Cells that line the inner and outer surfaces of the body by forming cellular layers (epithelium) or masses. epithelial cells lining the skin; the mouth; the nose; and the anal canal derive from ectoderm; those lining the respiratory system and the digestive system derive from endoderm; others (cardiovascular system and lymphatic system) derive from mesoderm. epithelial cells can be classified mainly by cell shape and function into squamous, glandular and transitional epithelial cells.