The structure of animal tissue is directly related to its function. Animals have groups of cells in the tissue, which are combined together to perform as an organ.
Tissues are groups of cells with a basic structure and function.
Tissues are woven together with a sticky extra cellular matrix. Epithelial tissue is a tissue that is made up of tightly packed cells that line organs and body cavities. The cells contained in the epithelium are closely joined without much material between them. Inside the epithelia the cells are locked together by tight junctions. The reasons for the tightly packed cells are to act as a barrier against mechanical injury, invading microorganisms, and fluid loss.