Hypertrophy Hypertrophy is the opposite of atrophy. Hypertrophy describes an increase in cell mass/size. Again, this process can be either physiological or pathological and can be reversible. A normal process such as exercise increases the metabolic demand on muscle tissue and causes a subsequent increase in tissue mass. Pathological conditions that increase the workload of a particular tissue or organ cause hypertrophy in that tissue in an attempt to for deficiencies in another area of the body. For example, the left ventricle of the heart becomes hypertrophic in an attempt to compensate for a leaky heart valve. Hyperplasia Hyperplasia is an increase in the number of cells in a tissue or organ. There is no change in the size or function of the cells that make up a hyperplasic tissue. Hyperplasia can be a normal physiological phenomenon when it is associated with normal growth or repair of an organism but can also be pathological when there is an uncontrolled increase in the number of cells in a tissue. Cell types that normally undergo , such as the epithelial cells of the skin, urinary track and gastrointestinal tract tend to become hyperplasic in response to stress or injury. The mechanism controlling hyperplasia involves over expression of the gene controlling proliferation. Hyperplasia may be reversible if the injurious agent is eliminated. Metaplasia Metaplasia describes a change from one type of cell to another. Often with a change in cell type there is some change in function. Chronic and inflammation is the most common cause of metaplasia. The irritated cells become a type of cell that can tolerate the irritation better. During a metaplastic change, the undifferentiated stem cells of the tissue undergo a shift that results in the differentiation of a cell type that is more able to withstand a particular stress. Metaplasia is reversible once the inflammation and irritation are removed. An example of metaplasia can be seen in the epithelial lining of the respiratory tract of smokers. In a non-smoker the airways are lined with ciliated columnar epithelium while the airways of smokers are made of non-ciliated squamous cells. Dysplasia Dysplasia refers to a loss of cell or tissue organization. In dysplastic tissue the cells are not uniform in morphology or function. Many of the cells in dysplastic tissue are immature cells while the number of functional, mature cells is decreased. Dysplasia is considered a precursor to , however when the causative pathogen is removed there is potential for the affected tissue to return to normal. Anaplasia Anaplasia is the loss of cell differentiation. Anaplastic cells have no discernable functional characteristics. The morphology of the cells in anaplastic tissue no longer resembles normal cells of any type. Anaplastic cells tend to have enormous nuclei with abnormally bunched chromatin. Anaplastic cells tend to undergo frequent mitosis with abnormalities in that process. Anaplasia is always considered as anaplasia is a cancerous change. Anaplasia is irreversible.
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