Cellular Stress occurs when a cell is put in an inhospitable environment or required to do something it can’t currently do. Overtime cells put under stress will either adapt to the new situation or die. Cells will also undergo changes if the amount of stress placed on them decreases or the type of stress changes.
Hypertrophy is an increase in the size of individual cells. Examples would include muscles getting bigger due to weight training or the heart wall thickening due to hypertension (and an increased work needed by the heart).
Hyperplasia is an increase in the number of cells in a tissue via mitosis. CNS and muscle (including heart tissue) cannot undergo hyperplasia as they are permanent tissues “stuck” in the G0 phase. These tissues can only undergo hypertrophy to meet increased demand. An example would be hyperplasia of the adrenal cortex in Cushing’s Disease in order to be able to produce more cortisol.
Metaplasia is when one cell type is replaced with a different cell type that is better equipped to handle new kind of stress placed on the tissue. This is a type of adaption is reversible. It can be physiologic or pathologic, but is not cancerous. However, prolonged Metaplasia can turn into Dysplasia which can become cancerous. An example would be Barrett’s Esophogus. GERD leads to stomach acid in the esophagus and as a result stratified squamous esophageal cells are replaced by Goblet cells which can better handle stomach acid. Another example would be the replacement of pseudostratified columnar cells in the respiratory system with squamous epithelium as a result of cigarettes.
Atrophy is a decrease in cell size or decrease in cell number (apoptosis) in response to a decreased level of stress placed on the cells