Tissues

Multicellular organisms are made up of groups of cells called tissues. Tissues in both plants and animals are made of cells that share similar structures and perform specific functions.

In animals, tissues can be classified into four main types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

Epithelial tissue covers body surfaces, such as the surface of the skin and the lining of internal cavities. Some epithelial tissues form protective barriers, while others allow the exchange or absorption of nutrients.

Epithelial tissues also form specialized structures called glands, which release secretory substances.

The cells of epithelial tissues are tightly packed in either a single layer, called simple epithelium, or multiple layers, called stratified epithelium.

Connective tissue supports and connects other tissues. Its cells lie within an extracellular matrix that may be fluid, as in blood; flexible, as in arteries; or rigid, as in bone.

Muscle tissue helps the body move. Its cells contain proteins that slide past each other to shorten the cell.

There are three types of muscle tissue. Skeletal muscle attaches to bones and allows voluntary movement. Smooth muscle lines internal organs and controls involuntary movement. Cardiac muscle forms the heart and contracts rhythmically to pump blood.

Nervous tissue includes neurons that transmit electrical signals in response to stimuli, helping coordinate body activities, along with supporting cells, such as Schwann cells, that help maintain and support neuronal function.

In multicellular organisms, tissues combine to form organs, and organs work together in body systems.

For example, in the stomach, epithelial cells secrete digestive enzymes. Connective tissue beneath them contains blood vessels that supply nutrients and oxygen. Smooth muscle tissue contracts to churn and mix food. Nervous tissue controls these muscle contractions and regulates secretion. Together, these tissues enable the stomach to digest efficiently.