DEVELOPMENT OF DIFFERENTIAL PREGANGLIONIC PROJECTIONS TO PREVERTEBRALAND PARAVERTEBRAL SYMPATHETIC-GANGLIA

Sympathetic preganglionic axons project to spatially distinct targets in the periphery. A precise topographic pattern exists within the thor acic preganglionic cell column relative to the direction of axonal pro jections within the sympathetic chain. In this study, the time course and pattern of axonal outgrowth from different populations of pregangl ionic neurons in the chicken embryo is examined in detail to clarify t he origin of the topography in this system. Projections to prevertebra l targets are established by development of the splanchnic nerves by s tage 25, well after the earliest somatic motor projections at stage 19 but at least two stages before the reported onset of paravertebral pr ojections. Further, preganglionic axons that project rostrally into th e sympathetic chain may do so earlier than those that project caudally in the chain. The separation of preganglionic axons into prevertebral , rostral paravertebral or caudal paravertebral directions occurs at a common site in the ventral mesenchyme, established by the initial ven tromedial projection of the splanchnic nerves. Analysis of the axonal trajectories of rostrally and caudally projecting cells reveals that p reganglionic axons are not selectively fasciculated before their point of separation at the sympathetic chain. The patterning of the pregang lionic cell column is specified before the establishment of functional connections within the chain, indicating that target contact is not a determinant of the segmental pattern. We suggest that the differentia l outgrowth of preganglionic axons to peripheral targets is determined by the unique identities of underlying subpopulations of preganglioni c axons. (C) 1997 Wiley-Liss, Inc.