Describe metaplasia of epithelium, which can occur in the respiratory tract (RT) and give two other examples of where metaplasia can occur in the body.

Metaplasia is a reversible change in which one differentiated cell type (epithelial or mesenchymal) is replaced by another cell type. It is often a response to chronic irritation and inflammation, and allows the tissue to better withstand the stress.

In the respiratory tract, metaplasia often occurs in response to irritants such as cigarette smoke. The normal columnar ciliated epithelial cells of the bronchial lining, which are sensitive to these irritants, may be replaced by stratified squamous epithelial cells, which are more capable of enduring the harmful effects of smoke and other irritants. However, while these squamous cells can protect the bronchial lining, they do not have the same functionality as the original columnar ciliated cells, such as moving mucus and debris out of the respiratory tract. This can lead to complications such as chronic bronchitis.

Apart from the respiratory tract, metaplasia can also occur in other parts of the body.

1. Esophagus: In a condition known as Barrett's esophagus, chronic exposure to acid reflux can lead to the normal stratified squamous epithelium of the esophagus being replaced by columnar epithelium, similar to that found in the stomach. This is the body's attempt to protect the esophageal lining from the damaging effects of the acid.

2. Cervix: The cervix is another common site for metaplasia. The normal columnar epithelial cells of the endocervix can be replaced by squamous epithelial cells, a condition known as cervical squamous metaplasia. This is often a response to chronic inflammation, such as that caused by sexually transmitted infections.

It's important to note that while metaplasia is a benign process, if the underlying cause of the inflammation or irritation isn't removed, it can progress to dysplasia, which is a pre-cancerous condition.