Image supplied Akira Chiba, University of Illinois, IL, USA. © (2000) Macmillan Magazines Ltd.

The current view of synaptogenesis is of a one-sided affair with the axon leading the dance that eventually leads to synapse formation. Indeed, there is a large body of data describing the intricate and dynamic extension of motile filopodia that actively search the environment for suitable partners with which to form appropriate synapses. The role of the axon in actively seeking various molecular cues that facilitate axon guidance is a well established phenomenon that has been extensively studied and is, as a result, a relatively well understood process. In contrast, apart from the provision of the guidance cues, the role of the target cell is less well understood and it is, almost by default, characterized as a passive rather than a dynamic partner in synaptogenesis. Part of the problem is the very nature of the dendritic architecture, which presents substantial technological difficulties to the scientist wishing to study postsynaptic events in synaptogenesis.

As so often seems the case, the more tractable neuromuscular system now provides evidence of a more dynamic role for the postsynaptic tissue in synapse formation. In the October issue of Nature Neuroscience, Ritzenthaler, Suzuki and Chiba report that postsynaptic filopodia in muscle cells interact with innervating motor neurons and may force a reconsideration of the dynamics of pre- and postsynaptic events that lead to synaptogenesis.

Ritzenthaler and colleagues studied motor neuron targeting in Drosophila melanogaster embryos using high resolution in vivo time-lapse imaging to study the formation and properties of the postsynaptic microprocesses (myopodia) that are formed by the embryonic muscle tissue during neuromuscular synaptogenesis. Electron microscopy was used to study the morphology of muscle cells during the period when motor neuron axons start to make contact with the muscle tissue and form synapses. These experiments revealed that embryonic muscle cells extend numerous myopodia just before synaptogenesis and that these processes possess actin-based structural elements. Moreover, myopodia formed clusters at sites of motor neuron innervation and intermingled with innervating axon filopodia. Finally, the authors showed that the clustering was not observed in prospero mutants, which have severe delays in axon outgrowth, indicating that the clustering depends on axon innervation. These elegant studies suggest that the postsynaptic myopodia are dynamic rather than passive partners in the dance of synaptogenesis.