An identified cell is required for the formation of a major nerve during embryogenesis in the leech

Abstract

Investigations of the cues by which axonal growth cones navigate long distances to their targets have revealed the use of a rich and complex diversity of cellular and extracellular information. In the present study we describe one of the most conceptually simple pathfinding cues: a single identified cell in the leech, Hirudo medicinalis, that may guide axons several hundred micrometers to innervate a particular target. One of the stereotyped nerves of H. medicinalis is a “sex nerve” thatprojects from the anterior root of ganglion 6 [SNA (6)] tothe male reproductive structures in the adjacent anterior segment. The pathway for SNA (6) is completely underlainbya single peripheral cell, here called the axonal runway cell (ARC), before axons enter the pathway. The ARC is apparently a nonneuronal cell that stains with a monoclonalantibody that recognizes leech muscle cells. The importance ofthe ARC for establishing SNA (6) was tested by ablating itbefore axons entered the pathway. When the ARC was killed either by physical disruption with a microelectrode, or byphotoablation after filling it with the fluorescent dye Lucifer yellow, SNA(6) always failed to form, whereas all other nerves formed normally. Killing other peripheral cellsin proximity to the ARC did not interfere with SNA(6) formation. Ablation of possible “pioneer neurons” for SNA(6) also did not prevent its formation. These results show that formation of a particular nerve requires onlya single cell to serve as a guide for outgrowingprocesses.