Expansion
Hypertrophy, hyperplasia, atrophy, metaplasia and dysplasia
Expansion
- Hypertrophy: increase in cell size. Cardiac muscle in hypertension, skeletal muscle in training. The only option for cells that cannot divide
- Hyperplasia: increase in cell number. Endometrium under oestrogen, prostate with age. Requires a dividing population
- Atrophy: decrease in size or number. Disuse, denervation, ischaemia, loss of hormonal or nutritional support, ageing
- Metaplasia: replacement of one differentiated cell type with another better suited to the stress. Barrett oesophagus (squamous to columnar with acid reflux), bronchial columnar to squamous with smoking
- Dysplasia: disordered growth with cellular atypia. Not a true adaptation, and it is premalignant
The sequence matters clinically: chronic irritation drives metaplasia, which if sustained drives dysplasia, which may progress to carcinoma in situ and then invasive carcinoma. Barrett oesophagus to adenocarcinoma is the classic example, and it is why surveillance targets dysplasia rather than metaplasia.
Metaplasia is reversible if the stimulus is removed; dysplasia may be, but high-grade dysplasia usually is not, which is the rationale for treating it.