Physics of living systems

Understanding the behaviour of almost any biological object is a fundamentally multiscale problem — a challenge that biophysicists have been increasingly embracing, building on two centuries of biophysical studies at a variety of length scales.

It may look like little more than slime, but the glycocalyx coating our cells plays a key role in cell signalling. And changes to its physical structure have been linked to cancer, triggering emergent behaviours that form the focus of this Review.

Robust and responsive, the surface of a cell is as important as its interior when it comes to mechanically regulating form and function. New techniques are shedding light on this role, and a common language to describe its properties is now needed.

The behaviour of cells and tissues can be understood in terms of emergent mesoscale states that are determined by a set of physical properties. This Review surveys experimental evidence for these states and the physics underpinning them.

Evidence that ants communicate mechanically to move objects several times their size has prompted a theory that places the group near a transition between uncoordinated and coordinated motion. These findings and their implications are reviewed here.

Biofilms of rod-shaped bacteria can grow from a two-dimensional layer of founder cells into a three-dimensional structure with a vertically aligned core. Here, the physics underlying this transition is traced down to the properties of individual cells.

To perform key processes like division, many cells use star-shaped polymeric aster structures to find their centre. Force measurements now reveal that an active spring mechanism regulates this process, suppressing noise to ensure precise centration.

Magnetic tweezer measurements have revealed the forces associated with a star-shaped structure responsible for moving the sperm nucleus to the centre of the egg cell following fertilization.

The motor proteins and contractile forces involved in wound closure are both shown to be heterogeneously distributed around a wound. Theory suggests that this heterogeneity speeds up wound closure, as long as the proteins are mechanically regulated.

Cells in embryonic tissues generate coordinated forces to close small wounds rapidly without scarring. New research shows that large cell-to-cell variations in these forces are a key system feature that surprisingly speeds up wound healing.

Little is known about how a cell’s surroundings within tissue influence the mechanics of its division. Experiments on constrained dividing cells reveal that they create protrusive forces in order to undergo the shape changes required for division.

Epithelial cells are shown to scale via a shape distribution that is common to a number of different systems, suggesting that cell shape and shape variability are constrained through a relationship that is purely geometrical.

The cluster size distribution of cells’ progeny in developing organs is found to be universal. A new theory inspired by the physics of aerosols suggests that collective cell dynamics leads to a critical state balancing merger with fragmentation.

Wrinkling in human brain organoids suggests that brain development may be mechanically driven, a notion supported only by model gels so far. Evidence in this simple living system highlights roles for cytoskeletal contraction and nuclear expansion.

Swarms and statistical physics seem like natural bedfellows, but concepts like scaling are yet to prove directly applicable to insect group dynamics. A study of midges suggests they are, and that they may give rise to a new universality class.

The proteins that adhere cells together in tissue assemble in domains near the cell–cell interface. Experiments, simulations and theory show that formation of these domains is regulated by the membrane itself — with an explicit role for fluctuations.

The centennial celebrations for morphology masterwork On Growth and Form are just kicking off. We look at why physicists should get involved.