Past exam of the zoology course of the University of Cambridge / 2025 / Paper zl6 / 3 / a

Epithelial cell fate plasticity is a context-dependent change in developmental potential or differentiated identity. It includes dedifferentiation toward a progenitor cell state, transdifferentiation between differentiated identities, and reversible intermediate states. It is distinct from selecting a pre-existing rare stem cell, and from ordinary progression down a lineage. epithelial-mesenchymal plasticity is one important form, but changing epithelial subtype without losing epithelial organization is also cell fate plasticity.

The strongest physiological rationale is that a tissue needs both stable specialization and reserve routes for replacement. During ordinary adult tissue homeostasis, a stem-cell hierarchy can efficiently supply predictable cell types. After injury removes a component of that hierarchy, local signals and newly available stem-cell niche space may permit a committed descendant to reacquire self-renewal. In mouse intestine, Alpi marked enterocyte-lineage progenitor cells normally generate short-lived absorptive descendants. After targeted ablation of Lgr5 positive intestinal stem cells, labelled progenitor cells can generate long-lived crypt-villus lineages with renewed stem-cell output. Comparing the same labelled population before and after ablation distinguishes injury-dependent cell fate plasticity from its normal fate. The experiment concerns progenitor cells in the enterocyte lineage; it should not be generalized to every terminally mature villus enterocyte.^[ref]

A complementary model is the mouse airway. Genetic depletion of basal stem cells, combined with prior lineage tracing of luminal secretory cells, revealed secretory-cell descendants that acquired basal-cell properties and sustained subsequent epithelial production. Experiments with isolated cells and cocultures further showed that contact with basal cells inhibits this conversion. cell fate plasticity is therefore regulated by the tissue environment, not simply an intrinsic capacity unmasked whenever a marker changes. Its interpretation requires controls for residual basal cells and for whether the secretory-lineage driver labelled basal cells before injury.^[ref]

Development provides additional contexts. Embryonic epithelia can undergo epithelial-to-mesenchymal transition during processes such as gastrulation, and mesenchymal-to-epithelial transition can subsequently assemble epithelial structures. These are regulated changes in adhesion, cell polarity, motility and gene expression, rather than a simple binary switch read from E-cadherin alone. In wound repair, transient relaxation of epithelial organization can facilitate migration while retaining intercellular contacts. A partially migratory repair state need not entail a complete mesenchymal conversion or a permanent change of lineage. Following wound closure, restoration of barrier function and polarized architecture is as important as the initial ability to move.

The most convincing experimental design labels a defined starting population irreversibly before the inducing event, allows the inducer to clear, then follows labelled descendants through the transition. Check immediate labelling specificity, include no-inducer and uninjured controls, quantify labelling efficiency and rule out ectopic driver activation after injury. Demonstrate the new state with multiple markers, morphology and function, and persistence after the inducing signal is removed. Serial epithelial production or a second injury challenge tests stable self-renewal; a transient appearance of a basal marker does not. Sparse labelling and continuous live-cell imaging help distinguish conversion from contamination, cell fusion or selective expansion of a rare initially unrecognized progenitor cell.

single-cell RNA sequencing can identify intermediate cell states and candidate regulators, but inferred trajectories are not lineage histories. Combine it with genetic barcodes, spatial localization and timed perturbation of candidate transcription factors or niche signals. Organoids offer defined media, prospective cell isolation and manipulation of matrix stiffness. A sorted population's ability to make a structure in culture establishes potential under those conditions; dissociation and a permissive medium may induce a capacity rarely used in vivo. Compare culture results with intact-tissue fate mapping before claiming a normal physiological contribution.

The evolutionary argument is a trade-off, rather than a claim that every plastic response is an adaptation. Conditional access to alternative cell fates could buffer environmental injury and permit repair without maintaining a separate reserve stem-cell pool for every possible lesion. Conserved signalling modules can also be redeployed in new developmental contexts. These advantages must be weighed against loss of specialized function, energetic costs and the danger of inappropriate persistent growth. Comparative experiments should ask whether homologous lineages undergo similar transitions under comparable injuries; shared expression of a few regulators is insufficient to prove that an ancestral cell fate plasticity programme has been conserved. At the tissue level, cell fate plasticity supplies alternative states on which selection can act, but it does not itself establish heritable evolutionary change.

Clinically, controlled cell fate plasticity may improve epithelial repair, provide transplantable cells and make patient-derived disease models. The same capacity can undermine treatment: a tumour may switch to a less drug-sensitive lineage or regenerate a stem-like population after therapy. In carcinoma, epithelial-mesenchymal plasticity can contribute to invasion and treatment resistance, but neither a single EMT marker nor its absence establishes the mechanism of metastasis. In a lineage-traced mouse lung-cancer model, the tested EMT programme was dispensable for lung metastasis while contributing to chemoresistance; the conclusion is specific to that model and reporter coverage.^[ref] The useful clinical aim is therefore a defined, reversible transition that restores function, with evidence that it does not leave a persistently proliferative or malignant population.