Cell Migration and Epithelial-Mesenchymal Transition

Building an embryo requires cells to move, and a key way they gain the ability to move is the epithelial-mesenchymal transition (EMT), in which tightly joined epithelial cells lose their cell-cell junctions and apical-basal polarity and acquire the migratory, invasive properties of mesenchymal cells. EMT and the reverse process let cohesive sheets release individual cells that travel to new locations, as during gastrulation and neural crest formation. Together, EMT and directed migration reposition cells so that tissues form in the right places.

Definition

The epithelial-mesenchymal transition is a reversible cellular program in which an epithelial cell loses its junctions and apical-basal polarity and acquires a mesenchymal phenotype with front-rear polarity and increased motility and invasiveness; cell migration is the resulting directed movement of cells through the embryo.

Scope

The entry covers what EMT is, the molecular events that drive it, its roles in normal development, and how cells move once they become migratory. It treats EMT and developmental cell migration as molecular and cellular topics and is reference and educational, not clinical guidance.

Core questions

• What molecular changes convert an epithelial cell into a migratory mesenchymal cell?
• Which developmental events depend on EMT and cell migration?
• How do migrating cells sense direction and reach their destinations?
• How is EMT in development related to its reactivation in disease?

Key concepts

• Loss of cell-cell junctions and apical-basal polarity
• EMT transcription factors (Snail, Slug, Twist, ZEB)
• Downregulation of E-cadherin
• Acquisition of motility and invasiveness
• Mesenchymal-epithelial transition (the reverse process)
• Neural crest migration and gastrulation
• Cytoskeletal remodeling and cell-matrix adhesion

Key theories

EMT as a reversible developmental program

EMT is a coordinated, reversible switch in which signaling pathways activate transcription factors such as Snail, Slug, Twist, and ZEB that repress epithelial genes and induce mesenchymal ones, allowing cells to detach and migrate and, through the reverse transition, to re-epithelialize at their destination.

Classification of EMT contexts

EMT has been organized into types associated with development, with wound healing and fibrosis, and with cancer progression, sharing a common molecular core but occurring in different biological settings, which clarifies how a developmental program is reused in disease.

Mechanisms

EMT is driven by developmental signals — including TGF-beta/BMP, Wnt, Notch, and growth-factor pathways — that activate a core set of transcription factors such as Snail, Slug, Twist, and ZEB. These factors repress epithelial genes, notably the cell-cell adhesion molecule E-cadherin, and induce mesenchymal genes, so the cell dismantles its tight and adherens junctions, loses apical-basal polarity, and remodels its cytoskeleton to adopt front-rear polarity. The cell then extends protrusions, forms and turns over adhesions to the extracellular matrix, and moves by coordinated protrusion, attachment, and retraction. In the embryo this enables events such as the ingression of cells during gastrulation and the delamination and migration of neural crest cells, which travel along defined routes guided by extracellular cues before differentiating. Because EMT is reversible, migrating cells can undergo the reverse mesenchymal-epithelial transition to reassemble into epithelia at their new location.

Clinical relevance

The same EMT program that drives normal morphogenesis is reactivated in tissue fibrosis and in cancer invasion and metastasis, making developmental EMT a reference framework for understanding these processes. This entry explains the mechanism for reference and education and is not a basis for diagnosis or treatment.

Evidence & guidelines

Evidence comes from cell and developmental biology — studies of gastrulation and neural crest in model embryos, molecular dissection of EMT transcription factors, and live imaging of migrating cells — synthesized in review literature and textbooks rather than clinical guidelines.

History

The recognition that embryonic cells can switch between epithelial and mesenchymal states grew out of studies of gastrulation and neural crest migration. Over the following decades the molecular control of EMT was defined, with the identification of transcription factors that repress E-cadherin and drive the transition, and the realization that the developmental program is reused in fibrosis and cancer brought EMT to wide biomedical attention and prompted efforts to classify its contexts.

Debates

Is EMT an all-or-none switch or a spectrum of intermediate states?

Whether cells fully convert between epithelial and mesenchymal identities or pass through stable partial or hybrid states with mixed features is debated, with growing evidence that intermediate states are common and functionally important.

Key figures

• Jean Paul Thiery
• M. Angela Nieto
• Raghu Kalluri
• Robert Weinberg
• Rik Derynck

Seminal works

• thiery-2009
• kalluri-weinberg-2009
• lamouille-2014

Frequently asked questions

Why must epithelial cells undergo EMT to migrate?

Epithelial cells are held in place by junctions and apical-basal polarity; EMT dismantles these features and remodels the cytoskeleton, giving the cell the front-rear polarity and reduced adhesion it needs to detach and move.

Is EMT permanent?

No. EMT is reversible: cells that have migrated can undergo the reverse mesenchymal-epithelial transition and reassemble into epithelial tissue at their destination.