Metaplasia

-plasia
Anaplasia – dedifferentiation
Hyperplasia – physiological proliferation Increase in number of cells
Neoplasia – abnormal proliferation
Dysplasia – maturation abnormality
Metaplasia – cell type conversion
Prosoplasia – cell type develops new function
Desmoplasia – connective tissue growth

Metaplasia
Classification and external resources
Micrograph of a gastro-esophageal junction with pancreatic acinar metaplasia. The esophageal mucosa (stratified squamous epithelium) is seen on the right. The gastric mucosa (simple columnar epithelium) is seen on the left. The metaplastic epithelial is at the junction (center of image) and has an intensely eosinophilic (bright pink) cytoplasm. H&E stain.
MeSH	D008679

Metaplasia (Greek: "change in form") is the reversible replacement of one differentiated cell type with another mature differentiated cell type. The change from one type of cell to another may generally be a part of normal maturation process or caused by some sort of abnormal stimulus. In simplistic terms, it is as if the original cells are not robust enough to withstand the new environment, and so they change into another type more suited to the new environment. If the stimulus that caused metaplasia is removed or ceases, tissues return to their normal pattern of differentiation. Metaplasia is not synonymous with dysplasia and is not directly considered carcinogenic. It is also contrasted with heteroplasia, which is the abnormal growth of cytologic and histologic elements without a stimulus.

Causes

When cells are faced with physiological or pathological stresses, they respond by adapting in any of several ways, one of which is metaplasia. It is a benign (i.e. non-cancerous) change that occurs as a response to change of milieu (physiological metaplasia) or chronic physical or chemical irritation (pathological metaplasia). One example of pathological irritation is cigarette smoke that causes the mucus-secreting ciliated pseudostratified columnar respiratory epithelial cells that line the airways to be replaced by stratified squamous epithelium, or a stone in the bile duct that causes the replacement of the secretory columnar epithelium with stratified squamous epithelium (Squamous metaplasia). Metaplasia is an adaptation that replaces one type of epithelium with another that is more likely to be able to withstand the stresses it is faced with. It is also accompanied by a loss of endothelial function, and in some instances considered undesirable; this undesirability is underscored by the propensity for metaplastic regions to eventually turn cancerous if the irritant is not eliminated.

Specialized epithelial cells are already differentiated, and cannot simply transform their morphologies to change from one cell type to another. Metaplasia, then, does not occur as a result of any change in the pre-existing epithelial cells but rather as a result of reprogrammed stem cells present in the organ's connective tissue that are nudged along a different pathway of differentiation by cytokines, growth factors and other substances in the cell's environment. In a nutshell, metaplasia occurs by stem cells that reprogram differentiation of cells rather than by transdifferentiation.

Significance in disease

Normal physiological metaplasia - such as that of the ectocervix is highly desirable.

The medical significance of metaplasia is that in some sites where pathological irritation is present cells may progress from metaplasia, to develop dysplasia, and then malignant neoplasia (cancer). Thus, at sites where abnormal metaplasia is detected, efforts are made to remove the causative irritant, thereby decreasing the risk of progression to malignancy. The metaplastic area must be carefully monitored to ensure that dysplastic change does not begin to occur. A progression to significant dysplasia indicates that the area could need removal to prevent the development of cancer.

Examples

A micrograph of Barrett's esophagus.

Barrett's esophagus is an abnormal change in the cells of the lower esophagus, thought to be caused by damage from chronic stomach acid exposure.

The following table lists some common tissues susceptible to metaplasia, and the stimuli that can cause the change:

Tissue	Normal	Metaplasia	Stimulus
Airways	Pseudostratified columnar epithelium	Squamous epithelium	Cigarette smoke
Urinary bladder	Transitional epithelium	Squamous epithelium	Bladder stone
Esophagus	Squamous epithelium	Columnar epithelium	Gastro-esophageal reflux (Barrett's Esophagus)
Cervix	Glandular epithelium	Squamous epithelium	Low pH of vagina

Notes

• The AMA Home Medical Encyclopedia, Random House, p.683
• Robbins and Cotran - Pathologic Basis of Disease, 7th Edition, Saunders, p. 10