Epithelium has many functions that are accounted for by its structure. For example, epithelium can be protective. One way it can be protective is by being ______ through either being stratified or having taller cells make it up. Another way it can be protective is to not let anything slip through between cells by having ______. Some epithelia can also be protective by being ______ like skin or by having many ______ to secrete a protective layer of mucus.