Cellular Adaptations to Stress PDF

CELLULAR RESPONSES TO STRESS AND NOXIOUS STIMULI  Cells are active participants in their environment, constantly adjusting their structure and function to accommodate changing demands and extracellular stresses  As cells encounter physiologic stresses or pathologic stimuli,...

CELLULAR RESPONSES TO STRESS AND NOXIOUS STIMULI  Cells are active participants in their environment, constantly adjusting their structure and function to accommodate changing demands and extracellular stresses  As cells encounter physiologic stresses or pathologic stimuli, they can undergo adaptation, achieving a new steady state and preserving viability and function The principal adaptive responses are hypertrophy, hyperplasia, atrophy, and metaplasia. If the adaptive capability is exceeded or if the external stress is inherently harmful, cell injury develops  CELLULAR ADAPTATIONS TO STRESS Adaptations are reversible changes in the number, size, phenotype, metabolic activity, or functions of cells in response to changes in their environment. Physiologic adaptations usually represent responses of cells to normal stimulation by hormones or endogenous chemical mediators (e.g., the hormone-induced enlargement of the breast and uterus during pregnancy) Pathologic adaptations are responses to stress that allow cells to modulate their structure and function and thus escape injury. Such adaptations can take several distinct forms: Hypertrophy:  Is an increase in the size of cells resulting in increase in the size of the organ  There are no new cells, just bigger cells containing increased amounts of structural proteins and organelles  Hypertrophy can be physiologic or pathologic and is caused either by increased functional demand or by growth factor or hormonal stimulation. Hypertrophy:  The massive physiologic enlargement of the uterus during pregnancy occurs as a consequence of estrogen- stimulated smooth muscle hypertrophy  An example of pathologic cellular hypertrophy is the cardiac enlargement that occurs with hypertension or aortic valve disease Marked thickening of the left ventricle in the patient with systemic hypertension. Hyperplasia:  Is characterized by an increase in cell number because of proliferation of differentiated cells and replacement by tissue stem cells  Hyperplasia takes place if the tissue contains cell populations capable of replication; it may occur concurrently with hypertrophy  Hyperplasia can be physiologic or pathologic. In both situations, cellular proliferation is stimulated by growth factors that are produced by a variety of cell types  The two types of physiologic hyperplasia are: 1. Hormonal hyperplasia, exemplified by the proliferation of the glandular epithelium of the female breast at puberty and during pregnancy 2. Compensatory hyperplasia, in which residual tissue grows after removal or loss of part of an organ. For example, when part of a liver is resected This hyperplasia is physiologic with a normal menstrual cycle.  Most forms of pathologic hyperplasia are caused by excessive hormonal or growth factor stimulation  Example, after a normal menstrual period there is a burst of uterine epithelial proliferation that is normally tightly regulated by stimulation through pituitary hormones and ovarian estrogen and by inhibition through progesterone The prostate: increased in size, more than 70 gm (3-4gm). The pattern is not uniform, but nodular. Pathologic process that could interfere with emptying of the urinary bladder. The cells making up the glands are normal in appearance, but there are just too many of them. Atrophy: Shrinkage in the size of the cell by the loss of cell substance  When a sufficient number of cells are involved, the entire tissue or organ diminishes in size  Causes of atrophy include a decreased workload (e.g., immobilization of a limb to permit healing of a fracture)  Loss of innervation  Diminished blood supply  Inadequate nutrition  Loss of endocrine stimulation, and  Aging (senile atrophy) Cerebral atrophy in a patient with Alzheimer disease. The entire size of the brain is reduced (Gyri narrowing & Sulci widening) The testis at the right has undergone atrophy and is much smaller than the normal testis at the left. Some of these skeletal muscle fibers show atrophy, compared to normal fibers The cells have reduced in size or been lost from hypoxia. The pale brown-yellow pigment is lipochrome that has accumulated as the atrophic and dying cells undergo autophagocytosis. Metaplasia:  Is a change in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type  a cell type sensitive to a particular stress is replaced by another cell type better able to withstand the adverse environment  Epithelial metaplasia is exemplified by the squamous change that occurs in the respiratory epithelium of habitual cigarette smokers  The normal ciliated columnar epithelial cells of the trachea and bronchi are focally or widely replaced by stratified squamous epithelial cells. Normal Columnar epithelium (Right); Squamous epithelium (Left). Metaplasia is not a normal physiologic process and may be the first step toward neoplasia. Metaplasia of the normal esophageal squamous mucosa has occurred here, with the appearance of gastric type columnar mucosa.