The cortical cytoskeletal lattice in outer hair cells is a two-dimensional actin-based structure, which can be labelled with rhodamine/phalloidin and disrupted by the enzyme deoxyribonuclease I. Structural information from thin sectioned, freeze-etched and negatively stained preparations shows that it is based upon two types of filament that form a cross-linked lattice of circumferential filaments. The cross-links are 70–80 nm long. Measurements of the spacing between circumferential filaments suggest that the lattice is stiffer circumferentially than it is longitudinally. Analysis of the orientation of circumferential filaments shows that it is composed of discrete domains of up to 10 microns 2. Relative movements between domains could allow substantial changes of cell shape without disrupting the unit structure of the lattice, thus allowing the cell cortex to retain its elastic responses to high-frequency deformations.
Smooth septate junctions from the midgut of the cricket, Acheta, and the horseshoe crab, Limulus, as well as Hydra-type septate junctions from the epidermis of Hydra have been studied by freeze-fracture after direct freezing using the liquid helium-cooled copper block/slam freezing method. The exoplasmic fracture face at both types of septate junction exhibits rows of closely packed but irregularly shaped intramembrane particles. Complementary to these particle rows, on the protoplasmic fracture face, are sharply defined grooves with a periodic variation in depth and width that was conspicuous in Hydra but less well defined in arthropods. The closely packed, irregular particles on the exoplasmic faces could represent plastically deformed portions of transmembrane proteins pulled through the bilayer during freeze-fracture. On the basis of this interpretation, the grooves on the protoplasmic faces represent a confluence of the bilayer disruptions occurring during fracturing. The structures observed here are different from those reported in replicas of glutaraldehyde-fixed and glycerol-cryoprotected tissue, in which the intramembrane junctional components partition with the protoplasmic face and often assume the appearance of continuous cylinders. This comparison illustrates some of the artifacts associated with freeze-fracturing and shadowing. On the basis of a comparison of freeze-fracture replicas and sections of lanthanum-infiltrated tissues, the relationship between intramembrane junctional components and intercellular septal elements is analysed.