To help counteract this loss, tissues maintain stem cells to serve as a reservoir of undifferentiated cells. In fact, many differentiated cells lose this ability. Neurons are not the only cells that lose their ability to divide as they mature. Mammalian epithelial cells also turn over regularly, typically every few days. However, glial cells in the brain continue to divide throughout a mammal's adult life. For example, in adult mammal brains, neurons rarely divide. Inside every tissue, cells are constantly replenishing themselves through the process of division, although the rate of turnover may vary widely between different cell types in the same tissue. Without cell division, long-term tissue survival would be impossible. Of course, external signals from neighboring cells or from the extracellular matrix are also important influences on the arrangement of cells in a tissue. This organization depends in part on polarity, or the orientation of particular cells in their place. The spatial organization of the cells that form a tissue is also central to the tissue's function and survival. Proper regulation of these rates is essential to tissue maintenance and repair. They also have different transcriptional programs and may well divide at different rates. However, the multiple cell types within a tissue don't just have different functions. Some of these cells absorb nutrients from the digestive contents, whereas others (called goblet cells) secrete a lubricating mucus that helps the contents travel smoothly. For example, the endothelial tissue that lines the human gastrointestinal tract consists of several cell types. The exact role of a tissue in an organism depends on what types of cells it contains. Within multicellular organisms, tissues are organized communities of cells that work together to carry out a specific function.
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