Tissues of the body
The organs of the body are formed from four types of tissue: epithelial, connective, muscle, and nervous. Most types of tissue have several forms that perform different functions.
Tissue composed of one or more layers of cells protects all internal and external body surfaces. Such tissue is called epithelial tissue. Squamous epithelium is composed of flat, irregularly shaped cells. Squamous cells form the top layers of the skin, the protective covering of the heart and lungs, and the lining of blood vessels. Cuboidal epithelium is made up of cells that are basically cube shaped. They are found in many glands and in the ducts of some organs, such as the kidney, as well as in the middle ear and the brain. Columnar epithelium is composed of cells that are long, narrow, and tightly packed. They line much of the digestive system and the upper respiratory tract. Many columnar epithelial cells have tiny hairlike extensions called cilia. The wavelike motion of cilia helps move substances along these surfaces.
The most widely distributed tissue in the human body is connective tissue. It joints, supports, and protects the other types of tissue. Connective tissue is composed of relatively few cell embedded in a thick, nonliving material called the matrix. The matrix contains many tiny, living fibers.
Four kinds of connective tissue are found in the human body. Dense connective tissue makes up cartilage and bone. Cartilage is a flexible but tough material consisting of small clusters of cells embedded in the matrix. Bone consists of cells in a matrix that contains hard crystals. Loose connective tissue is found under the skin and around nerves, blood vessels, the heart and the lungs. Its matrix is semifluid. Liquid connective tissue forms blood and lymph, a clear fluid that comes from blood. The matrix in blood is a liquid called plasma. Fat tissue is composed of cells in which large droplets of fat are stored. This fat can be used for energy when needed.
Specialized cells with the ability to contract and thereby produce movement make up muscle tissue. Muscle tissue is classified into three types. Skeletal muscles are attached to bones and move the skeleton. Smooth muscles are found in the walls of many internal organs, such as digestive organs. Cardiac muscle is found only in the heart.
Cells that can transmit messages throughout the body make up nervous tissue. These cells are found in the brain, spinal cord, nerves, and sensory organs. Nervous tissue provides information about the environment. It also controls many body functions.
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