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3: Tissue Level of Organization
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3.1: Introduction to the Tissue Level of Organization
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The body contains at least 200 distinct cell types. These cells contain essentially the same internal structures yet they vary enormously in shape and function. The different types of cells are not randomly distributed throughout the body; rather they occur in organized layers, a level of organization referred to as tissue.
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3.2: Types of Tissues
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The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions. From the evolutionary perspective, tissues appear in more complex organisms. For example, multicellular protists, ancient eukaryotes, do not have cells organized into tissues.
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3.3: Epithelial Tissue
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Epithelial tissues serve the main function of forming linings, or covering, for areas exposed to the outside world and lumens of organs. Depending on the cell shape and number of layers in an epithelial tissue, they can also serve to absorb and secrete materials.
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3.4: Connective Tissue
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Connective tissues provide the structural framework of the human body. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The characteristics of these three components will ultimately determine the classifications and functions of the different types of connective tissues.
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3.5: Muscle Tissue
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Muscle tissue is characterized by properties that allow movement. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth.
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3.6: Nervous Tissue
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Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia. Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information propagation.
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3.7: Tissue Injury and Aging
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Tissues of all types are vulnerable to injury and, inevitably, aging. In the former case, understanding how tissues respond to damage can guide strategies to aid repair. In the latter case, understanding the impact of aging can help in the search for ways to diminish its effects.