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How Groups of Cells Cooperate to Build Organs and Organisms

How Groups of Cells Cooperate to Build Organs and Organisms

Efforts to use regenerative medicine—which seeks to address ailments as diverse as birth defects, traumatic injury, aging, degenerative disease, and the disorganized growth of cancer—would be greatly aided by solving one fundamental puzzle: How do cellular collectives orchestrate the building of complex, three-dimensional structures?

While genomes predictably encode the proteins present in cells, a simple molecular parts list does not tell us enough about the anatomical layout or regenerative potential of the body that the cells will work to construct. Genomes are not a blueprint for anatomy, and genome editing is fundamentally limited by the fact that it’s very hard to infer which genes to tweak, and how, to achieve desired complex anatomical outcomes. Similarly, stem cells generate the building blocks of organs, but the ability to organize specific cell types into a working human hand or eye has been and will be beyond the grasp of direct manipulation for a very long time.

But researchers working in the fields of synthetic morphology and regenerative biophysics are beginning to understand the rules governing the plasticity of organ growth and repair. Rather than micromanaging tasks that are too complex to implement directly at the cellular or molecular level, what if we solved the mystery of how groups of cells cooperate to construct specific multicellular bodies during embryogenesis and regeneration? Perhaps then we could figure out how to motivate cell collectives to build whatever anatomical features we want.

New approaches now allow us to target the processes that implement anatomical decision-making without genetic engineering. In January, using such tools, crafted in my lab at Tufts University’s Allen Discovery Center and by computer scientists in Josh Bongard’s lab at the University of Vermont, we were able to create novel living machines, artificial bodies with morphologies and behaviors completely different from the default anatomy of the frog species (Xenopus laevis) whose cells we used. These cells rebooted their multicellularity into a new form, without genomic changes. This represents an extremely exciting sandbox in which bioengineers can play, with the aim of decoding the logic of anatomical and behavioral control, as well as understanding the plasticity of cells and the relationship of genomes to anatomies.

Deciphering how an organism puts itself together is truly an interdisciplinary undertaking. Resolving the whole picture will involve understanding not only the mechanisms by which cells operate, but also elucidating the computations that cells and groups of cells carry out to orchestrate tissue and organ construction on a whole-body scale. The next generation of advances in this area of research will emerge from the flow of ideas between computer scientists and biologists. Unlocking the full potential of regenerative medicine will require biology to take the journey computer science has already taken, from focusing on the hardware—the proteins and biochemical pathways that carry out cellular operations—to the physiological software that enables networks of cells to acquire, store, and act on information about organ and indeed whole-body geometry.

In the computer world, this transition from rewiring hardware to reprogramming the information flow by changing the inputs gave rise to the information technology revolution. This shift of perspective could transform biology, allowing scientists to achieve the still-futuristic visions of regenerative medicine. An understanding of how independent, competent agents such as cells cooperate and compete toward robust outcomes, despite noise and changing environmental conditions, would also inform engineering. Swarm robotics, Internet of Things, and even the development of general artificial intelligence will all be enriched by the ability to read out and set the anatomical states toward which cell collectives build, because they share a fundamental underlying problem: how to control the emergent outcomes of systems composed of many interacting units or individuals.

(Re)Building a body

Many types of embryos can regenerate entirely if cut in half, and some species are proficient regenerators as adults. Axolotls (Ambystoma mexicanum) regenerate their limbs, eyes, spinal cords, jaws, and portions of the brain throughout life. Planarian flatworms (class Turbellaria), meanwhile, can regrow absolutely any part of their body; when the animal is cut into pieces, each piece knows exactly what’s missing and regenerates to be a perfect, tiny worm.

The remarkable thing is not simply that growth begins after wounding and that various cell types are generated, but that these bodies will grow and remodel until a correct anatomy is complete, and then they stop. How does the system identify the correct target morphology, orchestrate individual cell behaviors to get there, and determine when the job is done? How does it communicate this information to control underlying cell activities?

Several years ago, my lab found that Xenopus tadpoles with their facial organs experimentally mixed up into incorrect positions still have largely normal faces once they’ve matured, as the organs move and remodel through unnatural paths. Last year, a colleague at Tufts came to a similar conclusion: the Xenopus genome does not encode a hardwired set of instructions for the movements of different organs during metamorphosis from tadpole to frog, but rather encodes molecular hardware that executes a kind of “error minimization loop,” comparing the current anatomy to the target frog morphology and working to progressively reduce the difference between them. Once a rough spatial specification of the layout is achieved, that triggers the cessation of further remodeling.

The deep puzzle of how competent agents such as cells work together to pursue goals such as building, remodeling, or repairing a complex organ to a predetermined spec is well illustrated by planaria. Despite having a mechanistic understanding of stem cell specification pathways and axial chemical gradients, scientists really don’t know what determines the intricate shape and structure of the flatworm’s head. It is also unknown how planaria perfectly regenerate the same anatomy, even as their genomes have accrued mutations over eons of somatic inheritance. Because some species of planaria reproduce by fission and regeneration, any mutation that doesn’t kill the neoblast—the adult stem cell that gives rise to cells that regenerate new tissue—is propagated to the next generation. The worm’s incredibly messy genome shows evidence of this process, and cells in an individual planarian can have different numbers of chromosomes. Still, fragmented planaria regenerate their body shape with nearly 100 percent anatomical fidelity.

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