In Situ Structure of Intestinal Apical Surface Reveals Nanovilli on Microvilli

Abstract Microvilli are actin bundle-supported membrane protrusions essential for absorption, secretion, and sensation. Microvilli defects cause human diseases, including gastrointestinal disorders and inherited deafness; however, mechanisms controlling microvilli formation and organization remain unclear. Here, we study microvilli by vitrifying the C. elegans larvae and mouse intestinal tissues with high-pressure freezing, thinning them by cryo-focused ion beam milling, cryo-electron tomography, and sub-tomogram averaging. We uncover that hundreds of previously unrecognized stick-like structures, which we refer to as nanovilli, decorate the lateral surface of C. elegans and mouse microvilli. The C. elegans 37.5-nm long and 4.5-nm wide nanovilli are composed of the protocadherin family protein CDH-8. Loss of nanovilli slows down animal growth and increases the number of Y-shaped microvilli, intermediate structures when a microvillus splits from its tip and separates into two. Our results show that nanovilli space microvilli and suggest a microvilli division model through which microvilli assemble with striking uniformity.