A biological tissue is a collection of cells that work together to perform a specific function. Animals and plants have tissues that allow them to carry out the various processes necessary to sustain them. In animals, for example, four types of tissue—epithelial, connective, nervous, and muscle—combine to make up all the organs of the body. Two or more tissues often combine and function together as an organ, such as a heart or lung.

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Tissues and organs have distinctive shapes that reflect how they work and what they do. In the same way, the different cell types that make up these organs have distinctive shapes based on what they do in the organ. In animals, the shape of cells is determined and maintained by structural protein networks in the cytoplasm called the cytoskeleton (section 10.2). The shape and structural integrity of tissues and organs depend on the ability of cells to connect to one another. In turn, the connection of cells to one another depends on structures called cell junctions (section 10.3). Equally important to a strong, properly shaped tissue or organ is the ability of cells to adhere to a meshwork of proteins and polysaccharides outside the cell called the extracellular matrix (section 10.4).