In the beginning there are just a few cells. What emerges is a brain with a precise structure and intricate circuitry.
Development is a process of spatial and temporal gene expression .
Perhaps surprisingly, the initial development of the nervous system doesn't produce exactly the desired number of neurons, but evidently it produces many more than is needed. After injection of Neurobiotin, a low molecular mass tracer known to cross gap junction channels, into a single identified lumbar motor neuron, several labeled motor neurons were found in spinal cord sections after staining with fluorescein-conjugated strepavidin. From E. Bryan Crenshaw's lab. A retinal growth cone loses its normal motile structure and is paralyzed when it touches a sympathetic axon in culture.