Motile cells are any cells capable of self-propelled movement, and they are found across all three domains of life. Bacteria swim using tiny rotary motors, sperm whip their tails to reach an egg, immune cells crawl through tissue to fight infection, and embryonic cells migrate in coordinated streams to build organs. The mechanisms behind these movements differ dramatically, but they share a common thread: converting chemical energy into physical force that pushes, pulls, or propels the cell forward. How each cell accomplishes that trick depends on its evolutionary toolkit, its environment, and the job it needs to do.
How Crawling Cells Move
Many animal cells get around by crawling along surfaces, a process that unfolds in a repeating cycle of three steps: pushing the front edge forward, gripping the surface, and pulling up the rear. Each step relies on different molecular machinery, and all three have to work in concert for the cell to make progress.
The forward push comes from actin, a protein that assembles into long filaments just beneath the cell’s leading membrane. When a cell receives a signal to move, membrane-bound activators trigger rapid branching and growth of actin filaments near the front edge, generating a local force that physically shoves the membrane outward.1PubMed Central. Dynamics of membranes driven by actin polymerization This creates a thin, fan-shaped extension called a lamellipodium, or sometimes finger-like projections called filopodia. Think of it as a crowd of people all pressing against a curtain from behind: no single person moves the curtain far, but millions of tiny pushes add up to a smooth, steady advance.
Once the front edge extends, the cell needs to anchor itself so it doesn’t just snap back. That grip comes from proteins called integrins, which span the cell membrane and bind to molecules in the surrounding material. Research has shown that increasing the number of integrin bonds engaged with the surface directly increases the pulling forces the cell can exert.2PubMed Central. Activation of beta 1 but not beta 3 integrin increases cell traction forces These anchoring sites, called focal adhesions, are clusters of integrins and associated proteins that collectively transmit force from the cell’s internal skeleton to the external surface. Within moving focal adhesions, integrins can transmit forces exceeding 54 piconewtons per molecule, enough to maintain traction while the rest of the cell catches up.3Biophysical Journal. Visualizing and Quantifying Integrin Tension at the Cell-Substratum Interface The adhesion process isn’t perfectly smooth, though. Studies tracking individual focal adhesion proteins found that when the cell’s internal tension kicks in, the entire adhesion complex briefly slides backward as a unit before locking onto the surface, a transient slip that covers roughly half a micrometer before the grip catches.4PubMed Central. Transient frictional slip between integrin and the ECM in focal adhesions under myosin-II tension
The final step is hauling up the tail. The motor protein myosin II, working with actin filaments inside the cell, contracts the rear like a drawstring, pulling it forward and completing the movement cycle.5PubMed. Mechanism of cell rear retraction in migrating cells This retraction step is so central to our understanding of cell migration that it dominates textbook illustrations of the process.6PubMed. Actomyosin forces in cell migration: Moving beyond cell body retraction The whole cycle, extending, gripping, and retracting, repeats over and over, propelling the cell forward in a slow but persistent crawl.
Amoeboid Movement and Blebbing
Not every motile cell crawls flat along a surface. Some cells, particularly those navigating three-dimensional environments like tissue interiors, use a faster and more fluid style of movement called amoeboid migration. Instead of building the elaborate actin-rich protrusions and strong adhesion points that crawling cells rely on, amoeboid cells squeeze and flow, changing shape rapidly to push through gaps.
One striking version of this involves blebs, spherical bulges that pop out from the cell surface. Blebs form when the cell’s internal contractile machinery squeezes the cytoplasm hard enough to push the membrane away from the underlying skeleton. The force inflating these protrusions comes not from actin assembly at the front edge but from actomyosin contraction elsewhere in the cell, generating hydrostatic pressure that blows the membrane outward like a balloon.7PubMed Central. Blebs-Formation, Regulation, Positioning, and Role in Amoeboid Cell Migration Experiments with the soil-dwelling amoeba Dictyostelium confirmed that bleb expansion is driven by this hydrostatic pressure produced by the cortex contracting with the help of myosin II.8PubMed. Blebbing of Dictyostelium cells in response to chemoattractant
Cancer cells exploit both strategies. Tumor cells invading surrounding tissue can use either a mesenchymal mode, which resembles the crawling cycle described earlier, or an amoeboid mode based on blebbing. The two modes involve different levels of adhesion and different interactions with the surrounding tissue matrix, and cells can switch between them depending on conditions.9PubMed Central. The molecular mechanisms of transition between mesenchymal and amoeboid invasiveness in tumor cells This plasticity is one reason metastatic cancer is so difficult to contain: block one mode of invasion, and the cells may simply switch to the other.
