Permanent tissues are tissues that are not capable of further division. Such includes Neural, skeletal and muscles tissues. Stable tissues are tissues that proliferate in response to injury i.e. in GO stage. Such includes hepatocytes, lymphocytes, smooth muscles, etc. Labile tissues constantly reproduce and regenerate. Such includes skin, mucosa and bone marrow
Hyperplasia is defined as increase in number of cells but not cell size and occurs only when the cells are capable of dividing further (i.e., in stable and labile tissues). Most common one is seen during the proliferative phase of the menstrual cycle; the uterine endometrium undergoes hyperplasia in response to a hormonal trigger leading to wound healing. Secondly, changes in female breast at puberty, pregnancy and pregnant uterus. Another example is where older men frequently develop benign prostatic hyperplasia: enlargement of the prostate gland due to hyperplasia triggered by alteration of hormonal signaling. Also, to note, if a patient moves to a high altitude, their bone marrow may respond to lowered oxygen levels by manufacturing additional erythrocytes (red blood cells) to carry oxygen. Regeneration of liver after partial resection is also an example of physiologic hyperplasia.
Hypertrophy on the other hand is defined as an increase in size of cell leading to increased organ size and occurs when there is increased demand on permanent tissues. For example, body building creates an increase in muscle mass by increasing the size of each individual myocyte via hypertrophy. Secondly, if a patient is obese, they may develop cardiac enlargement because their cardiac myocytes individually enlarge in response to an increased workload.
Both hypertrophy and hyperplasia work together to meet an increased functional demand on a group of cells or an organ. One example of this is the increase in size of the uterus during pregnancy, in which the uterine muscle undergoes hypertrophy and the endometrium proliferates.
Figure showing hypertrophy of uterus during pregnancy
Metaplasia is a reversible change in where adult cell types (epithelial or mesenchymal) is replaced by other adult cell types. The injured cells can transform from their original cell type to another cell type that is mature to help withstand certain environmental changes. It is due to re-programming of stem cells. It can be epithelial or mesenchymal. Epithelial can also be squamous or columnar. For example, when faced with chronic irritant like cigarette smoke, the epithelial lining of the respiratory tract loses the soft linings and change to a hardier form made up of multiple horizontal layers of cells (stratified squamous epithelium). Barrett’s esophagus, where gastric reflux results in columnar epithelium replacing squamous epithelium in the esophagus (dysplasia and adenocarcinoma may occur) that can lead to esophagus cancer. Vitamin A deficiency (in respiratory epithelium) or excess also leads to metaplasia
Atrophy is the shrinkage in cell size by loss of cellular substance. Entire organ can become atrophic with the involvement of a sufficient number of cells. Causes of atrophy include decreased workload, pressure, diminished blood supply or nutrition, loss of endocrine stimulation, and aging. Mechanism of atrophy consists of combination of decreased protein synthesis & increased protein degradation by Ubiquitin proteasome pathway. Atrophic cells usually contain increased autophagic vacuoles with persistent residual bodies such as lipofuscin.
Dysplasia is disordered cellular development. It affects only epithelial cells. Most common examples are uterine cervix and respiratory tract.
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