Students of the Ambystoma genus of salamanders are aware of the transplantability of the animals' eyes, especially when the experimental subject is in the larval stage --and in terms of both donor and host considerations (e. g., rapid healing; tissue viability before revascularization; sluggish rejection mechanisms). We have exploited these attributes on numerous occasions since the late 1960's to pursue otherwise unanswerable questions concerning the biology of vision1. Incidental to our main observations, we have also observed a conspicuous but small number of subjects (some 7 percent) with a permanent, laterally directed, spastic flexure of the spine; i. e., a scoliosis.
Ambystoma opacum larvae Animals were 30 mm in length at operation; photo taken 90 days postoperatively. Left to right: Animals under MS 222 narcosis. MS 222 temporarily darkens a pigmented animal's skin. |
 Here is the same scoliotic animal but at 37 days postoperatively. The animal was unanesthetized during photography. |
The observed scoliosis seemed to be induced by the eye transplant. For record-filing purposes we were prompted to use the term Optoscoliosis to catalog it.
At first, we thought optoscoliosis was confined to what we called the 'cyclops' preparation -- bilaterally enucleated animals with a single eye mounted atop the head (with the membranous skull cap removed and the donor optic nerve stump aimed at the host's optic tectum). But our photographic records showed that optoscoliosis also occurred in some animals when the host site was an enucleated orbit ('orthoclops' operations).
 Here is a photograph of
three albino axolotl larvae (A. mexicanum). Each subject has
one eye; the animal in the middle is an orthoclops: its right eye was
removed and discarded; its left eye was removed and then returned to
the orbit. On
either side of the latter animal is a cyclops: the cyclops on the left
exhibits optoscoliosis whereas the cyclops on the right side has an
normal spine (click here
for reference.)
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With or without optoscoliosis, our subjects exhibited the visually guided behavior patterns and the optically elicited reflexes of control animals; i. e., the transplanted eye became functional, irrespective of optoscoliosis. Therefore, even with the bent spine, the regenerating optic nerve fibers of the donor must have reestablished optic pathways in the brain, at least in part.
On the basis of the literature2 our working hypothesis concerning optoscoliosis was that among the regenerating retinal fibers of the donor eye, some, on rare occasion, had taken an aberrant course down into the spinal cord, there to make abnormal connections.
We fully appreciate that our data did not, and do not, furnish unequivocal evidence for a causal connection between the transplanted eye and the observed scoliosis. Unfortunately, we no longer have the means to conduct the requisite experiments.
But optoscoliosis could conceivably provide a useful and exploitable model for investigating malformations in the central nervous system that arise when growing axons take a deviant course (e. g., Probst bundles in agenesis of the corpus callosum3). With the advent of reliable tracing methods, such as horseradish peroxidase (HRP), pursuit of the antecedents of optoscoliosis may also help answer questions, many still extant, that Roger Sperry raised a generation ago.4
2 Constantine-Paton, M. and Capranica, R.T. Central projection of optic tract from translocated eyes in the leopard frog (Rana pipiens). Science 189:480-482, 1975; Katz, M. J. and Lasek, R. J. Eyes transplanted to tadpole tails send axons rostrally in two spinal-cord tracts. Science 199:202-204 1978; Harris, W. A. and Cole, J. Common mechanisms in vertebrate axonal navigation: retinal transplants between distantly related amphibia. J. neurogenetics 1:127-140, 1984; Godment, P. and Bonhoeffer, F. Cross-species recognition of tectal cues by retinal fibers in vitro. Development 106:313-320 1989.
3Pirola, B. et al Agenesis of the corpus callosum with Probst bundles owing to haploinsufficiency for a gene in an 8 cM region of 6q25. J. Med. Genet. 35: 1031-1033, 1988; Hori A. Stan A. C. Supracallosal longitudinal fiber bundle: heterotopic cingulum, dorsal fornix or Probst bundle? Neuropathology 24:56-59, 2004
4Sperry, R. W., Mechanisms of neural maturation. In S. S. STEVENS (Ed.), Handbook of Experimental Psychology, New York, , pp. 236-280, 1951.