Intracellular surface of intercalated disk of human heart seen by HRSEM

Intercalated disks (ID) are specialized areas of cardiomyocytes membranes which serve to serially bind adjacent fibers far adhesive and electrical coupling purposes. The transmission electron microscopy (TEM) demonstrated that three main types of intercellular junctions characterize each disk: fasciae adherentes, desmosomes, and gap junctions. Although these components are well defined in a two dimensional sense, the 3-D morphology by high resolution scanning electron microscopy (HRSEM) inspected from the cytoplasmic side of their plasmalemma is still unknown. In this report we have taken advantage of a modified osmium maceration procedure that, removing the contractile apparatus from cardiomyofibers, leaves the sarcolemma and associated ID intact. Needle-biopsy of left ventricle were obtained from seven thalassemia patients with no clinical signs of cardiac failure and were treated as previously reported. Informed consent was obtained and permission was granted by the local ethical committee. By HRSEM, most of the cytoplasmic surfaces of the disk membranes is covered by particles with a diameter of 20-35 nm, resembling those we have found to be associated with junctional complexes and desmosomes in other human tissues. Far the topographical correspondence between the particles seen by HRSEM and the adhesive junctions seen by TEM in the intercalated disks, we believe that those particles may represent the protein complexes linking desmosomes and fasciae adherentes to their cytoskeletal complement. However, the finding that particles of different sizes are distributed at random in ID makes it impossible to distinguish between desmosomes and fasciae adherentes. This is in agreement with the finding of Franke et al. (2006) and of Borrmann et al. (2006) which recognized, by immuno-electron-microscopical methods, typical desmosomal proteins also in areas corresponding to fasciae adherentes. As a consequence, the same authors have proposed the comprehensive term "area composita" to indicate the junctions of intercalated disks. Other zones completely devoid of particles might correspond to gap junctions because the thickness (30nm) of the joined cell membranes in these areas seem to match that calculated at the gap junction level.