When cells are faced with physiological or pathological stresses, they respond by adapting in any of several ways, one of which is<unk> . It is a benign (i.e. non-cancerous) change that occurs as a response to change of milieu (physiological<unk>  or chronic physical or chemical irritation. One example of pathological irritation is cigarette smoke, which causes the mucus-secreting ciliated pseudostratified columnar respiratory epithelial cells that line the airways to be replaced by stratified squamous epithelium, or a stone in the bile duct that causes the replacement of the secretory columnar epithelium with stratified squamous epithelium (squamous<unk> .<unk>  is an adaptation that replaces one type of epithelium with another that is more likely to be able to withstand the stresses it is faced with. It is also accompanied by a loss of endothelial function, and in some instances considered undesirable; this undesirability is underscored by the propensity for metaplastic regions to eventually turn cancerous if the irritant is not eliminated. The cell of origin for many types of<unk> s are controversial or unknown. For example, there is evidence supporting several different hypotheses of origin in Barrett's esophagus. They include direct transdifferentiation of squamous cells to columnar cells, the stem cell changing from esophageal type to intestinal type, migration of gastric cardiac cells, and a population of resident embryonic cells present through adulthood.
Metaplasia
Hyperplasia
Dysplasia
Atrophy
Aplasia
Hypoplasia
Hypertrophy
Agenesis