Hyperplasia - Hyperplasia is an increase in the number of cells in an organ or tissue which occurs in response to a stimulus and which ceases when the stimulus is removed. A good example of this is the erythroid hyperplasia which accompanies various hemoglobinopathies. Alterations in the globin chain reduce oxygen delivery to peripheral tissues, causing increased erythropoietin secretion which stimulates proliferation of erythroid elements in the bone marrow. Metaplasia - Metaplasia is the replacement of one type of differentiated cell by another in response to a stimulus, usually a noxious stimulus. It can thus be seen as an adaptive response.
Epithelial surfaces are a common site of metaplasia.
A good example is the replacement of bronchial columnar epithelium by squamous epithelium in cigarette smokers. Metaplasia is often reversible, but restoration of the original cell type may take some time after removal of the stimulus. The features of dysplasia are increased numbers of cells with variable size and shape, large, darkly staining nuclei and reduced cytoplasm. Increased numbers of mitoses are seen, and the normal pattern of cell maturation is usually disrupted. Serial studies of cervical dysplasia suggest that untreated dysplasia frequently progresses to carcinoma.