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Tissues form part of an organ. Tissues are groups of cells that work together to do a job in the body. They are a group of cells having the same origin, structure and function. The cells look the same or almost the same. The work the cells do is also the same or almost the same.
Organs are made up of more than one kind of tissue. The heart is an organ. It is made up of different tissues. It has muscle tissue. It has connective tissue like the endocardium. The endocardium is the thin layer of tissue that surrounds the heart. The heart has valves that make sure the blood goes the right way through the heart. So the heart is an organ made from several tissues.