TISSUES AND HISTOLOGY
A tissue (tish′ ̄u ) is a group of cells with similar structure and func-tion that have similar extracellular substances located between them. The microscopic study of tissue structure is called histology (his-tol′ ō -je; histo-, tissue + -ology, study). Knowledge of tissue structure and function is important in understanding how individual cells are organized to form tissues and how tissues are organized to form organs, organ systems, and the complete organism. The structure of each tissue type is related to its function, and the struc-ture of the tissues in an organ is related to the organ’s function.
Changes in tissues can result in development, growth, aging, trauma, or disease. For example, skeletal muscles enlarge because skeletal muscle cells increase in size in response to exercise. Reduced elasticity of blood vessel walls in aging people results from gradual changes in connective tissue. Many tissue abnor-malities, including cancer, result from changes in tissues that can be identified by microscopic examination.
The four basic tissue types are epithelial, connective, muscle, and nervous.