What is fate commitment?

Fate commitment does not mean that a cell has irrevocably become something. It means that the space of futures available to that cell has begun to collapse. Early on, a cell can be reprogrammed into many outcomes if you perturb its environment. As development proceeds, fewer interventions succeed. Commitment is about loss of options, not acquisition of identity.

The earliest signs of commitment are rarely visible as single marker genes. They show up instead as subtle biases. Slight but consistent shifts in transcription factor levels, asymmetric exposure to signaling gradients, changes in chromatin accessibility, and altered competence to respond to future signals. Two cells may look nearly identical, but one will respond to a signal and the other will not. That difference is already fate information.

The logic behind this is constraint accumulation. Development does not assign fates by labeling cells. It reshapes the energy landscape in which cells move. Gene regulatory networks become wired so that certain states stabilize and others become inaccessible. Feedback loops lock in expression patterns. Epigenetic modifications raise the energetic cost of switching trajectories. What looks like a decision is often the point at which reversal becomes dynamically implausible.

This is why fate commitment is gradual and context dependent. A cell can be committed with respect to one axis and still plastic with respect to another. Neural progenitors, for example, may be committed to being neural long before they are committed to being hippocampal, cortical, or interneuronal. Commitment is always relative to the question being asked.