Metaplasia comes from the Greek meaning “change in form.” It is the reversible replacement of one differentiated cell type with another differentiated cell type. The change implies the reprogramming of a common stem cell, and not the metamorphosis of one differentiated cell type into another. Metaplasia occurs as a result of reprogrammed stem cells that are nudged along a different pathway of differentiation by cytokines, growth factors, and other substances in the cell’s environment. Where these stem cells come from—either stem cells inherent to the tissue in question, derived from a circulating pool, or generated through a process of regressive differentiation—is a matter of speculation.
The change from one type of cell to another is generally caused by some sort of abnormal stimulus. In simplistic terms, the original cells are insufficiently robust to withstand the environmental change, and so they are replaced by another cell type that is within the repertoire of a common stem cell, but more suited to the new, more rugged environment. If the stimulus that caused metaplasia is removed or ceases, tissues generally return to their normal pattern of differentiation.
Metaplasia may occur in benign, atypical, or malignant tissue. Although metaplasia does not represent carcinogenesis, it is accompanied by a loss of normal epithelial function, and this is undesirable. The undesirability is enhanced by the propensity of metaplastic regions to eventually turn dysplastic or cancerous ifthe irritant remains. Although endometrial epithelial metaplasia is most commonly encountered among postmenopausal women with abnormal uterine bleeding, it is unlikely that the bleeding can be ascribed to metaplasia.
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Maksem, J.A., Robboy, S.J., Bishop, J.W., Meiers, I. (2009). Endometrial Epithelial Metaplasias and Foam Cells. In: Endometrial Cytology with Tissue Correlations. Essentials in Cytopathology, vol 7. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-89910-7_8
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