Cellular Interactions in the Development of Annelid Neuromuscular Systems

The ontogeny of muscles, motor neurons, and the central circuitryinvolved in producing patterned motor outputs is often thoughtof as a series of relatively independent and stereotyped events.Although mature neuromuscular systems are indeed often highlystereotyped, there is mounting evidence that the developmentalmechanisms that give rise to such stereotypy may often be interactive.The medicinal leech, Hirudo medicinalis, has stereotyped neuronsand muscles, yet at least some of the neuromuscular componentsseem to depend upon a particular sequence of cell-cell interactionsto differentiate, express identifiable phenotypes, or selectsynaptic partners. Three examples are summarized to illustratethese possibilities. First, a pattern-forming cell (the C-cell) develops at an earlystage and projects parallel processes that are used as a scaffoldupon which myocytes assemble. In the absence of the C-cell,oblique muscle fascicles never become organized. Second, atleast one way to match neuronal phenotype to that appropriatefor a segment-specific target is for homologous neurons to receivea signal from that target locally to partially respecify furtherdifferentiation of that neuron. Third, how do identified motorneurons select appropriate target muscles and how do interneuronalcircuits become matched to particular muscles when an interposedmotor neuron disallowsdirect interactions? A well-defined pattern-generatingsystem driving the muscular heart tubes in this leech is beginningto provide insights into these issues.