sell adaptations are reversible changes of cells in their size number phenotype metabolic activity or functions in response to changes in their environment such adaptations may take several distinct forms they are hypertrophy hyperplasia atrophy and metaplasia let's discuss one by one in detail first let's discuss about hypertrophy it refers to an increase in the size of cells resulting in an increase in the size of the organ this image shows a normal myocardial cell along with normal ventricular walls when the cell undergoes hypertrophy the size of the myocardial cell increases consequently the thickness of the ventricular wall also increases hypertrophy Dorgan has no new cells just larger cells increased size of the cells is due to the synthesis of more structural components a cell undergoes hypertrophy when there is an increased demand for the function of that particular cell or due to the stimulation by hormones or growth factors cells which are capable of division may respond to these stimuli by undergoing both hypertrophy and hyperplasia however non-dividing cells like myocardial cells undergo hypertrophy only but in many organs hypertrophy and hyperplasia coexist and contribute to the increased size of the organ hypertrophy could be either physiological or pathological and muscles the most common stimulus for hypertrophy is increased workload for example the bulging of muscles and bodybuilders is due to the increased workload which causes enlargement of individual muscle fibers in cardiac hypertrophy it is due to chronic hemodynamic overload generated by long-standing hypertension or valve incompetence in both occasions the muscle cells synthesize more structural proteins and this will result in increased number of myofilaments ultimately the muscle size and force of contractility of the muscle increases smooth muscle hypertrophy occurs when there is an obstruction to the outflow of the contents of a hollow muscular organ for example hypertrophy of the stomach occurs in polaric stenosis hypertrophy of intestine occurs in Crohn's disease and bladder hypertrophy occurs in conditions like prostatic hyperplasia sometimes hypertrophy is caused by hormonal stimulation such as massive growth of the uterus during pregnancy the cell undergoes hypertrophy due to the increased production of cellular proteins hypertrophy can be induced by the linked actions of mechanical sensors which are activated by increased workload growth factors including transforming growth factor beta insulin-like growth factor and fibroblast growth factor and vasoactive agents likened offal in one and angiotensin ii these factors activate two major signal transduction pathways phospho in acetyl 3 kinase Akt pathway which is more prominent in physiological hypertrophy and g-protein coupled receptor pathway which is more prominent in pathological hypertrophy signal transduction along these pathways causes expression of certain genes and synthesis of more structural proteins hypertrophy may also be associated with a switch of contractile proteins from adult forms into fetal or neonatal forms for an example during muscle hypertrophy the alpha isoform of myosin heavy chain is replaced by the beta isoform which is a slower more energetically economical contraction in addition some genes that are expressed only during early development re-expressed in hypertrophic cells for an example gene for at real natural erotic factor it is a peptide hormone that increases the salt secretion from the kidney by doing this it increases the urine output and helps to reduce blood volume and hemodynamic overload although hypertrophy usually refers to an increase in size of cells or tissues sometimes a sub cellular organelle may undergo selective hypertrophy for instance individuals who treated with drugs like barbiturates show hypertrophy of the smooth endoplasmic reticulum into pata sites which is an adaptive response that increases the amount of enzymes available to detoxify the drug hyperplasia is an increase in the number of cells in an organ or tissue usually resulting in an increase in the size of the organ or tissue however hyperplasia does not always increase the size of an organ or tissue especially in compensator e hyperplasia which we will discuss later in this video as we discussed earlier frequently both hypertrophy and hyperplasia occur together a tissue undergoes hyperplasia only if the cell population is capable of dividing hyperplasia could be either physiological or pathological first let's discuss about physiological hyperplasia it can be divided into hormonal hyperplasia which increases the functional capacity of an organ and compensate or hyperplasia which increases the tissue mass after damage or partial resection hyperplasia of the breast tissue in pregnancy is a good example for hormonal hyperplasia regeneration of the liver after partial hepatectomy is an example for compensator hyperplasia most forms of pathological hyperplasia are caused by excess secretion of hormones and growth factors endometrial hyperplasia is an example for pathological hyperplasia it occurs due to a disturbance between estrogen and progesterone balance and it is frequently associated with abnormal menstrual bleeding benign prostatic hyperplasia is another example in this case hyperplasia is due to excess secretion of androgens although these forms of hyperplasia are abnormal the process remains controlled because there are no mutations in genes that regulate cell division therefore hyperplasia is distinct from cancer however uncontrolled hyperplasia may leave the individual at risk of developing cancer atrophy is reduced size of an organ or tissue resulting from a decrease in cell size and number it can be physiological or pathological physiological atrophy is common during early development and the fetus thyroglossal duct node occurred an branchial clefts undergo atrophy and disappear during infancy ductus arteriosus and umbilical vessels undergo atrophy