Cilia, Flagella, and Rotary Motors
Crawling and blebbing work well on surfaces and inside tissues, but cells that need to swim through liquid use entirely different structures. Eukaryotic cells, which include everything from single-celled pond organisms to human sperm, rely on cilia and flagella. These are hair-like extensions built around a scaffold of microtubules, and their movement is powered by dynein motors that cause the internal microtubule pairs to slide against each other, bending the whole structure in a coordinated wave or whip.10PubMed Central. Fifty years of microtubule sliding in cilia Multiple types of dynein motors work together, and their coordinated switching on opposite sides of the cilium is what generates the rhythmic beating pattern.11PubMed Central. Asymmetric distribution and spatial switching of dynein activity generates ciliary motility
Bacteria took a completely different approach. A bacterial flagellum is not a flexible whip but a rigid, corkscrew-shaped filament attached to a rotary motor embedded in the cell membrane. This motor literally spins the filament like a propeller. The energy source is not a molecule like ATP but the flow of protons (hydrogen ions) across the membrane, a protonmotive force.12PubMed Central. A protonmotive force drives bacterial flagella In E. coli, the motor’s speed scales linearly with this proton-driven force, reaching up to about 270 revolutions per second under light loads.13PubMed Central. The speed of the flagellar rotary motor of Escherichia coli varies linearly with protonmotive force When the motor spins counterclockwise, the cell swims in a smooth run; when it briefly reverses, the cell tumbles and reorients before running again in a new direction.
Though eukaryotic flagella and bacterial flagella share a name, they share almost nothing else. They are built from different proteins, powered by different energy sources, and assembled in different ways. Even the flagella of archaea, the third domain of life, are structurally unrelated to either. This is a striking case of convergent evolution: the problem of swimming through liquid has been solved independently at least three times.14PubMed Central. The evolution of archaeal flagellar filaments
How Cells Know Where to Go
Having the machinery to move is only half the challenge. Cells also need to figure out which direction to go. A cell that crawls or swims randomly is far less useful than one that can navigate toward a wound, a food source, or an invading pathogen. Cells accomplish this by sensing gradients in their environment and orienting their movement accordingly.
The best-studied guidance system is chemotaxis, in which cells detect differences in chemical concentration across their surface and migrate toward (or away from) the source. A neutrophil chasing a bacterium, for example, senses tiny differences in chemoattractant concentration between its front and back, amplifies those differences through intracellular signaling, and reorganizes its internal machinery so that actin polymerization concentrates at the edge facing the highest concentration.15PubMed Central. The signaling mechanisms underlying cell polarity and chemotaxis This creates a stable front-back polarity: the front pushes forward, the back contracts, and the cell moves up the gradient with surprising accuracy.
Chemical gradients are not the only cues cells respond to. Cells can also sense and follow electrical fields, a behavior called galvanotaxis. The body naturally generates small electric fields at wound sites, and these fields guide stem cells and other repair-related cells toward the damage.16PubMed Central. Environmental Factors That Influence Stem Cell Migration: An “Electric Field” Then there is durotaxis, the tendency of cells to migrate toward stiffer surfaces. When researchers created substrates with a sharp boundary between soft and stiff regions, cells crossing from soft to stiff moved right across, but cells approaching from the stiff side turned around or pulled back rather than venturing onto the softer material.17Biophysical Journal. Cell movement is guided by the rigidity of the substrate This preference for stiffness influences how cells distribute themselves in tissues with varying mechanical properties and plays a role in processes from wound healing to tumor invasion.
Moving as a Group
Cells don’t always migrate as lone individuals. In many biological contexts, cells move in coordinated groups, maintaining physical connections with their neighbors while traveling together. This collective migration shows up throughout embryonic development, wound repair, and cancer metastasis, and it poses a coordination problem: how does a group of connected cells agree on a direction?
A widely studied model is the leader-follower arrangement, where cells at the front edge of the group take on a distinct “leader” identity. Leaders tend to be larger, extend more prominent protrusions, and generate stronger traction forces than the followers behind them. But followers are not passive passengers. They transmit forces through cell-cell junctions, help maintain the group’s polarity, and can even influence which cells stay in the lead.18PubMed Central. Roles of leader and follower cells in collective cell migration The dynamic between leaders and followers is not rigidly fixed; cells can swap roles depending on signals and position, making collective migration remarkably adaptable.
