Nexus of frog ventricle.

Abstract

In Rana pipiens ventricle a nexus with very unusual morphology was demonstrated. This tissue was reported previously to lack nexuses. The nexus appears in thin sections of ventricle, fixed in aldehyde and OsO4 or permanganate, as a series of punctate membrane appositions regularly alternating with regions of membrane separation. The junctional width at membrane appositions, as determined by microdensitometry and optical measurements, is 15-17 nm, and the width of the electron-translucent region between the junctional membranes is 1.8 nm. These values correspond closely to similar measurements of the more typical nexuses in frog liver. Along the nexus the mean distance between punctate appositions is 74.5 nm. Freeze-cleave replicas of the nexuses between myocardial cells show particles 10.4 nm in diameter arranged in arrays of up to 9 linked circles or partial circles on the PF[protoplasmic face]-face and similar arrays of pits of shallow grooves on the EF[endoplasmic face]-face. The mean diameter of the circles on both membrane fracture faces is 76.7 nm. A comparison of the thin-sectioned and freeze-cleaved nexuses demonstrates an excellent correspondence between the spacing of membrane appositions along the junction and the diameters of the freeze-cleaved circles of particles and pits or grooves.