For decades, the prevailing narrative in biology has been that our genes are the sole architects of life, dictating every aspect of growth and development through intricate chemical pathways. We’ve been taught to think of DNA as the ultimate blueprint, with proteins acting as the builders. While this genetic imperative is undoubtedly fundamental, a growing body of scientific research is revealing a more complex and fascinating story – one where physics, the very same force that governs the motion of planets and the flow of water, plays an equally crucial, albeit often overlooked, role in shaping living organisms.
Imagine looking at a glass of wine. As it sits, you might notice little rivulets of liquid weeping down the sides. This seemingly simple phenomenon, observed and explained by James Thomson (brother of the renowned physicist Lord Kelvin) in 1855, is known as the Marangoni effect. It arises from differences in surface tension between liquids. Little did Thomson know that this same physical principle might be orchestrating one of the most fundamental processes in life: the development of an embryo.
Recent groundbreaking research from a team of biophysicists in France has unveiled that the Marangoni effect is directly responsible for a pivotal moment in embryonic development. It’s the instant when a simple, undifferentiated blob of cells elongates and begins to establish a head-and-tail axis – the very first defining features that will guide the organism’s future form and function. This discovery is a powerful testament to the increasing appreciation for mechanical forces in biology, forces that push and pull tissues, influencing their growth and development in ways that purely genetic instructions cannot dictate alone.
Beyond the Genes: The Rise of Mechanical Biology
The traditional view of biological processes has largely focused on chemical signals and genetic cues. However, this picture has often felt incomplete, like a symphony missing its percussion section. Modern imaging and sophisticated measurement techniques are now flooding the scientific landscape with unprecedented data, compelling researchers to interpret these observations through a mechanical lens. The ability to witness biological processes unfold in real-time, observing the intricate dance of cell movement, rearrangement, and tissue growth, has revolutionized our understanding.
"What has changed over the past decades is really the possibility to watch what happens live, and to see the mechanics in terms of cell movement, cell rearrangement, tissue growth," explains Pierre-François Lenne of Aix Marseille University, a key figure in the recent study on embryonic development. This shift toward understanding the mechanical underpinnings of life has also sparked renewed interest in the pioneering work of scientists from a bygone era.
Echoes of D’Arcy Thompson: Physics as a Sculptor
In 1917, D’Arcy Thompson, a Scottish biologist, mathematician, and classics scholar, published his seminal work, "On Growth and Form." In this monumental tome, Thompson argued against the prevailing tendency to explain all biological phenomena solely through Darwinian natural selection. He highlighted striking similarities between the shapes and forms found in living organisms and those that emerge naturally in non-living matter, suggesting that fundamental physical forces were significant sculptors of life. Thompson’s thesis – that physics, too, shapes us – is experiencing a powerful resurgence.
"The hypothesis is that physics and mechanics can help us understand the biology at the tissue scale," says Alexandre Kabla, a physicist and engineer at the University of Cambridge. The grand challenge now is to unravel the intricate interplay between these forces, understanding how genes and physics collaborate harmoniously to sculpt the astonishing diversity of organisms we see on Earth.
Growing with the Flow: The Marangoni Effect in Embryos
While the idea of mechanical forces influencing growth isn’t entirely new, the ability to rigorously test these theories has been limited. Observing embryos has historically been a challenge. They are microscopic, their forms diffuse, and light scatters unpredictably, much like looking through frosted glass. However, advances in microscopy and image analysis have provided a clearer window into the fascinating processes of development.
Lenne and his colleagues employed these cutting-edge techniques to study mouse gastruloids – essentially, bundles of stem cells that, as they grow, remarkably mimic the early stages of embryonic development. Their meticulous observations revealed a consistent pattern: cells within the gastruloid flowed upwards along its sides, converging into a distinct stream of tissue that then moved down the center. This pattern immediately reminded Lenne of a droplet, prompting him to delve into the scientific literature on surface tension in moving liquids. It was there that he rediscovered the Marangoni effect.
Thomson’s 1855 explanation of the Marangoni effect is elegantly simple: when two liquids with different surface tensions interact, the liquid with the higher surface tension will exert a pulling force on the liquid with the lower surface tension. This occurs because the surface tension of a liquid is a measure of the inward pull experienced by its outermost molecules due to neighboring molecules. When two liquids meet, the one with a stronger inward pull (higher surface tension) will draw the fluid with the weaker pull (lower surface tension) towards it.
In the wineglass analogy, alcohol evaporates more readily from the sides, leaving a more watery solution. Since water has a higher surface tension than alcohol, this watery liquid pulls the rest of the wine upwards along the glass’s wetted surface. Eventually, gravity overcomes this upward pull, and the wine drips back down, creating the characteristic "tears."
