Microvilli are not the same as epithelium; rather, they are found on the surface of epithelial cells. Epithelium is a tissue type, while microvilli are microscopic structures.
Epithelium: The Tissue Layer
Epithelium is a sheet-like tissue that covers body surfaces, lines body cavities and forms glands. Cleveland Clinic's explanation of epithelium highlights its diverse types, including simple squamous, simple columnar, and stratified cuboidal epithelium, each with unique structures and functions. Epithelial cells, depending on their location and function, may or may not possess microvilli.
Microvilli: Surface Extensions
Microvilli are tiny, finger-like projections found on the apical (free) surface of some epithelial cells. As stated in multiple sources, including this ScienceDirect article, they are non-motile and significantly increase the surface area of the cell. This increased surface area is crucial for absorption, as seen in the small intestine where microvilli facilitate nutrient uptake. A study comparing microvilli in mouse intestines (PubMed) demonstrates variations even within a single organism. Furthermore, research indicates differences in membrane protein composition between microvilli of epithelial and non-epithelial cells (PNAS). The presence of microvilli is a characteristic of some epithelial cells, not a definition of the tissue type itself.
In short:
- Epithelium: A tissue type forming linings and coverings.
- Microvilli: Microscopic, non-motile projections on some epithelial cells, increasing surface area for absorption.
Examples where this distinction is important:
- The lining of the small intestine is epithelial tissue composed of cells with many microvilli.
- The skin's epidermis is epithelial tissue but typically lacks microvilli.
