The scheme illustrates the initiation and extinction of expression of growth factor receptors and markers of sympathetic neurons, blood vessel mediated path finding of axons to the target region, organogenesis of the NEM and PEM, onset of expression of growth factor ligands in the target erector muscles and concomitant extinction in blood vessels, cell body growth and specialization of NEM and PEM neurons and lastly, innervation of erector muscle target tissues as we interpret our results. Arrows indicate onset (upwards arrow) or extinction (downward arrow) of expression. Note that Ret is expressed in the neuroblasts, which extend axons along blood vessels towards the target, but it is thereafter downregulated in the newly born neurons and it is not turned on again until postnatal stages. Shortly after birth, ARTN is downregulated in blood vessels and upregulated in the nascent erector muscles. Ret re-expression in prospective EMNs participates in erector muscle specialization and target innervation. Abbreviations: BV, blood vessels; NEM, nipple erection muscle; PEM, piloerection muscle.