Cell motility is the ability of a cell to move itself from one location to another, and it underpins nearly everything that keeps a complex organism alive and developing. Immune cells chase down invaders, embryonic cells travel long distances to build a face, and skin cells crawl across a wound to seal it shut. The machinery behind this movement varies wildly depending on the cell type, from bacteria spinning tiny rotary motors to human white blood cells reshaping their internal skeleton on the fly to squeeze through tissue. What ties these systems together is a shared basic challenge: converting chemical energy into directed physical force.
How Crawling Cells Actually Move
Most animal cells move by crawling, and the core of that process is a protein called actin. Actin molecules link together into long filaments that push the front edge of the cell forward, forming a thin, sheet-like structure called the lamellipodium. This structure acts as the cell’s leading foot. A protein complex called Arp2/3 plays a key role by branching new actin filaments off existing ones, creating a dense, pushing network right at the cell’s front edge. Simulations of this process show that the branching pattern of Arp2/3 has to be spatially restricted during each branching event to match the filament arrangements researchers actually observe in real cells.
1PubMed Central. Morphology of the lamellipodium and organization of actin filaments at the leading edge of crawling cellsPushing the front forward is only half the job. The cell also needs to grip whatever surface it is moving on. It does this through structures called focal adhesions, which are clusters of proteins that physically link the cell’s internal skeleton to the external environment. These adhesions are anchored by integrins, proteins that span the cell membrane and grab onto molecules outside the cell. High-resolution force maps have revealed that a specific integrin, αvβ3, concentrates in the regions where force is highest, and that paxillin, a scaffolding protein not traditionally considered a direct force-transmitter, correlates with force distribution more closely than proteins that were long assumed to be the main force carriers.
2PubMed Central. Visualizing the interior architecture of focal adhesions with high-resolution traction mapsThe pulling force itself comes from myosin motors, molecular machines that walk along actin filaments and contract them, somewhat like tiny muscles. Myosin activity at the rear of the cell, combined with the forward push of new actin at the front, produces a coordinated cycle: extend the front, grip the surface, pull the body forward, release the back. Researchers have modeled this traction force generation and shown that myosin motor activity, together with the sliding of surface-attached molecules, produces a controlled pulling force at adhesion sites.
3PubMed Central. Friction-controlled traction force in cell adhesionBacteria Use a Completely Different Motor
Bacterial movement looks nothing like crawling. Many bacteria swim using flagella, which are long, whip-like filaments attached to a rotary motor embedded in the cell wall. This motor is genuinely a spinning engine, not a metaphor. It consists of a rotor and surrounding stator units, each of which is a transmembrane ion channel that converts the flow of ions (usually protons) across the cell membrane into mechanical rotation.
4PubMed Central. Structure and Dynamics of the Bacterial Flagellar Motor ComplexIn Escherichia coli, the stator units themselves are now understood to be small rotary motors that mesh with the larger rotor like intermeshed gears. Each stator unit is an asymmetric assembly of five MotA proteins surrounding two MotB proteins; the MotB dimer anchors to the cell wall while the MotA pentamer rotates around it, transmitting torque to the central rotor. Measurements show the MotA pentamer spins roughly six times faster than the flagellar motor itself, revealing a gear ratio between stator and rotor.
5PubMed Central. Torque-generating units of the bacterial flagellar motor are rotary motorsEarly measurements in Streptococcus showed that proton flow through the motor is directly proportional to motor speed, supporting the idea that each proton’s passage is tightly coupled to rotation. At low speeds, the motor converts energy at close to perfect efficiency; at the high speeds seen in freely swimming bacteria, efficiency drops to a few percent.
6Cell. Constraints on flagellar rotationCilia and Eukaryotic Flagella
Eukaryotic cells (the kind that make up animals, plants, and fungi) also have flagella and cilia, but these work on an entirely different principle from their bacterial counterparts. Instead of a rotary motor, eukaryotic cilia contain an internal scaffold called the axoneme, which usually consists of nine pairs of microtubules arranged in a circle. Dynein motor proteins, organized into inner and outer rows along these microtubule pairs, drive sliding between adjacent microtubules, and that sliding is converted into the characteristic beating or rotating motion.
7PubMed Central. Fifty years of microtubule sliding in cilia8PubMed Central. Axoneme Structure from Motile Cilia
In your body, cilia line the airways and sweep mucus upward to clear debris and pathogens. Sperm cells use a single long flagellum to swim. And during embryonic development, cilia in a structure called the node generate a leftward fluid flow that helps the body establish its left-right asymmetry. Defects in ciliary motility cause a group of conditions known as primary ciliary dyskinesia, which can lead to chronic respiratory infections, infertility, and in some cases a mirror-image reversal of the internal organs.
