Cellular and molecular features of axon collaterals and dendrites

Neural geometry is the major factor that determines connectivity and, possi bly, functional output from a nervous system. Recently some of the proteins and pathways involved in specific modes of branch formation or maintenance , or both, have been described. To a variable extent, dendrites and axon co llaterals can be viewed as dynamic structures subject to fine modulation th at can result either in further growth or retraction. Each form of branchin g results from specific molecular mechanisms. Cell-internal, substrate-deri ved factors and functional activity, however, can often differ in their eff ect according to cell type and physiological context at the site of branch formation. Neural branching is not a linear process but an integrative one that takes place in a microenvironment where we have only a limited experim ental access. To attain a coherent mechanism for this phenomenon, quantitat ive in situ data on the proteins involved and their interactions will be re quired.