Microvilli are tiny finger-like projections found on the surface of certain cells. Their purpose is to increase the surface area of the cell's apical surface, resulting in more effective absorption or secretion of substances.

These microvilli are predominantly present in cells lining the small intestine, kidney tubules, and certain cells in the respiratory and reproductive systems. By significantly expanding the surface area of the cell membrane, microvilli enhance the cell's capacity to exchange materials with its surroundings.

Each microvillus is composed of a core of actin filaments that provide them with structural support. The actin filaments are connected to one another by other proteins, such as villin and fimbrin, giving the microvilli their characteristic shape. The cell membrane covering the microvilli contains various transport proteins and enzymes that assist in moving substances across the cell surface. The energy required for the transport processes is catered by the numerous mitochondria present at the basal ends of the cells.

In the small intestine, microvilli on the surface of epithelial cells significantly enhance the transport and absorption of nutrients from digested food. However, in individuals with celiac disease, an immune response to gluten damages microvilli, hampering nutrient absorption and resulting in malnutrition, cramping, and diarrhea.

In addition to their involvement in absorption and secretion, microvilli also play a part in sensory perception in some specialized cells. For instance, hair cells in the inner ear have microvilli which help to detect mechanical stimuli, convert them into electrical signals, and transmit them to the nervous system.

Overall, microvilli are vital structures that contribute to the functionality of various cells by increasing their surface area and enhancing their ability to interact with the surrounding environment.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.3: Eukaryotic cells