Human Body Tissues
The structure and function of an organ are dependent on the tissues that make it up. A tissue is a group or layer of cells with similar structures, plus the intercellular substances, the material packaged between them, all of which function together for the same specialized purpose. The four basic types of tissue in the human body are epithelial, connective, muscle, and nervous.
The epithelial tissue in the human, also called epithelium, covers body surfaces and lines internal body cavities. A type of epithelial tissue makes up the outer layer of the skin, called the epidermis. Epithelium provides protection. For a substance to enter or exit the body at the digestive tract, the genital tract, or the lungs, it must cross epithelial tissue. Epithelial tissues have a nerve supply, but no direct blood supply. Cells in epithelial tissue are packed so closely together there is virtually no room between them for any extra material.
- Squamous epithelium, such as that lining the air spaces of the lungs and blood vessels, is composed of flattened scale-shaped cells. Squamous epithelial tissue that lines the sacs of the lungs allows for the diffusion of oxygen and carbon dioxide across its surface.
- Cuboidal epithelium, is made up of cube-shaped cells. Cuboidal epithelial tissue lines the inside of the kidney tubules.
- Columnar epithelium has cells resembling tiny columns, or rectangular pillars. The nuclei in columnar epithelial cells are usually located near the bottom of each cell. This type of tissue lines the cavity (lumen) of the digestive track.
- Stratified (layered) epithelium, such as makes up the epidermis, the outer layer of the skin, contains cells reinforced by keratin, a fibrous protein that provides strength. The skin's stratified epithelium provides protection from injury and possible pathogen (virus and bacterium) invasion, as well as protects the underlying tissues from drying out. Stratified epithelial tissues also covers the tongue.
- Transitional epithelium, found for example in the lining of the urinary bladder, stretches and contracts, having a structure that suits function.