Often you will see cells in orderly arrangements. When cells adhere to one another in order to cover an interior or exterior surface, we call them epithelium. The cells are attached tightly to one another to protect the integrity of the surface and keep tissue fluids from leaking out. Epithelial cells cover every healthy body surface except the gliding surfaces of joints and the surfaces of the teeth.
Epithelial cells stick tight to one another.
There are many different kinds of epithelium. Beginning students are often disappointed that they cannot always see the cell borders. This should not surprise you. Often they are only a few molecules thick. Also remember that animal cells do not have cell walls.