The cell is the fundamental unit of living organisms (Chapter 1). Some estimates place the number of cells in an adult human being at between 50 and 75 trillion, whereas others place the number at well above 100 trillion. Whichever estimate is more accurate, humans, as well as all complex multicellular organisms, are made up of a lot of cells! What then keeps us (or any other multicellular organism) from slumping into a pile of cells? And what keeps cells organized into tissues, and tissues into organs?

Tissues are held together and function as a unit because of cell junctions. Cell junctions physically connect one cell to the next and anchor cells to the extracellular matrix. Some tissues have cell junctions that perform roles other than adhesion. For example, cell junctions in the outer layer of the skin and the lining of intestine provide a seal so that the epithelial sheet can act as a selective barrier. Other cell junctions allow communication between adjacent cells so that they work together as a unit. In this section, we look more closely at the roles of the various types of cell junction in tissues, as well as their interaction with the cytoskeleton.