Two-tailed fish may help solve the riddle of how embryos are put together during the first stages of development. For years scientists thought that early cells use a common plan to build an animal from top to bottom. But a new study shows that zebrafish tails have their own organizing blueprint and are assembled independently of the head and body.
Like American football players lined up for a complex play, every cell in a developing embryo needs to be at the right place at the right time. After fertilization, certain clusters of cells always morph into a head, a torso, or a tail. But cells in these "organizer" clusters seemed to contain a complete set of instructions for building the entire body. Transplanting a head organizer from one embryo into another embryo, for instance, created two complete animals, attached to each other like Siamese twins. Ditto for cells from the torso. But Bernard Thisse, an embryologist at the Institut de Génétique et de Biologie Moléculaire et Cellulaire in Cedex, France, noticed something fishy: When head and torso organizer cells were grafted onto other embryos in zebrafish, the tail region never quite developed completely.
To investigate how zebrafish tails take shape, Thisse's team removed some of the presumed pre-tail cells from early-stage embryos, then transplanted them into the middle of another embryo. The resulting zebrafish grew an extra tail--and only a tail--that extended from its trunk. These findings implied that the tail has its own group of organizing cells different from those of the head and torso.
In another set of experiments, Thisse and his colleagues found that three molecules called BMP, Wnt8, and Nodal team up to create zebrafish tails. Wherever they injected RNA for the trio into the zebrafish embryos, tails popped up, the team reports in the 24 July issue of Nature.
“The cool thing about the paper is that they can get the signals to actually mimic what the cells do,” says Marnie Halpern, an embryologist at the Carnegie Institution of Washington in Baltimore, Maryland. The next step, she thinks, is to figure out how the molecular trio builds up correct levels in the pre-tail region without spreading elsewhere.