Eyes are among the earliest recognisable structures in an embryo; they start off as bulges on the sides of tube-shaped tissue that will eventually become the brain. Researchers from the European Molecular Biology Laboratory (EMBL) in Heidelberg have now discovered that cells are programmed to make eyes early in development and individually migrate to the right place to do so. The study, published in this week's issue of the journal Science, overturns the textbook model of the process and suggests that also other organs might be formed by the movement of single cells rather than sheets of entire tissues.
Jochen Wittbrodt and his lab at EMBL made the discovery using advanced microscope techniques to track individual cells in the transparent embryos of a small fish called Medaka.
"You can think of the tube as a deflated balloon shaped like a Mickey Mouse," Wittbrodt says. "As the fish grows, the eyes gradually bulge out from the tube, the way Mickey Mouse ears expand as a balloon is filled with air. Most scientists have thought that cells in the neighbouring regions grow to make the bulges. What we've seen is that individual cells migrate to this area from the central region of the tube as if to make ears, tiny rubber particles had to fly out from the air inside the balloon."
In 2001, Felix Loosli from Wittbrodt's laboratory discovered a protein called Rx3 that is required for eye formation. Only cells that will become the eye begin producing this molecule early on in development. Martina Rembold, also from Wittbrodt's group, labeled these cells with a fluorescent marker and tracked them using advanced software developed by Richard Adams at the University of Cambridge. Following the cells required recognizing them under the microscope and assembling tens of thousands of images into 3D movies.
"Rx3 plays a crucial role in giving the cells their identity and telling them where to go," says Rembold. "Normally, single cells migra
Contact: Yvonne Kaul
European Molecular Biology Laboratory