Which apical modification increases surface area for absorption and has an actin core?

• A. Cilia
• B. Stereocilia
• C. Keratinization
• D. Microvilli

Answer: (D)

Increasing apical surface area for absorption is achieved by microvilli, tiny finger-like projections on the apical surface of absorptive epithelial cells. Each microvillus contains an actin filament core that provides structural support, with the membrane wrapped around these actin bundles and anchored to the cell interior by linker proteins. Collectively, many microvilli form a brush border that dramatically expands the surface area available for nutrient uptake, which is essential in tissues like the intestinal lining and kidney tubules.

Cilia, in contrast, have a core built from microtubules arranged in an axoneme and are mainly used to move mucus or cells rather than to increase surface area for absorption. Stereocilia also have actin cores and resemble microvilli, but they are typically longer and found in specialized tissues (such as parts of the male reproductive system and the inner ear) where their role isn’t to maximize absorptive surface in the same way as the intestinal brush border. Keratinization involves forming a tough, protective outer layer rather than expanding the absorptive surface.