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Tissues are a group of similar cells that perform a similar function inside a living organism and consist of epithelial tissue, muscle tissue, nervous tissue, and connective tissue in complex multicellular animals.

Epithelial cells provide a protective barrier against injury and pathogens. Muscle tissue attributes all types of body movement, and nervous tissue has nerve cells that help transmit information and form a brain.

Connective tissue is responsible for holding various tissues and organs together and in place and providing protection and structural support to the tissue within the organisms.

Epithelial tissue has tight junctions and serves as a role of a barrier against mechanical injury or microorganisms. They include basic characteristics such as polarity, cellularity, cell arrangements with distinct functions, vascularity, attachment, and regeneration.

The shape of its cells categorizes epithelial tissue, and this tissue helps in forming protective boundaries between different environments. Epithelial tissue allows nearly all substances to pass through and includes three different types of epithelial tissue such as squamous, columnar and cuboidal.

Simple epithelial tissue lines blood vessels and air sacs of the lungs that help exchange nutrients and gases by diffusion. Simple columnar epithelium lines the intestine where secretion and absorption occur.

Cuboidal epithelium lines kidney tubules and thyroid as well as salivary glands that are specialized for secreting mucus. All types of epithelia are closely packed and form an active and protective barrier on external and internal surfaces. The bodies of animals are characterized by their cells, organs, or tissues.