Long-distance signals are required for morphogenesis of the regenerating Xenopus tadpole tail, as shown by femtosecond-laser ablation

Hauser published evidence that the source of the information, which requires the spinal cord, is actually in the brain [17]. The regenerate phenotypes he induced by damaging the subcommissural organ in the brain, or the spinal cord at what he termed the base of the tail (equivalent to antSC) match the phenotypes caused by laser ablation at antSC (compare Fig. 3D to Fig. 1b in [17]); it is not clear from the published images if there was left-right bending in those tails. His data suggest that the information is carried by Reissner's fiber, a continuously renewing strand of large-molecular mass, core-glycosylated proteins that starts in the brain and grows down the entire length of the central canal of the spinal cord. It is clear from our histological results that the laser disrupts or destroys the central canal (Fig. 2) thus the hypothesis that this fiber is the information conduit is consistent with our data; it does not, however, provide insight into the AP coding of the information.

The potential contributions to medicine promised by a greater understanding of regeneration are truly exciting, and there are an ever-growing number of studies of vertebrate regeneration. Surprisingly, despite the requirement for information to flow from undamaged tissue to the regeneration machinery, most work has focused on understanding only the tissue that is local to the amputation. We expanded the research scope to also include long-distance signals in Xenopus tail regeneration. Our method consisted of targeting many individual melanocytes on and inside the tadpole tail with fs pulses from a Ti:sapphire laser such that the absorbed heat damaged cells in their vicinities. We observed that the largest change in regenerate shape occurred along the AP axis where histological sections showed spinal cord damage. Quantifying the resulting shapes using Geometric Morphometrics allowed us to analyze these effects with much greater precision than has been possible with observations by eye. Our results suggest the existence of a long-distance, non-graded signal that affects morphogenesis of the regenerate. This work highlights the critical importance of long-distance signals for normal regeneration, and illustrates the need for more studies on the role of the entire animal, not just the cells that participate directly in replacing lost tissue. Such an approach could lead to better understanding of how to induce regeneration, and thus represents a new approach to the design of biomedical treatments for lost or damaged tissue.

The authors wish to acknowledge the contributions of Punita Koustubhan, Amber Currier, Bryan Pennarola, and Ryan Morrie for Xenopus care; Claire Stevenson for histology; N. Banks for laser sample preparation.