Three-dimensional model of tight junction fibrils based on freeze-fracture images.

To study the three-dimensional structure of tight junction fibrils, the epithelia of the jejunum and epididymis of adult mice were examined by the freeze-fracture technique in unfixed and in aldehyde-fixed specimens. The fibrils have a stronger affinity for the protoplasmic (P) face of the lipid bilayer in fixed material, and for the external (E) face in unfixed and rapidly frozen material. Therefore we can observe the fibrils both from the outside and inside of the cell. Fibrils appearing on the P-face are smoothly contoured ridges and rows of hemispherical particles, while those appearing on the E-face are exclusively rows of hemispherical particles. Based on these observations, we wish to propose a new fibril model for the tight junction. There are two distinctive types of junctional elements. One type is composed of a smooth and continuous strand in the external view of the cell, but is studded with hemispherical bulgings in its internal view. This type will be referred to as the "continuous type". The other type is bead-like, and will be referred to as the "particle type". The relative proportion of these two types of elements appearing within a tight junction network differs among tissues.