Metaplasia

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Metaplasia (change in form, Greek) is the replacement of one differentiated cell type with another mature differentiated cell type. The change from one type of cell to another is generally 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 considered carcinogenesis. It is also contrasted with heteroplasia, which is the abnormal growth of cytologic and histologic elements without a stimulus.

1 Causes
2 Therapy
3 Examples
4 Notes

[edit] Causes

There are three main theories as to how metaplasia occurs:

Differentiated cells may change their pattern of differentiation in a process termed transdifferentiation.

Local stem cells, which are believed to generate new cells in most tissues, become reprogrammed by the changed environment to form a new form of cell.

Circulating stem cells in the blood, which are recognised to colonise tissues, may start to grow in damaged tissues and produce a new form of cell.

[edit] Therapy

The medical significance of metaplasia is that in some sites cells may progress from metaplasia, to develop dysplasia, and then neoplasia (cancer). Thus, at sites where 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.

[edit] Examples

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.

Metaplasia of the cervix, which occurs in cervical erosion, can be detected by a cervical smear test. The normal endocervical columnar epithelium is replaced by a squamous epithelium in an area termed the transformation zone. This is a normal physiological event that takes place around puberty. The stimulus is believed to be a change in the vaginal environment, which becomes acidic.

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

Tissue	Normal	Metaplasia	Stimulus
Airways	Columnar epithelium	Squamous epithelium	Cigarette smoke
Urinary bladder	Transitional epithelium	Squamous epithelium	Bladder stone
Oesophagus	Squamous epithelium	Columnar epithelium	Gastro-esophageal reflux