Growth cones stall and collapse during axon outgrowth in Caenorhabditis elegans

During nervous system development, neurons form synaptic contacts with dist ant target cells. These connections are formed by the extension of axonal p rocesses along predetermined pathways. Axon outgrowth is directed by growth cones located at the tips of these neuronal processes. Although the behavi or of growth tones has been well-characterized in vitro, it is difficult to observe growth cones in vivo. We have observed motor neuron growth cones m igrating in living Caenorhabditis elegans larvae using time-lapse confocal microscopy, Specifically, we observed the VD motor neurons extend axons fro m the ventral to dorsal nerve cord during the L2 stage. The growth cones of these neurons are round and migrate rapidly across the epidermis if they a re unobstructed. When they contact axons of the lateral nerve fascicles, gr owth cones stall and spread out along the fascicle to form anvil-shaped str uctures. After pausing for a few minutes, they extend lamellipodia beyond t he fascicle and resume migration toward the dorsal nerve cord. Growth cones stall again when they contact the body wall muscles, These muscles are tig htly attached to the epidermis by narrowly spaced circumferential attachmen t structures. Stalled growth cones extend fingers dorsally between these hy podermal attachment structures. When a single finger has projected through the body wall muscle quadrant, the growth cone located on the ventral side of the muscle collapses and a new growth cone forms at the dorsal tip of th e predominating finger. Thus, we observe that complete growth cone collapse occurs in vivo and not just in culture assays. In contrast to studies indi cating that collapse occurs upon contact with repulsive substrata, collapse of the VD growth cones may result from an intrinsic signal that serves to maintain growth cone primacy and conserve cellular material.