Why metaplasia is a double sword?

Metaplasia is a double-edged sword because in respiratory tract respiratory epithelium is changed into squamous epithelium to protect against noxious stimulant but lost its ability mucus secretion and cilliary action which is important for protection against infection.
Moreover, the influences that predispose to metaplasia, if persistent, can initiate malignant transformation in metaplastic epithelium.

What is metaplasia? Short note.

Defenition:

Metaplasia is a reversible change in which one differentiated cell type (epithelial or mesenchymal) is replaced by another cell type.

Mechanism of metaplasia:

Metaplasia does not result from a change in the phenotype of an already differentiated cell type; instead it is the result of a 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 a particular lineage is brought about by signals generated by cytokines, growth factors, and extracellular matrix components in the cells’ environment.

Example:

1. Columnar to squamous: Occurs in the respiratory tract in response to chronic irritation. In the habitual cigarette smoker, the normal ciliated columnar epithelial cells of the trachea and bronchi are often replaced by stratified squamous epithelial cells.

2. Squamous to columnar: Occur, as in Barrett esophagus, in which the esophageal squamous epithelium is replaced by intestinal-like columnar cells under the influence of refluxed gastric acid.

3. Connective tissue metaplasia: The formation of cartilage, bone, or adipose tissue in tissues that normally do not contain these elements.

How hyperplasia differs from hypertrophy?

1. Hypertrophy is increase in the size of cell which result in increase in organ size. whereas hyperplasia is increase in number of cell due to a stimulus.
2. Hypertrophy is the result of increase production of cellular protein, in hyperplasia there is active cell division.
3. Hypertrophy can take place in any cell type whereas hyperplasia take place only in cells capable of division.