Cut off one finger from a salamander and one will grow back. Cut off two and two will grow back. It sounds logical, but how the salamander always regenerates the right number of fingers is a biological mystery.

The salamander isn't the only animal with this regenerative ability. Take the sea squirt, Ciona intestinalis, a cylindrical marine creature about the size of a small cucumber that regularly loses its siphons, or feeding tubes, to hungry predators.

At the base of each siphon are eight photoreceptors, cells used to detect light. Whenever the sea squirt experiences a violent loss at the siphon base, the number of photoreceptors that grow back is always eight.

Understanding the molecular pathway responsible for this phenomenon is a research objective for MBL investigator William R. Jeffery, a former director of the MBL Embryology course and professor of biology at the University of Maryland. "The question I'm interested in is not only what mechanisms are involved in regeneration, but how exact [photoreceptor] patterns are formed," Jeffery says.

Following up on previous research, in which he experimentally induced variations in the number of photoreceptors that regenerate by manipulating the siphon's diameter, this summer Jeffery will test the role of the Notch signaling pathway, a highly conserved molecular cascade that determines how an embryo forms.

If Jeffery is on the right track, not only will he develop a model of regeneration in sea squirts, but in salamanders as well. Basic research on animal regeneration is a foundation for a major goal in medicine: Learning how to guide human stem cells to regenerate new tissues or organs.