A developmental clock ticks faster in some vertebrates than others, producing a longer spine.

With 315 somites—the precursors to vertebrae—this corn snake embryo will have many more segments in its spine that most other vertebrates.

When it comes to the number of vertebrae that wind along their spines, snakes have other vertebrate organisms beat, with a whopping 300 vertebral sections in many snake species. But how a developing snake embryo forms so many more segments than other vertebrates has never been clear.

Researchers led by HHMI investigator Olivier Pourquié of the Stowers Institute for Medical Research knew that somites—the embryonic precursors to vertebrae—develop from rhythmic waves of signaling molecules that move through vertebrate embryos. As the embryo grows, precursor cells are deposited in a head-to-tail sequence, forming somites periodically in response to a traveling oscillator that produces regular signaling pulses. So the somites end up developing at regular intervals, like a plant stem sprouting branches at regular intervals as it grows.

The researchers assumed that in all vertebrates, this rhythmic wave moved at the same rate. A lengthy snake spine, therefore, would just take longer to develop than a short frog spine, for instance.

To test this hypothesis, the researchers compared the speed of somite formation in corn snakes with that of three other vertebrates: zebra fish, with 31 vertebrae; chickens, with 55; and mice, with 65. They observed both the physical pinching off of each somite, and the movement of the signaling molecules along the embryos.

Pourquié and his colleagues found that it actually takes a similar number of cell generations for the whole length of somites to develop in all four species. “What is very different is the rhythm of the clock,” says Pourquié. “Within the same amount of growth, the snake's clock ticks faster so it can make more somites.” The results appear in the June 17, 2008, issue of Nature.

The next big question, Pourquié says, is what makes the clock tick at such a different pace in snakes, and how this change happened in evolution.

Human genes related to the same clock mechanism have been linked to severe deformations of the spine such as scoliosis, and the scientists hope their snake research could eventually lead to ways to prevent these deformations in humans.

Image: Olivier Pourquié