New work on zebrafish development is about to complicate a debate already brewing among embryologists. According to the mainstream dogma, the notochord, a rod-like structure that precedes the vertebral column in vertebrate embryos, signals neural cells to induce the formation of floor plate, a crucial row of cells that lies along the floor of the ventral brain and spinal cord. This belief has come under fire recently, as new data about floor plate development cast doubt on the authoritative role of the notochord.
A group of scientists at the Carnegie Institution of Washington, Vanderbilt University, and the National University of Singapore bring forth more evidence in opposition to the classic model with their report appearing in the September 10 Nature. The authors argue that the first steps in floor plate development occur earlier than was initially thought, in reaction to inducing factors other than those emitted by the notochord. The new data, based on studies of zebrafish mutants, may also shed light on certain neurological defects that affect humans as well as fish namely, holoprosencephaly. This syndrome, which can result in severe deformity and death, prevents the forebrain hemispheres from separating normally, in turn preventing the initial eye field from separating into two eyes during embryonic development.
The mainstream model for patterning of the central nervous system was
demonstrated in the 1980s and much supportive evidence has been obtained
ever since. In this model, the notochord instructs neural tube precursor
cells to differentiate into floor plate during the early stages of
somitogenesis (i.e., the time of formation of somites, blocks of similar
cells that differentiate into muscles, bone, and other tissues). The
command arrives via a potent messenger protein, Sonic hedgehog. Once
formed, the floor plate then induces motor neurons to develop on either
side (motor neurons will ultimately relay
Contact: Catherine Whittenburg