Intercalated Disc

When observing cardiac muscle tissue through a microscope, intercalated discs are an identifying feature of cardiac muscle. Cardiac muscle consists of single heart muscle cells (cardiomyocytes) which have to be connected by intercalated discs to work as a functional organ. By contrast, skeletal muscle consists of multinucleated muscle fibers and therefore exhibit no intercalated discs. Intercalated discs support synchronised contraction of cardiac tissue. They can easily be visualized by a longitudinal section of the tissue.

Cell biological textbooks mention three types of cardiomyocyte adhering junctions within an intercalated disc — fascia adherens, macula adherens and gap junctions.

• Fascia adherens are anchoring sites for actin, and connect to the closest sarcomere.

• Macula adherens stop separation during contraction by binding intermediate filaments, joining the cells together. Macula adherens junctions are also called desmosomes.

• Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.

However, molecular biological and comprehensive studies have shown that intercalated discs consist for the most part of mixed type adherens junctions termed composite junctions or areae compositae (singular area composita). These represent an amalgamation of typical desmosomal and fascia adherens proteins (in contrast to various epithelia). Thus cardiomyocyte adherens junctions differ from epithelial zonula adherens and desmosomes.