= Solution
External chemical signals can guide <cell migration>, but the statement is too strong if “determined” means that they uniquely specify whether, how fast and in which direction a cell moves. A <chemoattractant> <gradient> biases a cell's internal polarity machinery; it does not directly supply the mechanical work of locomotion. <Chemotaxis> is directional bias, whereas a change in speed or persistence without directional preference is <chemokinesis>.
A chemical response depends on receptor abundance, signalling adaptation and the cell's existing <cell polarity>. The same external field can therefore yield different trajectories in different cell states. Uniform stimulation can increase motility without providing a direction, and spontaneous fluctuations of the <actin> system can initiate movement without a spatial chemical instruction. Persistent migration combines feedback within the cell with a cue that biases that feedback.
Mechanical and geometrical inputs also matter. <Durotaxis> describes a bias associated with substrate stiffness, while <haptotaxis> follows spatial variation in adhesive ligands. <Contact guidance> by fibre alignment can channel motion even when soluble signals are uniform. Narrow spaces constrain the available paths and can force a switch of protrusive mechanism. These effects are not wholly separate from chemistry: cells sense mechanical forces through molecular interactions. Nevertheless, their directional information resides in mechanics or geometry, not necessarily in an imposed soluble <concentration gradient>.
Contacts between cells supply another source of organization. Experiments on <neural crest> cells showed that collisions can cause protrusion collapse and repolarization, and that perturbing noncanonical <Wnt signalling> disrupts this behaviour and group directionality.<Contact inhibition of locomotion in vivo controls neural crest directional migration by Carlos Carmona-Fontaine and others>[Collision assays in vivo and in vitro; perturbation of noncanonical Wnt signalling and local RhoA activation]{ref} <Contact inhibition of locomotion> therefore helps generate collective trajectories through local interactions, rather than each cell independently reading a complete external chemical map. Cohesion and other attractive interactions can prevent such repulsion from simply dispersing the group.
A discriminating experiment independently varies soluble <gradient>, adhesive pattern and mechanical confinement. Measure direction, speed and persistence separately; reverse the <gradient>, remove it, and then block the relevant receptor while checking viability and the ability to move. If a directional bias follows the reversed chemical field, that supports chemical guidance. If speed rises under uniform stimulation, that supports <chemokinesis>. If trajectories remain aligned with fibres after chemical reversal, physical guidance is influential. Manipulating intracellular polarity can test whether a cue is instructive or merely permissive.
The useful conclusion is that motion emerges from an internally powered, state-dependent cell interacting with chemical, mechanical and cellular surroundings. External chemical signals are often decisive inputs, but the trajectory is a property of the coupled system.
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