Quick-freeze, deep-etch visualization of the cytoskeleton beneath surface differentiations of intestinal epithelial cells.

The cytoskeleton that supports microvilli in intestinal epithelial cells was visualized by the quick-freeze, deep-etch, rotary-replication technique (Heuser and Salpeter. 1979. J. Cell Biol. 82: 150). Before quick freezing, cells were exposed to detergents or broken open physically to clear away the granular material in their cytoplasm that would otherwise obscure the view. After such extraction, cells still displayed a characteristic organization of cytoskeletal filaments in their interiors. Platinum replicas of these cytoskeletons had sufficient resolution to allow us to identify the filament types present, and to determine their characteristic patterns of interaction. The most important new finding was that the apical "terminal web" in these cells, which supports the microvilli via their core bundles of actin filaments, does not itself contain very much actin but instead is comprised largely of narrow strands that interconnect adjacent actin bundles with one another and with the underlying base of intermediate filaments. These strands are slightly thinner than actin, do not display actin's 53A periodicity, and do not decorate with myosin subfragment S1. On the contrary, two lines of evidence suggested that these strands, could include myosin molecules. First, other investigators have shown that myosin is present in the terminal web (Mooseker et al. 1978. J. Cell Biol. 79: 444-453), yet we could find no thick filaments in this area. Second, we found that the strands were removed completely in the process of decorating the core filament bundles with the myosin subfragment S1, suggesting that they had been competitively displaced by exogenous myosin. We conclude that myosin may play a structural role in these cells, via its cross-linking distribution, in addition to whatever role it plays in microvillar motility.