The precision of pathway selection by developing peripheral axons in the axolotl

Abstract

At the time of hindlimb development in the axolotl there is a well-established but still developing trunk innervation. The trunk innervation is primarily composed of the segmental nerves, each of which consists of a dorsal and a ventral ramus and its branches. At a few segmental levels in the region of the hindlimb a large number of additional axons arise to innervate the limb. To reach the limb, they must grow via the ventral rather than the dorsal rami. The precision with which this pathway is selected was determined by counting axons in the dorsal and ventral rami at limb segmental levels, prior to and during the period of maximum axon proliferation. If outgrowth is highly directed rather than random, then the ratio of ventral/dorsal ramus axons should increase significantly during the period when large numbers of additional axons are produced. In addition, since the dorsal trunk varies little in size between limb segments and immediately caudal 'non-limb' segments, the number of axons in the dorsal rami can be compared at the two levels. Mistaken projections should result in inordinately large axon numbers in dorsal rami at limb compared to non-limb levels. The results show that there is approximately a tenfold increase in the ratio of ventral/dorsal ramus axons at the time of maximum outgrowth to the limb, thus the mode of distribution at the ventral-dorsal branch point is significantly altered in favour of growth toward the limb, and outgrowth appears to be highly directed rather than random. Moreover at this time there is no discernible increase in the number of dorsal ramus axons at limb levels while those at non-limb levels increase fourfold. The apparent growth of all axons into the ventral ramus suggests the presence of a strong, non-specific attraction.