The neural tube is a structure of the embryo of vertebrates that develops into the brain and spinal cord. It forms from the genetically guided enclosure of a plate of tissues inside of the embryo. These subdivide into sections at different places in the tube that correspond to the location of the brain regions that they will become. The peripheral nervous system develops outside of the neural tube through the migration of neural crest cells. Incomplete closure of the tube in either the brain or spinal column results in serious birth defects.
Neural tube formation takes different lengths of time, depending on the organism. In humans, it commences three to four weeks after conception. Early in development, the neural groove arises from a plate of tissue and deepens as it folds over a central axis. The genes that guide this process, called hox genes, are a subset of the system organizing the embryo into structural divisions. Along with the Cdx family of proteins moderating their action, the hox genes are critical in directing neural cells to their appropriate locations in the embryo.
Development of the neural tube is called neurulation. Primary neurulation occurs as soon as the neural plate forms, causing the plate's edges to fold up and enclose the center of the plate. Motor/neuromuscular control centers derive from tissues in the ventral portion of the tube, a region associated with the basal plate. Somatosensory functions, including perception and other forms of sensation, are conversely affiliated with the alar plate in the tube's dorsal section. Secondary neurulation occurs when the cells along the neural plate form a hollow space inside of it, completing the tube.
Fluid-induced bulging divides the neural tube into four sections, the first three of which become the brain while the last forms the spinal cord. Hox genes guide the division of the nervous system's different regions, as the hindbrain, midbrain, and forebrain develop respectively from the tube's rhomebencephalon, mesencephalon, and prosencephalon. New neurons form throughout this process, as a tissue called the neural crest separates from the tube and migrates throughout the embryo. Neural crest cells eventually become the nerves of the peripheral nervous system.
Spina bifida is a common neural tube defect occurring early in pregnancy during the brief time the tube is open, and resulting in incomplete closure of the fetal spinal column. The vertebrae that surround the spinal cord fail to fully surround it, so the cord, and sometimes a fluid-filled membrane, protrude out of the column. When the upper part of the neural tube does not close, a condition called anencephaly results in a partially exposed and deformed brain. Folic acid supplements before conception reduce the risk of defects like spina bifida.