# Tissue --- A **tissue** is a level of organization in [[anatomy & physiology]] that is made up of similar [[cell|cells]] that function as a unit. The next level most complex level of organization is the [[organ]], which is made up of multiple types of tissues working together. The study of tissues is called **histology**. Membranes made out of tissue are [[tissue membrane]]. ## Types of Tissue There are four primary types of tissues based on their roles in the body, *epithelial*, *connective*, *muscle* and *neural* tissue. - [[Epithelial tissue]] is used to cover and protect different parts of the body, and even the body entirely in the form of our skin. It is highly cellular, meaning it's just jam-packed with cells that are all closely connected with one another. The cells come in a wide variety of shapes and configurations. - [[Connective tissue]] is used for connecting parts of the body together (no surprise) and for support as well. It is characterized by being composed mostly of inorganic, non-cellular material (the [[extracellular matrix|ECM]]), and few living cells. - [[Muscle tissue]] is used for *movement*, not just of the body as a whole (in the form of *skeletal muscle*), but also in the form *smooth muscle* found in the internal organs that have to move to function, such as the intestines that have to move material along. The only other type of muscle is the highly specialized muscle of the heart, the *cardiac muscle*. - [[Neural tissue]] is is for *perception*, *control*, and *communication* in the body. Neurons are the workhorse of the nervous tissue, and they are supported by the glial cells. - Special classifications - **Parenchymal tissue** is a subset of mostly epithelial cells in organs that does the specialized work of that organ. It is opposed to the **stroma** of an organ, which is the connective tissue that makes up the structural elements of the organ. It's also frequently used when discussing different [[plants|plant]] tissues. Examples of human parenchymal cells/tissues are: - Hepatocytes in the liver - Nephrons in the kidneys - Etc. ## Changes in Tissue Tissues can change for many reasons, and it's called different things depending on why the change is occurring, or what exactly is happening to it. - **Hypertrophy**: Increase in size of existing cells of a tissue - **Hyperplasia**: Increase in number of cells of a tissue - **Atrophy**: Shrinkage of tissue by decrease in cell number or size - **Metaplasia**: Change of mature epithelium to a different form - May occur as epithelium adapts to environment: E.g., smokers experience metaplastic changes in trachea epithelium - **Dysplasia**: Abnormal tissue development - May be precancerous, or revert back to normal. E.g., cervical dysplasia due to exposure to human papillomavirus - **Neoplasia**: Tissue growth is out of control, i.e. **cancer** - **Benign:** localized growth and does not spread - **Malignant:** metastasizes, spreads and invades other tissues - [[Necrosis]]: Tissue death due to irreversible tissue damage ## Aging of Tissues All tissues change with aging Proper nutrition, good health, normal circulation, infrequent wounds—all promote normal functioning support, maintenance, replacement of cells and extracellular matrix. It's less efficient after middle age. The structure and chemical composition of many tissues are altered. For example: Epithelia thins -> old people get cold quickly, you loose a lot of subcutaneous fat also CT loses pliability and resiliency Collagen declines Bones become brittle Muscles atrophy ___