When cells are stressed, they can undergo a number of changes. It is part of normal adaptation. Cell adaptive changes are atrophy, hypertrophy, hyperplasia, and dysplasia. Atrophy is a reversible reduction in the size of the cell, use it or lose it. That idiom defines well the idea of atrophy. If your teeth work out for several days, it's enough to start muscular atrophy. Disuse, insufficient blood flow, malnutrition, denervation, or reduced endocrine stimulation causes atrophy. Hypertrophy is an increase in the size of a cell due to increased workload. It can be pathologic or normal reaction. For example, hypertrophy of a heart can be a disease, but a muscular hypertrophy resulted from vigorous exercise is a normal adaptive reaction. Hyperplasia, an increase in the number of cells, is caused by increased workload, hormonal stimulation, or decreased tissue, for example, breast enlargement and hyperplasia during pregnancy. Metaplasia is the replacement of one adult cell with another adult cell that can better endure the change or stress. It's usually response to chronic inflammation or irritation. For example, Barrett's esophagus. Squamous epithelium is changed by columnar epithelium in the lower part of esophagus in response to constant irritation of esophagus by gastric acids. Dysplasia is dearranged cell growth of specific tissue results in abnormal size, shape, and appearance. Dysplastic cell changes can be preceded carcinose changes. For example, cervical dysplasia refers to abnormal cells on the surface of the cervix.