Dysplasia versus neoplasia

Dysplasia is disordered growth of the epithelium, the sheet of cells that covers or lines a surface such as the cervix, the airways or the gut. Its cells vary in size and shape, lose the neat, regular arrangement of the layers (their normal orientation) and show frequent mitotic figures (cells caught in the middle of dividing). A persistent stimulus (the trigger) is usually behind dysplasia, for example HPV (human papillomavirus) infection of the cervix.

What separates dysplasia from neoplasia (autonomous new growth, which may be benign or malignant) is reversibility. Dysplasia can regress if the stimulus is removed, whereas neoplasia carries on after the stimulus has gone. Variation in cell size and shape and frequent mitotic figures do not separate the two, because malignant neoplasms can show them too.

Reversible does not mean harmless. The abnormal cells stay above the basement membrane, the thin supporting layer beneath the epithelium, so the change is not yet invasive cancer, which would mean cells breaking through that layer. If the stimulus persists, dysplasia can progress to carcinoma in situ, a malignant change still confined to the epithelium. That is why dysplasia is described as sitting between normal tissue and cancer.

Cervical screening is built on this risk: samples are tested for high-risk HPV, and HPV-positive samples are then examined for abnormal cells, because persistent dysplasia can progress to cervical cancer.

Compare it with the other cell changes. Hyperplasia means more cells that stay neatly organised, metaplasia means one cell type replacing another, and anaplasia is a marked lack of differentiation in malignant cells, which no longer resemble their tissue of origin.