Neural crest cells offer a dramatic example. These embryonic cells break away from the developing nervous system and migrate in streams to populate distant parts of the body, eventually forming structures as varied as facial bones, pigment cells, and parts of the heart. Depending on where they originate along the body axis, neural crest cells move in broad sheets or single-file chains, using a combination of chemotaxis and cell-cell contact to stay on course.19PubMed. Mechanisms of Neural Crest Migration
Motile Cells in Daily Physiology
The immune system is one of the most motility-dependent systems in the body. White blood cells constantly patrol the bloodstream and, when they detect signs of infection or injury, exit the blood vessels to enter the affected tissue. This exit, called extravasation, is a multi-step process. Leukocytes first tether to and roll along the vessel wall, then adhere firmly, crawl to a suitable exit point, and finally squeeze between or through the endothelial cells lining the vessel. Each step is governed by a distinct set of molecular interactions, with the final transmigration step regulated by molecules that play no role in the earlier tethering and rolling phases.20PubMed Central. How endothelial cells regulate transmigration of leukocytes in the inflammatory response
Sperm motility is another case where movement is mission-critical. A sperm cell’s tail, a single long flagellum, propels it through the reproductive tract. But simple swimming isn’t enough for fertilization. Near the egg, sperm switch into a hyperactivated state characterized by asymmetric, high-amplitude tail bending. This vigorous whipping motion helps sperm push through the protective layers surrounding the egg. The shift to hyperactivation depends on a calcium channel called CatSper; without it, sperm can swim but cannot complete the final penetration needed for fertilization.21PubMed Central. Sperm hyperactivation and the CatSper channel: current understanding and future contribution of domestic animals
Parasites That Glide
Some of the most medically important motile cells don’t crawl, swim, or bleb. Apicomplexan parasites, the group that includes the organisms responsible for malaria (Plasmodium) and toxoplasmosis (Toxoplasma), use an unusual form of movement called gliding motility. These parasites have no cilia, flagella, or visible shape changes during movement. Instead, they slide smoothly across surfaces and into host cells using an internal motor apparatus wedged between their outer membrane and an underlying double-membrane layer unique to this group of organisms.22PubMed. Gliding motility in apicomplexan parasites
The gliding motor, called the glideosome, works by anchoring myosin proteins to the inner membrane complex while short actin filaments at the surface are linked to adhesive molecules that grip the external surface. When myosin pushes the actin backward, the adhesins are dragged toward the rear of the parasite, and the whole cell moves forward. It is fundamentally different from how any other eukaryote moves, a system built from familiar components (actin and myosin) but arranged in a way that produces smooth substrate-dependent gliding rather than crawling or swimming.23PubMed. Actin/myosin-based gliding motility in apicomplexan parasites The same motor that drives gliding also powers host-cell invasion, making it one of the most attractive drug targets in malaria research.
Why Motility Evolved More Than Once
Given how central movement is to survival, it might seem obvious that motile structures would have evolved once and then been inherited by all living things. That is not what happened. Flagellar motility arose independently at least three times: once in bacteria, once in archaea, and once in eukaryotes. Bacterial and archaeal flagella are both rotary propellers, but they are built from unrelated proteins and assembled in opposite ways. Eukaryotic cilia beat or wave rather than rotate and contain hundreds of proteins compared to the handful found in prokaryotic filaments.24FEMS Microbiology Reviews. Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia Even their energy sources differ: bacterial flagella run on ion flow across the membrane, while archaeal flagella and eukaryotic cilia use ATP.
Researchers have compared the three flagellar systems and concluded that the structural and mechanistic differences are so vast that a gradual transition between any of them is hard to imagine. The more likely scenario is that each system evolved from scratch in lineages that originally had no flagella at all.25eLife. Flagellar energy costs across the tree of life This parallel invention of swimming mirrors other famous examples of convergent evolution in larger organisms, where the same functional need produced completely independent solutions.
Hijacking Host Motility
Some pathogens skip building their own movement systems altogether and instead commandeer the host cell’s machinery. Listeria monocytogenes, the bacterium responsible for listeriosis, is a well-known example. After invading a host cell, Listeria triggers the host’s own actin to polymerize into a tail behind the bacterium, propelling it through the cytoplasm and even into neighboring cells. Researchers have reconstituted this actin-tail propulsion in simplified laboratory systems, confirming that the force comes from actin polymerization pushing against the bacterial surface.26PubMed. Reconstitution of Listeria motility: implications for the mechanism of force transduction The bacterium essentially turns the host’s cytoskeleton into a jet engine, allowing it to spread without ever re-entering the hostile environment outside the cell.
When the Nucleus Gets in the Way
For cells migrating through the dense, three-dimensional environments found inside the body, speed and direction are not the only challenges. The nucleus, the largest and stiffest organelle in most cells, has to come along for the ride. During migration, the nucleus must be repositioned and sometimes physically deformed to fit through narrow gaps in the surrounding tissue. Its movement has to stay coordinated with the cytoskeletal dynamics driving the leading edge and the trailing end, and changes in nuclear shape in turn affect cell polarity and overall migration efficiency.27PubMed Central. Nuclear mechanics during cell migration Cells with stiffer or larger nuclei have a harder time squeezing through tight spaces, which is one reason nuclear mechanics has become a focus of research into metastatic cancer. A tumor cell that can deform its nucleus more readily may be better equipped to navigate the narrow passages between healthy tissues, making nuclear stiffness a potential factor in how aggressively a cancer spreads.