Synaptogenesis and its relation to growth of the postsynaptic cell: a quantitative study of the developing Mauthner...

We describe the relation between growth and branching of an identified dendrite and the formation of synapses on its surface during a 3 1/2-day period early in development. We studied the lateral dendrite and the adjacent lateral perikaryon of the Mauthner cell (M-cell) during embryonic stages 39-43 in the axolotl Ambystoma mexicanum. Reconstructions from light micrographs of serial sections through the cell revealed that during this interval the dendrite elongates rapidly, and large numbers of ventrally directed branches are formed. Samples of the same material by electron microscopy showed that large numbers of synaptic contacts appear during the same interval. We quantitatively estimated changes in local synapse densities (the number of contacts/100 micrometers2 of M-cell surface) and local surface areas of the M-cell and found that synapses were most densely clustered, and accumulated most rapidly, on regions of the cell that were rapidly expanding. These data are in accord with previous evidence from work in this and in other systems that synaptic contacts induce local growth of dendrites. Furthermore, the data are consistent with a proposal that outgrowth of new dendritic branches is induced or stabilized by synapses in a concentration-dependent fashion.