Do you remember your first day in elementary school? Everything and everyone was new and nothing was impossible.
But as you went through your education, you got further and further differentiated from your original classmates. That’s analogous to what happens to a cell that undergoes cellular differentiation.
An undifferentiated stem cell can become pretty much any tissue, influenced by both genes and the environment. Now sometimes, environmental stresses can alter that developmental path.
In metaplasia what happens is that a mature, differentiated cell type is replaced by another mature, differentiated cell type.
Often, this happens because there’s an environmental stressor, that the new cell type is better suited to handle. One example, is switching from breathing clean air to inhaling tobacco smoke each day.
Our airways are lined with columnar respiratory epithelial cells, which generally work well with air breathing, but not so well when faced with an irritant, such as tobacco smoke.
In response to the toxins in the smoke, already differentiated, mature columnar respiratory epithelial cells are replaced by stem cells undergoing differentiation into sandbag-shaped squamous epithelial cells, which become stratified - meaning that they form layers on top of another.
This replacement of already differentiated, mature cells into another type of cell is known as metaplasia.
Another example is our esophagus, which is lined with a nonkeratinizing squamous epithelium. These cells are adapted to withstand the passage of food going down to our stomach.
However, in case of gastroesophageal reflux disease, acid from the stomach makes its way up into the esophagus on a regular basis. Esophagus cells are not well-suited for chronic contact with acid and can get damaged.
Normally, when there’s occasional damage, stem cells differentiate into new squamous epithelial cells to replace the damaged ones.
But when there’s regular exposure to acid, stem cells begin to adapt by differentiating into nonciliated, mucin producing columnar epithelial cells.
These cells are far better suited to withstand the acid - after all, they’re the same types of cells that are found in the small intestine. This is an example of metaplasia, and the condition is known as Barrett’s esophagus.
Now, metaplasia is technically reversible - so if the gastroesophageal reflux disease is treated, stem cells will begin to divide into regular esophagus epithelial cells again. On the other hand, if the problem persists, the cells can become dysplastic.