In a primtive relative of the humans

Mar 6, 2007 10:48 GMT  ·  By

Vertebrates compass all the animals with backbone, from fishes to mammals and birds.

But the first vertebrates originated in more primitive invertebrates. From living species, our closest invertebrate relative is the humble sea squirt, which lives attached to the sea bottom, feeding by filtering just like the clams do.

Now Israeli researchers discovered that these animals possess the ability of regenerating its entire body from small blood vessel fragments. The process, which imitates the early phases of embryonic development, generates an adult sea squirt in as little as a week.

Scientists could not only solve the evolutionary origins of organism regeneration, but also how it modified during vertebrate evolution. Some primitive vertebrates, like salamanders, lizards or frogs, can regenerate limbs or Tails, and even humans can regenerate portions of skin, lungs and livers and children until 6 years old can regenerate the tip of their fingers. "However, in general, the more complex the animal, the lower the regeneration abilities are, relatively," said biologist Ram Reshef at Technion Israel Institute of Technology in Haifa. "No vertebrate could regenerate their whole bodies if you cut them in two."

Regenerating the entire body from a fragment is a trait usually found in less complex invertebrates, like sponges, worms and jellyfish. But Reshef's team focused on the sea squirt Botrylloides leachi, a much more complex invertebrate, living in colonies under the stones in shallow waters along the eastern Mediterranean coast. "Massive regeneration is not just confined to low complexity animals, but rather can take place in highly evolved animals," said Reshef.

Each colony is made of up to thousands clones, each 2-3 mm long and embedded in a gelatinous matrix, connected by a network of blood vessels. The researchers cut fragments of blood vessels, each roughly 1 mm long, containing one or more ampullae (the pear-shaped endpoints of the vessels) and about 100 to 300 blood cells. 80 out of 95 fragments developed into an entire adult within one to three weeks, in a very different way from other creatures. "When less complex groups regenerate their bodies, they do so through what we call a blastema, which is a kind of tissue that forms right at the place where you want to regenerate an organ or body," said Reshef.

In contrast, the sea squirts did not employ blastemas. But sea squirts regenerated starting from stem cells, not blastemas, found in tiny lodges filled with them, named regeneration niches. "In mammals, many adult organs and tissues contain specific stem cells that are involved in repair and some restricted regeneration abilities," Reshef said.

The stem cells generated a hollow sphere organized into a thin and thick layer on opposite sites, exactly like in the early stages of the embryo, and followed an embryonic development to adults capable of sexual reproduction. These stem cells are similar to stem cells in adult mammals and humans that generate our tissues and organs, "the huge difference is that they culminate in an entire organism," Reshef said. "The most important implication of our finding is the possibility that vertebrate adult tissue stem cells may exhibit the same capabilities to generate any cell in the body," he added.

The research team is now investigating the molecular mechanisms that enable the sea squirt to have this powerful regeneration, and will compare it to similar processes in other invertebrates and vertebrates. "We speculate that vertebrates altered or suppressed parts or all of this ability," Reshef said