How Cells Know Where to Go
Moving is one thing; moving in the right direction is another. Cells navigate by detecting gradients of chemical, mechanical, or electrical signals in their environment. The best-studied form is chemotaxis, in which cells move toward (or away from) a chemical attractant. Neutrophils, the first-responder white blood cells, detect attractant molecules using receptors on their surface. Even tiny differences in concentration from one side of the cell to the other get amplified internally, creating a clear “front” that drives extension and a “back” that contracts.
9PubMed Central. The signaling mechanisms underlying cell polarity and chemotaxisThe internal wiring that achieves this is surprisingly delicate. Positive feedback loops amplify the signal at the front, which should in theory cause the activation to spread everywhere and destroy spatial information. Cells solve this with negative feedback. In neutrophils, the protein Cdc42, a key coordinator of front-end polarity that predicts cell turning behavior, is kept spatially precise by at least two distinct braking mechanisms involving actin and Cdc42 itself. These limit both how long and how far the activation signal spreads.
10PubMed Central. Optogenetic control of receptors reveals distinct roles for actin- and Cdc42-dependent negative signals in chemotactic signal processingAt a broader level, the front-back distinction depends on asymmetric distribution of signaling molecules. Ras and PI3K activity concentrate at the leading edge, while molecules like PTEN dissociate from those regions and accumulate at the rear. This polarity is self-reinforcing and essential for sustained directional movement.
11PubMed Central. The excitable signal transduction networks: movers and shapers of eukaryotic cell migrationNot Just Chemicals: Stiffness and Electricity as Guides
Chemical gradients are not the only compass cells follow. In durotaxis, cells migrate along gradients of mechanical stiffness, generally heading toward stiffer regions. This matters in tissue engineering and in understanding how tumors interact with the stiffening tissue around them. Cells sense stiffness through their adhesion machinery and translate that information into biased movement.
12PubMed Central. Durotaxis: the mechanical control of directed cell migrationPerhaps more surprising is galvanotaxis, the ability of cells to follow endogenous electric fields. Natural voltage gradients exist in developing and regenerating tissues, and experiments have shown that applied electric fields can direct the migration of diverse cell types. Hippocampal neurons, for instance, migrate toward the cathode (the negative pole) at field strengths close to those found in living tissue, and reversing the polarity reverses their direction.
13PubMed. Small applied electric fields guide migration of hippocampal neuronsIn bone-forming cells, the directional information from electric fields is captured by charged membrane transporters and fed into signaling pathways that rearrange the cytoskeleton and motor proteins, producing persistent directional movement. Galvanotaxis can even override chemical or topographic cues, making it a dominant guidance signal under certain conditions.
14PubMed Central. Endogenous electric fields as guiding cue for cell migrationTwo Ways to Invade Tissue
When individual cells move through three-dimensional tissue rather than across a flat surface, they face a more complex challenge. Two broad strategies have been identified. In mesenchymal migration, cells adopt an elongated shape, form strong adhesions, and often secrete enzymes that digest the surrounding matrix to clear a path. In amoeboid migration, cells are rounder, less adhesive, and squeeze through gaps using contractile force rather than cutting through them.
15PubMed Central. The molecular mechanisms of transition between mesenchymal and amoeboid invasiveness in tumor cellsWhat makes this particularly relevant to cancer biology is that some tumor cells can switch between modes depending on their environment, a capacity called migration plasticity. This switch is triggered by the microenvironment itself, allowing cells to adapt to changing tissue conditions.
16PubMed Central. Amoeboid-mesenchymal migration plasticity promotes invasion only in complex heterogeneous microenvironmentsRecent work has challenged the traditional view that amoeboid cells do not digest their surroundings. Using cancer spheroid models and single-cell RNA sequencing, researchers found that both mesenchymal and amoeboid invasion programs involve active tunneling through matrix barriers, with both types expressing matrix-degrading enzymes. The distinction is real, but less clean-cut than once thought.
17PubMed Central. Amoeboid-mesenchymal transition and the proteolytic control of cancer invasion plasticityMoving as a Group
Not all migration is solo. During wound healing, embryonic development, and cancer invasion, cells frequently move as coordinated groups. Collective migration relies on a leader-follower hierarchy: leader cells at the front sense the environment, find paths, and transmit signals, while follower cells respond to those signals and transmit mechanical stress through cell-to-cell connections.
