Epithelial tissue

Epithelial tissue (epithelium) consists of sheets of tightly packed cells that cover the exterior surfaces of the body, line internal cavities and passageways, and form glands. For example, the outer layer of skin is an epithelial tissue, and so are the linings of the mouth, stomach, and blood vessels. This tissue primarily provides protection, absorption, secretion, and sensation.

• Functions of epithelium can be remembered by <u>PASE</u>: *Protection* (e.g., skin protecting underlying tissues), *Absorption* (e.g., intestinal epithelium absorbing nutrients), *Secretion* (e.g., glandular epithelium secreting hormones or enzymes), and *Excretion* (e.g., kidney tubules excreting waste). Different epithelial types specialize in different functions.
• Epithelial cells are characterized by polarity: they have an apical surface (free surface facing outward or into a lumen) and a basal surface attached to a <u>basement membrane</u> that anchors the epithelium to underlying connective tissue. Epithelia are avascular (contain no blood vessels), so they rely on diffusion from underlying tissues for nutrients.
• Epithelia are classified by cell shape and layers: squamous (flat cells), cuboidal (cube-shaped cells), columnar (tall cells); and simple (one cell layer thick) vs stratified (multiple layers). For instance, simple squamous epithelium in alveoli of lungs allows gas exchange, while stratified squamous epithelium in skin provides protection. Knowing examples of each type is high-yield.

• If a question asks which tissue type forms the epidermis (outer skin) or the lining of organs, it's epithelial tissue. For example, "Which tissue lines the inside of blood vessels?" Answer: an epithelial tissue (specifically simple squamous epithelium, often called endothelium).
• Glandular questions: Exocrine and endocrine glands are derived from epithelial tissue. A question might ask "What type of tissue is a gland?" - the answer is epithelial (glandular epithelium).
• Histology exam clues: Epithelial tissue will show a continuous layer of cells. Look for a free surface on one side and a basement membrane on the other. If an exam image shows tightly packed cells with one side facing a lumen or outside environment, that's epithelium.