Filament systems in the Purkinje fibers of the heart.

The presence of a specialized cell system designed to conduct the electrical impulses for the sequential contraction of heart muscle is well known. This conduction system of the heart exhibits special morphological features and differs from the contractile myocardium in several respects, including cell form and size, intercellular contacts, and the absence of T tubules. This review is focused on another distinguishing feature, the specific filament systems present in the conduction cells. The conduction cells contain myofibrillar material with different histochemical, biochemical, and morphological properties compared with the contractile myocardium. The conduction cells also contain specific myofilament-polyribosome complexes, of unknown function, never seen in ordinary myocardial cells. Intermediate filaments, present in all eukaryotic cell types, are extremely abundant in conduction cells, especially in birds and larger mammals. Other filamentous organelles of the conduction cells include leptofibrils, microtubules, and microfilaments. The combined results from several investigations indicate that the filamentous systems maintain the structural integrity of the conduction cell bundles functioning as a cytoskeleton. Extracellular elements, such as the surrounding connective tissue sheath, may aid in this respect. Other functions have been ascribed to the filamentous systems of the conduction cells, but these are unproven.