One differentiated epithelium replaced by another better suited to the stress
Mnemonic
Metaplasia is a reversible change of one differentiated cell type into another, and the classic examples each follow a chronic irritant:
| Site | Change | Cause |
|---|---|---|
| Oesophagus | Squamous to columnar (Barrett’s) | Acid reflux |
| Bronchus | Ciliated columnar to squamous | Smoking |
| Bladder | Urothelium to squamous | Chronic irritation, schistosomiasis |
| Cervix | Columnar to squamous | Normal, at the transformation zone |
| Stomach | Gastric to intestinal | Helicobacter, chronic gastritis |
“Barrett’s goes to columnar; the bronchus goes to squamous.” They run in opposite directions, which is the point most often confused.
The sequence to remember is metaplasia to dysplasia to carcinoma: metaplasia itself is adaptive and reversible, but it carries an increased malignant risk because the epithelium is under continuing stress, which is why Barrett’s oesophagus is surveilled.
Cervical metaplasia at the transformation zone is physiological, and it is precisely the zone sampled by cervical screening.
Expansion
| Site | Change | Stimulus | Risk |
|---|---|---|---|
| Oesophagus | Squamous to columnar with goblet cells (Barrett) | Acid reflux | Adenocarcinoma |
| Bronchus | Pseudostratified ciliated to squamous | Smoking | Squamous carcinoma; also loses mucociliary clearance |
| Cervix | Columnar to squamous at the transformation zone | Acid vaginal pH | The site where HPV drives dysplasia and carcinoma |
| Bladder | Urothelium to squamous | Chronic stones, catheter, schistosomiasis | Squamous carcinoma |
| Stomach | Gastric to intestinal | H pylori, chronic gastritis | Gastric adenocarcinoma |
| Connective tissue | Fibrous to bone (myositis ossificans) | Trauma | Benign |
Metaplasia is adaptive: the replacement epithelium is better suited to the new stress. Bronchial squamous metaplasia resists smoke better than ciliated epithelium, but the price is loss of mucociliary clearance.
It is reversible if the stimulus is removed, which is the argument for smoking cessation and for acid suppression.
The sequence that matters clinically is metaplasia, then dysplasia, then carcinoma in situ, then invasive carcinoma. Surveillance programmes target dysplasia, not metaplasia, since the annual progression rate from uncomplicated metaplasia is low.
The cervical transformation zone is the single most important example, being the site sampled by cervical screening.