Non-epithelial cells are all the other types of cells in the body that are not epithelial cells. Epithelial cells are those cells that line the surfaces of the body and its various cavities, such as the skin, the lining of the digestive tract, and the lining of the respiratory tract. Non-epithelial cells include all the other cells of the body, such as muscle cells, nerve cells, bone cells, and connective tissue cells.