18PubMed. Dynamic Leader-Follower Hierarchies in Collective Cell MigrationThe physical links between cells in a migrating group are provided by cadherins, adhesion proteins that bridge neighboring cells and connect to the actin cytoskeleton. These junctions are not just passive links. The cadherin complex actively senses fluctuations in mechanical force and converts them into biochemical signals that regulate cell polarity and movement, coordinating the group’s overall direction.
19PubMed. Cadherin mechanotransduction in leader-follower cell specification during collective migrationCell Movement in Embryonic Development
Some of the most dramatic cell journeys happen before birth. Neural crest cells are a temporary population that arises from the developing central nervous system and migrates extensively throughout the embryo. These cells give rise to a remarkably diverse set of tissues: the melanocytes that pigment your skin, the cartilage and bone of the face, peripheral nerves, the neurons that control your gut, and smooth muscle in blood vessels.
20PubMed Central. Development and evolution of the neural crest: an overviewThe patterning of the skull and face depends heavily on where these cells end up. Initial migration routes are conserved across species, but the interplay between the cells’ internal genetic programs and signals from the tissues they move through creates the enormous variety of facial structures seen across vertebrates.
21PubMed. Molecular mechanisms of cranial neural crest cell migration and patterning in craniofacial developmentImmune Cells on Patrol
Your immune system depends on cell motility at every stage of the inflammatory response. When tissue becomes infected or damaged, white blood cells circulating in the bloodstream must stop, attach to the vessel wall, and squeeze through the lining to reach the affected area. This process, called transendothelial migration, involves a carefully choreographed series of steps: tethering, rolling along the vessel wall, firm adhesion, and finally squeezing between or through endothelial cells.
22PubMed Central. How endothelial cells regulate transmigration of leukocytes in the inflammatory responseThat final step of transmigration is governed by molecules distinct from those involved in the earlier adhesion steps. On the leukocyte side, receptor polarization and signaling machinery must orient to drive directional movement. On the endothelial side, signaling driven by Rho-family GTPases induces a brief, localized loosening of cell-to-cell junctions, opening a gap just wide enough and long enough for the leukocyte to pass through.
23PubMed. Signaling in leukocyte transendothelial migrationWound Healing Depends on Coordinated Migration
When your skin is cut, closing the gap requires keratinocytes (the main cells of the outer skin layer) to migrate laterally across the wound bed. These migrating keratinocytes secrete a protein called extracellular heat shock protein 90 alpha (hsp90α), which has a dual signaling role. It acts on the keratinocytes themselves to drive further migration and also signals to deeper-layer cells, stimulating fibroblasts to move in and rebuild connective tissue and endothelial cells to form new blood vessels.
24PubMed Central. Keratinocyte Migration and a Hypothetical New Role for Extracellular Heat Shock Protein 90 Alpha in Orchestrating Skin Wound HealingThere is also a two-way conversation between cell layers. Keratinocytes stimulate fibroblasts to produce growth factors that in turn promote keratinocyte proliferation, creating a positive feedback loop between the skin’s surface and its deeper tissue that accelerates repair.
25Journal of Investigative Dermatology. Epithelial–Mesenchymal Interactions in Wound RepairWhen Motility Turns Dangerous: Cancer Metastasis
The same machinery that allows cells to migrate during development and wound healing can be co-opted by cancer cells to spread through the body. A central process in metastasis is the epithelial-mesenchymal transition, or EMT, in which tumor cells shift from a stationary, tightly connected state to a mobile, loosely attached one. This transition can promote a cancer cell’s ability to enter blood vessels and travel to distant organs.
26PubMed Central. The role of EMT and MET in cancer disseminationMetastatic and basal-like tumor cells exert higher traction forces through their integrins compared to less aggressive cells, consistent with models in which both the molecular motors and the adhesion “clutches” are ramped up together.
27PubMed Central. Integrin-mediated traction force enhances paxillin molecular associations and adhesion dynamics that increase the invasiveness of tumor cells into a three-dimensional extracellular matrixWhat fuels all this movement is also telling. In prostate and breast cancer cells, glycolysis (the quick-and-dirty breakdown of sugar without oxygen) appears to be the dominant energy source for motility and cytoskeletal rearrangement. Even when mitochondria are producing plenty of energy overall, they cannot compensate when glycolysis is blocked, because what matters is not total energy output but the location of energy production near the sites where the skeleton is being actively rebuilt.
