Developmentally Regulated Tissue-Associated Cues Influence Axon Sprouting and Outgrowth and May Contribute to Target Specificity

Abstract

The heart circuitry of the medicinal leech (Hirudo medicinalis) is a highly stereotyped circuit in the adult, but selection of the heart tube (HT) as a definitive target by heart excitor (HE) motor neurons during embryogenesis involves redirection of axonal arbors. In the present study we have confirmed the specificity of mature innervation using a retrograde marker and have used a combination of tissue/organ coculture and in situ manipulations to test the ability of HT and body wall to support axon outgrowth compared to CNS associated tissue. We also examined the temporal limits of target influence and the specificity of its action. Embryonic and young juvenile HT and body wall, but not adult HT, support or stimulate marked axon outgrowth from CNS ganglia, including those that would not innervate these tissues in vivo. Outgrowth support/stimulation by young tissue is largely contact based with little or no overt selectivity. Thus, outgrowth-supporting cues are developmentally regulated in the periphery, decreasing in efficacy with age while adult CNS-derived tissues consistently provide effective substrates supporting extensive axon outgrowth and regrowth. The HE motor neuron was very discriminating in that it showed little axon extension onto the HT compared to that of other neurons generally. These studies support a role for bidirectional communication in target selection. We suggest a working hypothesis that the HE motor neuron may initially select HT in response to a hierarchy of outgrowth supporting cues that have very broad influence and subsequently responds to selective signals for slowing or stopping growth and terminating on the functionally appropriate target.