Organization of the cross-filaments in intestinal microvilli

Abstract

We studied the arrangement of the cross-filaments in intestinal microvilli to understand how microfilaments interact with the membrane. Observations on thin-sectioned or negatively stained microvilli with the electron microscope demonstrate that the crossfilaments on the core bundle lie opposite to one another and are spaced 32.5 nm apart. In sections grazing through the membranes, the cross-filaments appear as transverse stripes in a barber-polelike arrangement. The cross-filaments point away from the microvillus tip. This orientation appears similar to that seen when microvilli are decorated with muscle myosin subfragments S1 or HMM. The cross filaments are associated not only with the microfilaments but also with electron-dense patches on the inside surface of the membrane. These results suggest the cross-filaments are arranged as a double helix around the core bundle. Furthermore, the cross-filements can serve as in situ markers for microvillar polarity. Lastly, the crossfilaments interact not only with specific portions on the actin filaments but also with dense patches on the membrane. These observations are summarized in a model of the microvillus cytoskeleton.