Abstract
Each muscle fiber in the segmented body wall of Drosophila larvae is innervated by anatomically stereotyped neuromuscular junctions. These synapses arise through the selective choices of motoneuronal growth cones at their peripheral targets. Using digital optical microscopy of staged intracellular dye fills, we have singly identified embryonic motoneurons and have examined individual growth cones when they contact and differentiate at the target cells. There is a precise connectivity between motoneuron and muscle fiber, which is the direct consequence of growth cone behavior. We have also found that Drosophila muscle fibers possess molecularly heterogeneous cell surfaces that may be involved in growth cone recognition of appropriate targets. Fasciclin III, a homophilic adhesion molecule, is coexpressed by several of the efferent growth cones and in a site-specific fashion by the target muscle fiber's membrane. The fasciclin III expression is transient, corresponding to the period in embryogenesis when the first neuromuscular contacts are made. Upon encountering the target cell surface, the growth cones can sprout stereotypically arrayed filopodial processes, orient along the anterior-posterior axis, and turn in predictable directions. Subsequently, terminal branches are established in a nonrandom order. These phenomena were found to occur in two motoneurons that innervate adjacent muscle fiber targets, and may be general features of neuromuscular synaptogenesis in Drosophila.