28Oncotarget. Glycolysis is the primary bioenergetic pathway for cell motility and cytoskeletal remodeling in human prostate and breast cancer cellsThe Nucleus as a Speed Limit
When cells migrate through tight spaces in tissue, the biggest physical bottleneck is the nucleus. It is the largest and stiffest organelle in the cell, and squeezing it through a narrow gap requires significant deformation. The nuclear envelope’s stiffness is largely determined by a family of proteins called lamins. Lamin-A, in particular, acts as a rate-limiting factor for three-dimensional migration across a range of human cell types, from glioma cells to mesenchymal stem cells. Cells with lower levels of lamin-A have softer, more deformable nuclei and migrate faster through tight pores.
29PubMed Central. Nuclear lamin stiffness is a barrier to 3D migration, but softness can limit survivalThere is a trade-off, though. Softer nuclei are more fragile. Excessive deformation during confined migration can rupture the nuclear envelope, exposing DNA to the cytoplasm and potentially causing damage. So cells face a balancing act: enough nuclear softness to squeeze through gaps, but enough stiffness to protect the genome.
30PubMed Central. Nuclear mechanobiology in confined cell migrationPathogens That Steal the Cell’s Engine
Some bacteria have evolved to hijack the host cell’s own motility machinery. The best-known example is Listeria monocytogenes, a food-borne pathogen. Once inside a host cell, Listeria uses a surface protein called ActA to recruit the host’s Arp2/3 complex and trigger actin polymerization directly on the bacterial surface. This builds a “comet tail” of actin filaments that propels the bacterium through the cytoplasm at speeds of several micrometers per minute.
31PubMed Central. Actin-based motility allows Listeria monocytogenes to avoid autophagy in the macrophage cytosolThe bacterium does not just move aimlessly. By riding its actin tail into the host cell’s membrane, it pushes out a protrusion that a neighboring cell engulfs, allowing Listeria to spread directly from cell to cell without ever being exposed to the immune system in the extracellular space. This subversion of host actin is a critical virulence strategy shared by several intracellular bacterial pathogens.
32PubMed Central. Actin-based motility and cell-to-cell spread of bacterial pathogensBiochemically, the trick works because ActA and the Arp2/3 complex synergize: neither efficiently nucleates actin on its own in purified conditions, but together they dramatically accelerate new filament formation.
33PubMed. Interaction of human Arp2/3 complex and the Listeria monocytogenes ActA protein in actin filament nucleationBuilding Cells That Move from Scratch
Understanding cell motility well enough to reproduce it from non-living components is one of the frontiers of synthetic biology. Researchers have recently constructed artificial cells with internal skeletons that mimic some features of the real thing. One approach uses coacervate droplets stabilized by a polymer membrane, with an artificial cytoskeleton built from polydiacetylene fibrils that bundle and form structures mimicking the internal scaffolding of a real cell.
34PubMed Central. Cytoskeleton-functionalized synthetic cells with life-like mechanical features and regulated membrane dynamicityAnother line of work focuses specifically on adhesion-driven motility. Using giant vesicles as stand-ins for cells and supported lipid bilayers as surfaces, researchers have systematically tuned adhesion properties with light-switchable protein interactions to map out the design rules for how receptor and ligand density affect movement. The results provide a framework for engineering synthetic cells capable of controlled, directional movement on dynamic substrates, with potential applications ranging from targeted drug delivery to programmable materials.
35PubMed Central. Design rules for adhesion-driven synthetic cell motility on dynamic membranesParasites With Their Own Gliding Motor
Malaria parasites and their relatives in the group Apicomplexa use yet another motility mechanism that does not fit neatly into the crawling or swimming categories. These parasites have a gliding motor sandwiched between their outer membrane and an inner double-membrane layer unique to the group. The motor consists of unconventional myosin proteins anchored inside the cell, short actin filaments, and transmembrane proteins that reach out and grab the surface the parasite is moving across. The result is a smooth, gliding motion that the parasite uses to invade host cells and traverse tissue barriers.
36PubMed. Actin/myosin-based gliding motility in apicomplexan parasitesThis system is a prime drug target. Because the gliding motor is essential for the parasite’s ability to invade red blood cells and the liver, blocking any of its key components could halt infection. The motor has no close equivalent in human cells, which means drugs targeting it are less likely to cause side effects. Research into the molecular details of this machinery continues to feed the pipeline for next-generation antimalarial compounds.