A matter of balance: the eyes have it.

A matter of balance: The eyes have it

Some Mexican salamanders are eyeless mutants. The defect does not seem to be in the eye region itself; previous studies have shown that if the eye region of such a mutant is transplanted into the flank of a normal salamander, the region will produce an eye. So what is it in a mutant that tells the eye primordium not to develop?

According to Rudolph Brun of Texas Christian University in Ft. Worth, normal development of sense organs in the salamander's head may depend on a proper balance of information from front and rear, perhaps signaled by different proteins produced by the two regions. The concentrations of the two proteins, like map coordinates, might tell the sense organs that they are in the correct place and are allowed to develop, Brun says. In the case of the eyeless mutants, an imbalance might tell the eyes that they should not develop.

Using both normal and mutant salamanders, Brun performed microsurgery on pairs of embryos very early in their development. He transplanted material from the nose primordium of one into the ear region of another; the excised ear material of the second was used to patch the nose region of the first. Brun found that, even though the eye regions were untouched during the operations, most of the mutants that received extra nose material implanted in their ear region went on to develop normal eyes. (Noses and ears developed normally.) "I really don't think the nose is that crucial,' Brun says. Instead, what seems to matter is strengthening the mutant's front side of the balance by implanting nose material from the front of the normal head.

Shifting the balance between anterior and posterior apparently prompts a total rearrangement of head organization, Brun says: While each operation involved only the right side of the head, mutants that received anterior material from normal salamanders developed eyes on both sides.

Researchers have postulated the existence of such concentration-based "guidance systems' for years, Brun says, but have had no way to investigate the mechanism; the mutant salamander might prove the model system. A report on the salamander experiments is in press in the JOURNAL OF NEUROGENETICS.