Epithelium (epi- + thele + -ium) is one of the four basic types of animal tissue. The other three types are connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of blood vessels and organs throughout the body.There are three principal shapes of epithelial cells: squamous, columnar, and cuboidal.he other three types are connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of blood vessels and organs throughout the body. There are three principal shapes of epithelial cells: squamous, columnar, and cuboidal.