and in later life thymus gland uterus after parturition and breast tissue after lactation undergo physiological atrophy pathological atrophy depends upon the basic cause and can be generalized or localized examples for generalized atrophy include the following starvation atrophy is characterized by wasting of skeletal muscle also known as cachexia it is seen in conditions like profound protein calorie malnutrition and in chronic inflammatory diseases and cancer in case of malnutrition body uses skeletal muscle as the source of energy leading to atrophy of muscle during inflammation and cancer there is an overproduction of cytokines like tumor necrosis factor high concentration of tumor necrosis factor causes appetite suppression and lipid depletion resulting in muscle atrophy senile atrophy is the type of atrophy which occurs in old-age the heart muscles undergoes atrophy and it becomes brown in color this is known as Brown atrophy of the heart in addition skeletal muscle also undergo atrophy in this condition loss of endocrine stimulation can also cause generalized atrophy for instance hypopituitarism leads to atrophy of thyroid glands adrenal cortex and doníts examples for localised atrophy include the following the scam akat Rafi occurs due to the slowly developing arterial occlusive disease which decreases the blood supply to an organ brain atrophy following atherosclerosis of cerebral arteries is an example disuse atrophy occurs when a fractured bone is immobilized in a plaster cast or when a patient is restricted to complete bed rest skeletal muscle atrophy is prominent in these people and the initial decrease in cell size is reversible however with prolonged disuse the number of skeletal muscle fibers decreases due to apoptosis of muscle fibers in addition prolonged disuse is also associated with increased bone resorption leading to us do Prosis of disuse the normal metabolism and function of skeletal muscle are depend on its nerve supply damage to the nerves leads to atrophy of the muscle fibers supplied by those nerves this type of atrophy is called denervation atrophy in conditions like carpal tunnel syndrome compression of the median nerve within the carpal tunnel causes loss of innervation to the thenor muscles so as a consequence of carpal tunnel syndrome thenar muscles undergo atrophy pressure atrophy occurs when a tissue is compressed by a solid mass such as a tumor atrophy of renal tissue in renal carcinoma is an example loss of endocrine stimulation also leads to atrophy of tissues many hormone responsive organs like breasts and reproductive organs are dependent on endocrine stimulation for their normal metabolism and functions loss of hormonal stimulation leads to atrophy of these tissues now let's discuss about the mechanism of atrophy it results from increased protein degradation and decreased protein synthesis in cells protein synthesis decreases because of the reduced metabolic activity the degradation of cellular proteins occurs by the ubiquitin proteasome pathway in addition increased Auto Fei Jie may also contribute to degrade the intracellular proteins autophagy is a condition where the starved cells eat their own components in order to survive this image shows the steps in autophagy in the step a the organelle which undergoes autophagy is bound by a membrane derived from the endoplasmic reticulum forming an early auto fat body these are also known as Auto fat jerk vacuoles in the step see lysosomes fuse with this vacuole and digest the organelle and next steps some of cellular components within these vacuoles are resistant to digestion by the lysosomal enzymes these will persist within the atrophic cells as residual bodies the major component in these residual bodies are lapis and granules when present and sufficient amounts these granules give a brown color to the atrophic tissue so called brown atrophy of the tissue metaplasia is a special type of cell adaptation in which one differentiated cell type is replaced by similarly differentiated another cell type metaplasia commonly occurs in epithelial and connective tissue first let's discuss about epithelial metaplasia transformation of simple columnar epithelium into stratified squamous epithelium is the communist form of epithelial metaplasia this is also known as squamous metaplasia these images are from the Robbins textbook of pathology showing squamous metaplasia squamous metaplasia commonly takes place in the respiratory tract in response to chronic irritation like cigarette smoke and vitamin A deficiency in addition stones in the excretory ducts of salivary glands pancreas and bile ducts also induced the metaplastic transformation of epithelial cells in all these instances the more rough stratified squamous epithelium is able to withstand the stress in Barrett's esophagus esophageal squamous epithelium is replaced by columnar cells due to the reflux gastric acid although metaplasia enables the cells to adapt to the changes in their environment there are some disadvantages as well for example in squamous metaplasia of the respiratory tract even though the epithelial lining becomes tough the essential functions of respiratory epithelium like mucus secretion and ciliary functions are lost and if the stimuli that predisposed to metaplasia persist for a long time this may induce malignant transformation of metaplastic cells in connective tissue metaplasia there is formation of cartilage bone and add up his tissue and tissues that normally do not contain these elements like bone formation and muscle mechanism of metaplasia is different from other types of cell adaptations it does not result from change of the phenotype of an already differentiated cell instead it is a result of reprogramming of stem cells along a new differentiation pathway extracellular signals mediated by cytokines growth factors and components of the extracellular matrix induce gene transcription of stem cells and differentiation into another type of cells you