The flow pattern observed in the gastruloid tissue closely mirrored this wine-dripping phenomenon. The researchers tested a model incorporating Marangoni-type flow within the gastruloid tissue, and the results showed a striking congruence with their experimental data. The implications are profound: a fundamental physical principle is driving the initial elongation and axis formation in developing embryos.
The Genetic Foundation of Mechanical Flow
Crucially, the Marangoni flow, while a mechanical effect, is not entirely divorced from genetic control. Genes are essential in establishing the initial difference in surface tension. They orchestrate the production of specific proteins in particular regions of the cell cluster. These proteins, in turn, alter the local surface tension, causing tissue to flow away from areas with lower surface tension. This directed movement, circulating around the periphery and then down the center, elongates the gastruloid – a process akin to the wine tears dripping back down the glass. As Kabla aptly puts it, "It’s a very nice example of how mechanics, coupled with all the intrinsic complexity of molecular and cellular biology, has a very important role in shaping organisms."
Feathers, Scales, and Mechanical Precision
The influence of mechanical forces isn’t limited to the earliest stages of development. Consider the intricate, regularly spaced patterns of bird feathers. For years, the leading hypothesis suggested that embryos secreted specialized molecules, known as morphogens, which then instructed genes to produce proteins precisely where feather follicles should form. However, researchers Amy Shyer and Alan Rodrigues, leading the Laboratory of Morphogenesis at Rockefeller University, struggled to find concrete genetic signals initiating this process.
Their persistent investigation led them to suspect that mechanical forces, rather than solely chemical signals, were playing a significant role. In a 2023 report published in Science, their team confirmed that morphogens were indeed secreted just before feather follicle budding. However, they discovered that these morphogens didn’t directly influence individual cells. Instead, they modified the material properties of larger tissue regions. This change in tissue mechanics set the stage for physical forces to push and pull the tissue, dictating the precise spacing and formation of feather follicles.
"What’s really amazed us is that you might be able to get by with a relatively simple amount of instruction from the genetic and molecular level," Rodrigues shared. "Because you have additional emergent processes and properties happening at other levels." This work challenges the long-held view in biology that causation and regulation always originate at the molecular level and then cascade upwards. In the case of feather development, molecular and tissue-level changes appear to co-emerge, influencing each other.
The Physics of Cellular Stretching: A Spring in Their Step?
Mechanical forces also operate at the cellular level, influencing how individual cells behave and interact. Certain proteins within cells can alter their material properties, preparing them for mechanical manipulation. During the embryogenesis of a fruit fly, for instance, cells don’t merely rearrange themselves; they also stretch. This cellular stretching has been directly linked to gene activity that influences the cells’ characteristic elasticity.
Think about stretching a spring or a rubber band. The amount of extension is generally proportional to the applied force – a principle known as Hooke’s Law. However, this simple relationship can become more complex when the material is suspended in a viscous fluid. Imagine stirring molasses; the rate at which you can stir is heavily influenced by the fluid’s viscosity and the time it takes for the material to deform.
Biological cells appear to exhibit a similar time-dependent stretching behavior. Experiments measuring the stretching of certain fruit fly embryo cells have shown that their extension depends on the square root of the time the stretching force is applied. This peculiar behavior has been explained by researchers Konstantin Doubrovinski and his colleagues at the University of Texas Southwestern Medical Center.
In a paper published in Physical Review Letters, they propose that the production of actin – one of the most abundant proteins within these cells – is responsible. As actin filaments are generated, they effectively act like internal springs, resisting the external force applied to the cell and contributing to the observed time-dependent stretching. The team validated this hypothesis by conducting experiments using drugs that inhibit actin assembly, observing that the elastic response of the cells largely disappeared.
While this study presents a compelling explanation, discussions about the exact drivers of cellular stretching continue within the scientific community. A persistent challenge in biology is discerning cause from effect: what is the primary driver of a particular phenomenon, what is a contributing factor, and what is merely a consequence?
A Timeless Synthesis: Physics and Genes in Harmony
These ongoing debates echo the questions surrounding D’Arcy Thompson’s observations more than a century ago. His central argument – that the beautiful geometric forms found in nature are not arbitrary but are instead a result of underlying physical forces – is proving remarkably robust under modern scientific scrutiny. The ability to visualize and quantify these forces at the cellular and tissue level provides compelling evidence for his holistic perspective.
As Kabla aptly summarizes, "To many of us, it seems natural that where there’s motion, mechanics is likely to be involved." The emerging picture of life is one of exquisite collaboration, where the precise instructions encoded in our genes are realized and sculpted by the fundamental laws of physics. This synthesis of genetic information and mechanical forces offers a more complete and awe-inspiring understanding of how living things grow, develop, and come to be the remarkable forms they are.
This article was originally published by Quanta Magazine, an editorially independent publication of the Simons Foundation. Quanta Magazine’s mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.