Metaplasia represents a reversible change in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type.
It is thought to involve the reprogramming of undifferentiated stem cells that are present in the tissue undergoing the metaplastic changes.
It usually occurs in response to chronic irritation and inflammation and allows for substitution of cells that are better able to survive under circumstances in which a more fragile cell type might succumb. However, the conversion of cell types never oversteps the boundaries of the primary groupus of tissue (e.g. one type of epithelial cell may be converted to another type of epithelial cell, but not to a connective tissue).
