Neurulation

Neurulation is the stage of vertebrate development in which the nervous system first takes shape. It follows gastrulation: signals from the notochord, a rod of mesoderm running along the embryo's back, cause the ectoderm above it to thicken into a flat neural plate. The edges of the plate rise as neural folds with a neural groove between them, and the folds bend toward each other and fuse along the midline, pinching off a hollow neural tube beneath the surface ectoderm, which goes on to form skin.

In humans this primary neurulation takes place in the third and fourth weeks after fertilization, often before a pregnancy is known. Closure starts in the middle and proceeds toward both ends, and the lowest part of the spinal cord forms later by a separate process called secondary neurulation. The front of the tube swells into the brain vesicles, the rest becomes the spinal cord, and its hollow center becomes the brain ventricles and central canal. Cells at the crests of the folds break away as the neural crest, which forms peripheral nerves, pigment cells and parts of the face.

If the tube fails to close, neural tube defects result: spina bifida at the lower end or anencephaly at the head end. Adequate folic acid before and in early pregnancy greatly reduces their risk.