Tissues are a group of cells that share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern to perform specific functions. From an evolutionary perspective, tissues appear in more complex organisms. Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall maintenance of the body.
Epithelial tissue refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities/passageways, and form certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and protects, supports, and integrates all parts of the body. Muscle tissue is excitable, responding to stimulation and contracting to provide movement. Muscle tissue has three major types: skeletal, smooth, and cardiac. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.
Different tissue types are organized into organs. Each organ has a specific arrangement of these tissues that helps perform their functions. A disruption of the structure in any of these tissue types is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.1: Types of Tissues
