Epithelial Tissue

There are four kinds of animal tissue: epithelial, connective, muscle, and nervous. Each tissue type comes in several forms that share functional and structural characteristics.

In animals, organs, spaces, and the outside of the body are covered with epithelial tissues. Epithelial cells are tightly packed together to form a protective barrier. On the outside of the body, they work to shield animals from injury and invasion from without, and from fluid loss from within.

External epithelial tissues may contain sensory cells that register stimuli, or cilia, which sweep away debris. Internally, some epithelial tissues secrete compounds such as enzymes, mucous, and lubricants, while others excrete wastes. The epithelial lining of the digestive tract is designed to absorb water and nutrients.

Connective Tissue

Connective tissue performs a diverse array of functions, including support, defense, material transport, and repair. All connective tissue is composed of specialized cells embedded in an extracellular matrix. The extracellular matrix, composed of protein fibers and other fluid or solid material secreted by the cells, helps anchor or support the tissues.

The cells in connective tissue are not connected directly to one another, but are suspended in water, plasma, bone, or cartilage.

Loose connective tissue holds internal organs in place and connects different kinds of tissue to one another. Some types form a weave of fibers that serve to store materials. Dense connective tissue, such as tendons and ligaments, binds tissues together.

Supportive connective tissue contains a solid to semisolid extracellular matrix, which acts to support and protect the body. It may be rigid, as in bone, or flexible, as in cartilage.

Blood and lymph are fluid connective tissues. They do not provide physical support, but connect body tissues physiologically. Blood and lymph cells are suspended within an extracellular matrix of plasma and interstitial fluid.

Muscle Tissue

Muscle tissue contracts and relaxes to cause movement of body parts, of materials within the body, and of the body itself.

Visceral, or smooth, muscle tissue contracts relatively slowly. It is found in most of the viscera, or internal organs, such as the stomach and intestines, bladder, and blood vessels. Visceral tissues help move substances within and between organs.

Cardiac muscle tissue is found in the heart. The cells that make up this tissue are highly coordinated, so that their carefully timed contractions provide a consistent and sustained beat.

Skeletal muscle tissue is attached to bones or chitin. These tissues provide voluntary movement, and also work to help support the body. Unlike visceral and cardiac muscles, skeletal muscles move voluntarily.

Nervous Tissue

Nervous tissue has sensory and signaling functions, and allows communication between tissues. Nerve cells detect internal and external stimuli, and carry the impulses that direct the appropriate responses.

Nervous tissue is embedded in epithelial and skeletal tissues, and bound into nerves and cords by connective tissues. It forms networks that transmit signals to and from these other types of tissues, helping to direct bodily functions and activities.