The disturbance results in the embryonic development of small shrimp

Consider the crustacean Parhyale hawaiensis, a small crustacean with an interesting appearance.

“It’s called the ‘Swiss army knife,'” said Dillon Cislo, lead author of the study Nature Physics. “It has different combinations and each one is characterized by its size and shape. Also, each of these legs has its own special function.”

Their impressive bodies and approachable size make these creatures the perfect choice for evolutionary studies. But more than that, according to Cislo and UC Santa Barbara researchers Mark Bowick and Sebastian Streichan, their embryos are a window to the world of muscle morphogenesis, a field that seeks to understand how the majority of embryonic cells become complex body parts. the oldest living thing. As a “direct producer,” or an organism that creates its own large – even small – forms as opposed to having the unique form of a larva and undergoing metamorphosis, this crustacean is one to watch.

“You’re going from random cells to all these crazy, super-realistic structures,” said Cislo, a Rockefeller University researcher who conducted research on the paper as a graduate student at UCSB under the guidance of physicists Bowick, Boris Schraiman, and Streichan, a biologist. Until recently, the observation of embryogenesis involved taking several embryos of a species – say, a fruit fly – at different stages of development and “processing” them to freeze over time. From there, scientists can calculate and reason about the sequence of events that lead to the development of their bodies. But what has become easier to see is how the young cells find their place and their place at all.

Figuring out how these all work together is a hot topic in biology. But it also falls into the field of material physics, a field that is interested in the behavior of groups of several independent “agents” that use local forces. Examples of active matter are diverse, from humming stars to clusters of bacteria to crowds of people. Active matter can also include non-natural phenomena where the components are not identical, such as a number of robots.

Order from Disorder
When the embryonic cells divide, they act differently along the axis, and then the daughter cells divide in opposite directions along their axes, and so on, although there is no reason why daughter division should depend on the division axis. of the parent. That seems to make things difficult for muscles whose structure and function depend on the structure and shape of their cells.

To see how the cells of P. hawaiensis coped with the problem that could appear due to their abundance, the researchers followed the development of the embryo, three days after fertilization.

“It looks like very few cells on top of the surrounding yolk,” Cislo said. To better visualize the process, they transformed the curved cells into a plane “in a way that respects the three-dimensional shape of the real cells,” the paper explains, and tracked the cells as they divided and moved around. in the first empirical analysis of this phase of early P. hawaiensis development.

Twelve hours after the start of the observation period, the number of proliferating cells not only increased a little, they formed themselves into a group, whose lines can match the parts of the adult body. From there, the monolayer cells, which resemble the region of the stomach of the crustacean, undergo waves of cell division, starting from the center and spreading laterally, dividing in a circle from the head to the tail of the animal. – to be.

The divisions were not random, Cislo said. That is, instead of just being a large mass of seemingly disorganized cells, these cells divide, so some daughters can straighten themselves 90 degrees before dividing again to join the head and tail.

“As it goes through its divisions, you start to see new lines being inserted between the lines, pushing the top and bottom lines apart,” he said. “And this is very dangerous, because in an inanimate body it is a very expensive operation.” In metals and crystals, this recycling process would require the material to be heated to thousands of degrees to make it work, Cislo said, “but here the shrimp is doing the heat.” According to the researchers, a large part of the cell division is often related to a biological signal that has not been revealed.

Although fragile and sometimes expensive, the four iterations at the beginning of the crustacean’s development are important, according to the researchers.

“There are some theories about how to interpret these results,” Streichan said. “The main idea is about the movement of the animal’s legs. Just like our hands or our legs, these limbs have a distinct shape…

“Imagine that your left hand is wrapped around you with your right hand, say 180 degrees to exchange the back of your hand with the palm,” he added. “Daily tasks can be very difficult!”

Shaking Things Up
One important thing to keep in mind is that the organization is very fluid — not liquid and not solid, Bowick said. “From a physics point of view, this part has the same shape as the bulk,” he said.

It turns out that the whole system is made up of a grid-like group of cells, the possibility of disorder caused by fluid and cell division is necessary for the evolution needed in nature, Bowick added. “Cells aren’t just dividing, they’re using each other’s energy when they’re doing it,” he said.

The researchers found that the cell, with its own little engine and its own independent “clock”, made a certain “noise” – variation and fluctuation – during the initial phase of cell proliferation and the later phase when the cells continued to grow. split but the muscles themselves were also long. The sound may at first seem unrelated to the creation of a very complex body with various accessories, but, according to researchers, the sound itself is necessary to make it strong. Using its four methods, the system has a “Goldilocks zone” between order and chaos: order enough to start building a creature, yet open enough to take a little variation in the process.

Through a series of experiments, he found that despite the difference in time, or division (to some extent), or the presence of cells that did not reproduce themselves during the propagation, it was possible to eventually reach the same level. final result.

“The bottom line is that biology doesn’t have to manipulate things so tightly to achieve what you want,” Cislo said — something that only dynamic analysis can do.

Bowick agrees. “Imagine that you want a system to arrive at a controlled location; if you are completely static, you will not find it,” he said. “But if you shake the system, you can allow it to settle into a better place. And what seems to be happening here is that the cells are shaking the system, allowing it to settle into a more controlled place.”

This study provides an interesting guide in a part of the development of biological sciences, which works according to the geometric organization, as seen by its four systems.

“The fruit fly, which is the hydrogen atom of evolutionary biology, forms the components of its body structure using biochemical signals,” said Cislo. “This is something very different.”

“What’s interesting about Dillon’s work is that the control system can be found at the cellular level, ensuring that the government is controlled,” Streichan said. Unlike the development of other animals in which embryonic cells rely on chemical signals to progress, in P. hawaiensis grid patterning is a mechanical process that passes through two regions – one near and the other away from the head, allowing both regions to connect. at the location of their cells. The grid also determines the location and position of the cells before the legs develop.

“In many ways, the Dillon project has provided another example of how biology can find ways to support science in its goals,” Streichan said.

“There could also be material science courses,” added Bowick. “If you want to make amazing devices, you might want to learn from biology and take some of these things out of proportion, and make amazing structures like this.”