Livers: better late than never.

After stealing fire for mankind from the gods, Prometheus faced the wrath of Zeus. The god chained Prometheus to a mountain crag where each morning an eagle would devour his liver, but the organ would grow back during the night. This ancient myth remarkably reflects the unusual capacity of a mature liver to regenerate lost tissue. Inspired by this tale, researchers have given the name prometheus to a mutant strain of zebrafish that appear to have no liver early in their lives.

Elke A. Ober of the University of California, San Francisco and her colleagues study the formation of internal organs in zebrafish, whose several-inch-long bodies are transparent. The scientists have genetically engineered the animals to produce a green fluorescent protein in cells throughout the abdomen.

In one mutant strain, the researchers found, an embryo has no apparent liver even 48 hours into its growth, although the organ is clearly visible by then in other zebrafish. Despite that, the mutant embryos survive and develop into healthy adults.

It turns out that a liver does eventually grow inside these mutant embryos. By adulthood, it reaches about 75 percent the size of a normal zebrafish liver. Ober and her colleagues are still looking for the mutated gene responsible for this glitch in liver development.--J.T.