Epithelial cells are one of four tissue types that make up the human body. The other three are nervous tissue, connective tissue, and muscle tissue. Epithelial cells fit tightly together, are no more than a few cells thick, and help to form a selectively permeable coating or covering for organs and ducts within the body. Epithelial cells are able to absorb substances, block other substances, help conduct the passage of substances within the body, and detect sensation directed towared the area of the body they are covering. Epithelial cells are replicated within the body by a process called mitosis, which makes duplicate copies of the cells. The duplicate copies take the place of other epithelial cells that die, once they have completed their life cycle. Epithelial cells can help make up an endoderm, a mesoderm, and an ectoderm, in terms of layers of covering.