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 line