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In the developing chordate (including ), the neural tube is the precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into the closed neural tube. In humans, neural tube closure usually occurs by the fourth week of pregnancy (the 28th day after conception).
Primary neurulation divides the ectoderm into three cell types:
Each organism uses primary and secondary neurulation to varying degrees.
Mammalian neural tubes close in the head in the opposite order that they close in the trunk.
For a short time, the neural tube is open both cranially and caudally. These openings, called , close during the fourth week in humans. Improper closure of the neuropores can result in neural tube defects such as anencephaly or spina bifida.
The dorsal part of the neural tube contains the alar plate, which is associated primarily with . The ventral part of the neural tube contains the basal plate, which is primarily associated with motor (i.e., muscle) control.
The spinal cord develops from the posterior neural tube. As the spinal cord develops, the cells making up the wall of the neural tube proliferate and differentiate into the neurons and glia of the spinal cord. The dorsal tissues will be associated with sensory functions, and the ventral tissues will be associated with motor functions. (2023). 9781947172043, OpenStax CNX. ISBN 9781947172043
Three main ventral cell types are established during early neural tube development: the floor plate cells, which form at the ventral midline during the neural fold stage; as well as the more dorsally located and . These cell types are specified by the secretion of the Shh from the notochord (located ventrally to the neural tube), and later from the floor plate cells. Shh acts as a morphogen, meaning that it acts in a concentration-dependent manner to specify cell types as it moves further from its source.
The following is a proposed mechanism for how Shh patterns the ventral neural tube: A gradient of Shh that controls the expression of a group of homeodomain (HD) and basic Helix-Loop-Helix (bHLH) transcription factors is created. These transcription factors are grouped into two protein classes based on how Shh affects them. Class I is inhibited by Shh, whereas Class II is activated by Shh. These two classes of proteins then cross-regulate each other to create more defined boundaries of expression. The different combinations of expression of these transcription factors along the dorsal-ventral axis of the neural tube are responsible for creating the identity of the neuronal progenitor cells. Five molecularly distinct groups of ventral neurons form from these neuronal progenitor cells in vitro. Also, the position at which these neuronal groups are generated in vivo can be predicted by the concentration of Shh required for their induction in vitro. Studies have shown that neural progenitors can evoke different responses based on the length of exposure to Shh, with a longer exposure time resulting in more ventral cell types.
At the dorsal end of the neural tube, BMPs are responsible for neuronal patterning. BMP is initially secreted from the overlying ectoderm. A secondary signaling center is then established in the roof plate, the dorsal most structure of the neural tube. BMP from the dorsal end of the neural tube seems to act in the same concentration-dependent manner as Shh in the ventral end. This was shown using zebrafish mutants that had varying amounts of BMP signaling activity. Researchers observed changes in dorsal-ventral patterning, for example, zebrafish deficient in certain BMPs showed a loss of dorsal sensory neurons and an expansion of interneurons.
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