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Differentiated cellA differentiated cell is a functional cell of a particular type, usually identifiable by its specific appearance under the light microscope. Examples include a neuron, an intestinal absorptive cell and a cartilage cell.
PluripotentAble to become one of several types of differentiated cell.
Transdetermination A term used in D. melanogaster genetics to indicate a change of commitment of imaginal disc cells such that they will become one disc type rather than another, for example wing instead of leg.
Transdifferentiation has attracted great controversy in recent years, mostly in regards to whether stem cells from the bone marrow can colonize other tissues after transplantation1,2. However, the general subject area of tissue-type switching is much wider than this specific controversy. It embraces some fascinating biological and pathological phenomena that deserve more attention than they have received.
There has been a recent tendency to use the word transdifferentiation to mean conversion of anything into anything else. But it is preferable to reserve the term for its original meaning transformation of one differentiated cell type into another3,4 and to use the term metaplasia for the more general transformation of one tissue type into another. This is because tissues generally consist of several differentiated cell types and metaplasia often involves the transformation of undifferentiated stem or progenitor cells such that they produce a repertoire of cell types that are characteristic of a different tissue5,6.
FIG. 1 shows a typical hierarchy of developmental decisions that cells pass through during the course of embryonic development. Superimposed on these are some different sorts of abnormal transformation. The transformation between differentiated cells is a trans-differentiation, whereas the transformation between tissue-specific stem cells is a metaplasia, but not a transdifferentiation. FIG. 1 also indicates the possibility of changes to abnormal phenotypes having no counterpart in the normal body (dysplasia).
Recently, there has been interest in the possibility of reprogramming cells from a differentiated state back to a
pluripotent state that resembles the embryonic stem (ES) cell, shown here as ES transformation. In this article, the term metaplasia will be used for any tissue-type-switching event and transdifferentiation will be used only when both the precursor and the product are differentiated cell types.
Because of scepticism about the reality of metaplasias, generally there is a...