Epithelium:
This is what the dictionary says:
epithelium: as cellular tissue that covers a free surface or lines a tube or cavity of an animal body and serves especially to enclose and protect the other parts of the body, to produce secretions and excretions, and to function in assimilation
It is a flexible covering that covers all body surfaces both inside and out.
In the mammary glands the epithelium cells are around the alveoli glands and if you are to become pregnant, these cells become fatty globules and will reduce once breastfeeding is over.
