngeal membrane, much like the related notochord. The plate also broadens, extending across most of the ectoderm. On Day 18, a groove forms along the central axis of the neural plate called the neural groove which has raised areas on either side of it called neural folds. The neural folds are the first signs of brain development at the cranial end, and by Day 21 the folds have begun to move towards each other to eventually fuse and form a tube called the neural tube. This tube forms the spinal cord and brain.

Neurulation 1

A/B = a foetus on Day 18, C/D = a foetus on Day 20, E/E = a foetus on Day 21

By Day 22 the neural folds have begun to fuse in the middle section of the groove forming the first part of the neural tube. The rest of the groove continues to close bi-directionally from this central point, with the rostral (head) and caudal (tail) neuropores being the final parts to fuse. As the tube forms it detaches from the surface ectoderm and sinks into the mesoderm. Some neuroectodermal cells which are on the crest of the fusing grooves detach and migrate dorsolaterally on each side of the forming tube. These are called neural crest cells and they form a flattened layer called the neural crest between the surface ectoderm and the newly formed neural tube. The neural crest splits into two parts, one on each side of the neural tube, and differentiates into various ganglia including the spinal ganglia and autonomic nervous system ganglia, as well as the meninges. This process is completed by Day 28.

Neurulation 2Neurulation 3Neurulation 4Neurulation 5

Formation of the neural groove and neural tube

As the neural tube forms, the intraembryonic mesoderm next to each side of the tube proliferates to become paraxial mesoderm. At the end of the third week this mesoderm differentiates and divides into somites, which are paired cuboidal bodies that gives rise to the axial skeleton and associated muscles. The somites are very prominent on the embryo and the number of them present is commonly used to determine the age of the embryo. The first pair forms by Day 20 along the middle of the embryo and, much like the neural tube, they form in a bi-directional manner from this central point. Eventually 36 pairs of somites form along the length of the neural tube by the end of the fifth week of development.

An animation of the events occurring during neurulation

As neurulation is a very complex process involving genes and other extrinsic factors, it is not uncommon to have birth defects associated with the neural tube. These defects are termed neural tube defects and are the most common birth defect, with an incidence of approximately 1/1000 live births. The two most common defects are formed as a result of the neural tube failing to close. These are anencephaly in which the rostral (head) neuropore fails to close resulting in the absence of the majority of the brain, and spina biffida in which the caudal (tail) neuropore fails to close resulting in a herniation of tissue out of the lumbar spine. Folic acid is usually prescribed to pregnant women to prevent neural tube defects. Check out the birth defects page for information on other defects.

AnencephalySpina Biffida

Spina Biffida