This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A squishy worm that starts its life as something of a floating head in the Pacific Ocean is changing what scientists know about metamorphosis at the cellular level. Unlike humans, about 80% of animal species undergo metamorphosis, a stepped development from egg to larva to adult, but how it works at the cellular level is not well understood.
Some theories and prior research suggested that the original cells in the larva die and are replaced with newly generated adult cells. Other work pointed to cells growing into the same function—for example, larval skin cells would become adult skin cells. Instead, a Stanford-led study has found strong evidence in an acorn worm called Schizocardium californicum that most larval cells were reprogrammed, with even neurons taking on a new role in the adult organism.
"Reprogramming is a bit of an exotic fruit in developmental biology," said Christopher Lowe, senior author of the study and biology professor in the Stanford School of Humanities and Sciences. "Generally, we used to think that as cells develop, they become increasingly restricted in their function. But when we start looking at more animals that go through metamorphosis, they may reveal that reprogramming is a much more common feature of development." Cellular reprogramming is thought to happen after an injury or in some species that regenerate organs or whole limbs—but not as a feature of normal development.
The study, published in Nature Communications, is the first known study to suggest extensive cellular reprogramming during normal development in an animal with a bilateral body plan, where one side of the body matches the other, as in humans. Previous research has found some of this type of developmental reprogramming in sponges and jellyfish, two organisms that are far from humans on the evolutionary tree. In contrast, these acorn worms are part of the phylum Hemichordata, considered an evolutionary link to vertebrate animals, including all mammals.
For this study, the team, led by first author Paul Bump, a former doctoral student in Lowe's lab at Stanford's Hopkins Marine Station, conducted genetic analyses on more than 87,000 cells from these acorn worms. The researchers performed single-cell RNA sequencing on samples from worms in five developmental stages: early and late larval stages, metamorphosis, and early and late juvenile stages. Using this information, they categorized the cells into 12 classes, such as cartilage, immune and skin cells.
This analysis found that many larval cells were more similar to each other than they were to the adult cells performing the same function. For example, larval neurons were more like larval gut cells than they were like adult neurons. This was true for more than half of the cells, suggesting extensive reprogramming.
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