One of the four primary tissues of the body, which constitutes the epidermis and the lining of respiratory, digestive, and genitourinary passages. The major characteristic of epithelium is that the cells are close together, separated by a very small amount of intercellular substance. The epithelium (endothelium) lining the inner cavities of the heart, blood vessels, and lymphatics differs from that of all the other groups because under abnormal conditions it may be transformed into a different type of cell characteristic of connective tissue, the fibroblast. Epithelium may be derived from any of the three primary germ layers of the very early embryo—ectoderm, endoderm, or mesoderm.