The establishment of cell-cell contacts through membrane extensions is a recurring theme during animal development. Neurons extend axons to contact distant cells, for example, and Drosophila and mouse muscle cells use specialized membrane extensions to contact motor axons. To understand how these muscle membrane extensions form, Dixon and Roy are investigating muscle arm development in Caenorhabditis elegans (see p. 3079). Muscle arms are specialized membrane extensions formed by the nematode's body wall muscles that contact the motor axons. The researchers report that in C. elegans muscle arm development is highly regulated: perturbing various cytoskeletal components and regulators, such as actin, ADF/cofilin and muscle myosin, disrupts muscle arm development. This reliance on proteins that remodel the actin cytoskeleton in other systems establishes C. elegans muscle arm development as an in vivo model in which to unravel the general details of membrane extension.