The body’s first line of defense against physical, chemical, and biological damage is epithelial tissues. An epithelium’s cells serve as bodily gatekeepers, limiting permeability by enabling selective material transfer across its surface. All substances entering the body must pass through first the epithelium.

1. Simple Squamous Epithelium – made up of a single layer of epithelial cells. They can be seen lining bodily cavities such as the pericardial, pleural, and peritoneal cavities, as well as in places where passive diffusion occurs, such as the glomeruli of the kidney and the alveoli of the respiratory system.

2. Simple Columnar Epithelium – It is made up of a single layer of tall and slender columnar epithelial cells with oval-shaped nuclei that are linked to the basement membrane in the basal region. Simple columnar epithelium lines the majority of the digestive tract organs in humans, including the stomach and intestines.

3. Transitional Epithelium – is a type of tissue that responds to stretching by changing shape (stretchable epithelium). When relaxed, the transitional epithelium appears cuboidal, and when stretched, it appears squamous.