Gap junction shape and orientation at the cardiac intercalated disk.

spaced) in length and may be subject to editing or abridgment. Gap Junction Shape and Orientation at the Cardiac Intercalated DiskIn a recent study on canine myocardium, Hoyt et al 1 present some fascinating new insights into the form and distribution of gap junctions in relation to the complex three-dimensional nature of the intercalated disk of working ventricular myocytes.Their conclusions, based on morphometric analysis of data from thin sections and on serial sectioning and scanning electron microscopy, include the following: 1) small gap junctions, accounting for -20% of gap junction membrane, are present in the plicate (i.e., fascia adherens-containing) regions of the intercalated disk; 2) gap junctions do not, as some had thought, occur at the true lateral surfaces of the myocyte body (i.e., in "nonintercalated disk regions"), and 3) each myocyte is connected via intercalated disks to an average of -9 other myocytes.These findings add substantially to the debate on the structural basis for coupling between myocytes, and draw attention to aspects of junction organization not adequately taken account of previously.However, one central conclusion from this study in particular merits further discussion.This concerns the shape and orientation of the large gap junctions located in the interplicate zones of the disk.According to Hoyt et al, 1 these junctions (which account for -80% of total gap junction membrane) occur in the form of ribbon-like structures (averaging 1.3x5.1 ^m), oriented with their long axes running transversely with respect to the longitudinal axis of the cell, that is, partially circumscribing, in discontinuous fashion, the "finger-like" processes making up the disk.This conclusion, summarized in the drawing in Figure 8 of their paper, is derived principally from measurements of gap junction profile length in sections cut in three orthogonal planes.