The four tissue types

A tissue is a group of similar cells that work together on the same job. And as varied as the body looks on the inside, all of it is made from four types: epithelial, connective, muscle and nervous.

Epithelial tissue is sheets of cells, like bed sheets. They cover the outer surfaces of the body, line the inside of body cavities and internal passages, and they also form some glands. Connective tissue does the opposite of covering: it joins. It ties cells and organs to one another, and in that joining it protects them, supports them and integrates them.

The other two share a property that sets them apart from the first two: they are excitable, meaning they respond when something stimulates them. Muscle tissue responds by contracting, and it comes in three varieties .. skeletal, the one you move at will, plus smooth and cardiac. Nervous tissue responds by propagating electrochemical signals, the nerve impulses one region of the body uses to talk to another.

And they combine. The next step up is the organ, and an organ is exactly that, several of these tissues gathered together to form a unit that does a job.