These layers are laid down during embryonic development, and each layer has characteristic mix of distinct subtypes of neurons that have different forms and sizes.
Previous evidence suggests that neurons that are born at around the same time end up in the same layer of the cortex, and tend to have the same form.
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However, further experiments that placed neurons with reduced levels of the protocadherin20 protein into layer 4 showed that these neurons acquire the characteristics of other layer 4 neurons, despite lacking a key layer 4 protein.
These results therefore suggest that the eventual form of a neuron is not determined just by when it is born, but also by the environment that it finds itself in.
It is also possible that the environment a newborn neuron finds itself in influences its eventual form.
However, many previous studies have investigated the function of molecules that work within the neurons, and the effect that a neuron’s surroundings have on its development still remains largely unknown.
Previous transplantation studies of ferret cerebral cortical neurons suggest that the ultimate laminar fate (whether they eventually come to reside in the superficial layers or in the deep layers) is determined, at least to some extent, in the progenitor cells (Mc Connell and Kaznowski, 1991).
The neocortex of mutant mice, such as homozygous mutant mice for Reln (also known as reeler),which contains an almost normal set of cortical neurons (Dekimoto et al., 2010; Hevner et al., 2003), but severely disorganized cortical lamination, shows some changes in subtype specification (Polleux et al., 2001).
In addition, several subtype-specific characteristics of cortical neurons appear in the post-migratory phase, when the cortical layers begin to develop, implying that additional events of subtype specification may also take place.
However, how much cell environment is crucial for subtype determination still remained unclear.
Here, we show that knockdown of Protocadherin20 (Pcdh20), which is expressed in post-migratory neurons of layer 4 (L4) lineage, caused the cells to localize in L2/3.