October 26, 2020

This Ancient Sea Creature Builds Its Body With a Whisper, Not a Scream

Building a body from scratch is a daunting task, one that requires careful coordination among all those involved. That’s why nature’s starting stuff — cells — have learned to be remarkably chatty to get the job done right.

Decades of experiments on embryos from fish, frogs and mice have painted a general picture of the way these cellular conversations often go. Cells will emit molecular signals that can diffuse deep into their environment, not unlike messages broadcast over radio waves. Such widely transmitted messages, which direct information to distant anatomical locales, have long been considered essential to the act of building a body.

“It is what is discussed in textbooks,” said Léo Guignard, a biologist at the Max Delbrück Center for Molecular Medicine in Berlin.

But nature has developed subtler ways of sending messages, too.

By eavesdropping on the embryos of sea squirts, saclike filter feeders that inhabit the world’s shallow ocean floors, Dr. Guignard and his colleagues may have identified another way that burgeoning cells correspond. During their earliest days, sea squirt cells seem to exchange signals only with their nearest neighbors, rather than dispatching signals to cells that are farther afield, according to a paper published Thursday in Science.

It’s the microscopic equivalent of passing notes between close friends — a talking tactic that may be more limited in reach than its long-range counterpart, but could allow for the delivery of especially precise instructions.

Most researchers in the field of developmental biology “have never really thought about cell signaling in this way,” said Chen Cao, who researches sea squirt development at Princeton University but wasn’t involved in the new study. “This is a brand-new angle” on how embryo cells find their fates, she said.

Sea squirts are more closely related to vertebrates — animals like humans with backbones — than you might expect from brainless blobs. And the embryos of some sea squirt species, such as Phallusia mammillata, are completely transparent, making them especially easy to observe.

The researchers used a highly sensitive form of microscopy to track 10 Phallusia mammillata embryos during six hours of their early development. Snapping images every two minutes, the cameras recorded the position and shape of each cell in the embryo through multiple rounds of division, until the sea squirts-to-be each contained several hundred cells apiece, about a third of the way through development.

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Cells in a developing sea squirt embryo, with different colors representing the tissues they will form, such as fuschia for muscles and dark green for the brain. Video by L. Guignard

Unlike frog or mouse embryo cells which may zip to and fro during development, young sea squirt cells stayed mostly in place. Mathematical modeling also showed that the cells signaled only to the cells they touched, almost like they were whispering to one another. In at least these early stages of development, sea squirt cells didn’t seem to need long-distance chatter.

The findings raise the possibility that “you can be a whole, sophisticated embryo” by way of local signals alone, said Patrick Lemaire, a developmental biologist at the University of Montpellier in France and a co-author of the study.

Remarkably, these patterns repeated themselves in all the sea squirts the team observed, to the point where the same cells occupied nearly identical neighborhoods in different embryos. Dr. Lemaire said he thinks this rigid consistency may have played some role in keeping the squishy forms of sea squirts mostly unchanged since they first appeared on the planet hundreds of millions of years ago.

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“This is a beautiful piece of work,” said Cassandra Extavour, a developmental biologist at Harvard University who wasn’t involved in the study. And though sea squirts are the only creatures in which these developmental patterns have been rigorously documented so far, Dr. Extavour said she expects “there will be many, many more animals that rely on a signaling principle like the one outlined here.”

Different types of signaling also aren’t mutually exclusive. Dr. Cao said she thinks even sea squirt cells might make some long-distance calls later in development.

Scientists have long “looked for a unifying view of how cell-cell communication works across animals,” said Didem Sarikaya, a developmental biologist at the University of California, Davis, who wasn’t involved in the study. But it’s becoming increasingly clear that is not the case, Dr. Sarikaya said.

“We tend to observe the things we have already known about before,” Dr. Extavour said. But “if we understand this lens is very narrow, we might not be surprised by findings like this.”