In vivo imaging of synapse formation on a growing dendritic arbor
The elaboration of the innumerable synaptic connections that functionally link the cells of the nervous system in highly specific ways, is one of the miracles of the development. It is still difficult to clearly differentiate between synapse formation and subsequent maturation, as the process of development is very complex. The form of a neuron's dendritic arbor determines the set of axons with which it may form synaptic contacts, thus establishing connectivity within neural circuits. However, the dynamic relationship between dendrite growth and synaptogenesis is not well understood. To observe both processes simultaneously Cristopher M. Niell, Martin P. Meyer and Stephen J. Smith (In vivo imaging of synapse formation on a growing dendritic arbor. Nature Neuroscience, Vol. 7, No. 3, 254-260, 2004) performed long-term imaging of non-spiny dendritic arbors expressing a fluorescent postsynaptic marker protein as they arborized within the optic tectum of live zebrafish larvae.
The authors results indicate that almost all synapses form initially on newly extended dendritic filopodia. A fraction of these nascent synapses are maintained, which in turn stabilizes the subset of filopodia on which they form. Stabilized filopodia mature into dendritic branches, and successive iterations of this process result in growth and branching of the arbor. These findings support a 'synaptotropic model' in which synapse formation can direct dendrite arborization.