<P> 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 . </P> <P> 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 . </P>

The posterior part of the neural tube develops into the