Metaplasia is a reversible change in which one adult cell type ( epithelial or mesenchymal ) is replaced by another adult cell type
Metaplasia occurs as a result of persistent chronic irritation/ injury
If the injurious stimuli is removed, metaplasia can be reversed
Metaplastic epithelium is susceptible for malignant change
Mechanisms of Metaplasia
Results from reprogramming of stem cells that are known to exist in normal tissues or of undifferentiated mesenchymal cells present in connective tissue
In a metaplastic change , these precursor cells differentiate along a new pathway
The differentiation of stem cells to particular lineage is brought by signal generated by cytokines, growth factors and extracellular matrix components in the cell environment
These external stimuli promote the expression of genes that drive the cells towards a specific differentiation pathway.
Examples
Columnar to squamous
In habitual smokers , the normal ciliated columnar epithelial cells of trachea & bronchi are replaced by squamous epithelium
Endocervical columnar epithelium to squamous epithelium
Squamous to columnar –
Barretts oesophagus – Squamous epithelium is replaced by mucin secreting columnar cells
Intestinal metaplasia – Lining epithelium of gastric glands is replaced by intestinal type of epithelium with Goblet cells
Connective tissue metaplasia is the formation of cartilage, bone or adipose tissue in tissues that normally do not contain these elements
E.g. Myositis ossificans
Reference
Vinay kumar, Abul K.Abbas, Nelson Fausto, Jon C. Aster. Robbins and Cotran Pathologic basis of disease. 8th edition.