Ultrastructure of the human atrioventricular conduction tissues.

The ultrastructure of the human atrioventricular conduction tissue has been studied by obtaining material from recipient hearts at transplant operations. The hearts were dissected immediately after surgical removal in order to expose the conduction system, and tissue samples were taken directly from the atrioventricular node, the penetrating bundle, the branching bundle, and both bundle branches. Examination with the electron microscope showed that the entire atrioventricular system throughout its length was composed of a spectrum of cells which ranged widely in size and in myofibril content from slim cells resembling cardiac muscle and packed with myofibrils to wide 'empty' cells containing relatively few myofibrils. The cells were polymorphic, and many branched with the branches varying greatly in width. Transverse junctions between cells or between their branches were made by intercalated discs. Lateral connections between cells were extremely rare; they were made by desmosomes only. Nerves were present throughout the axis. The striking features of the atrioventricular conduction system as a whole were firstly that the constituent cells were so widely heterogeneous as to defy any classification into cell types, and secondly that totally dissimilar cells established direct continuity by means of intercalated discs.

The developing atrioventricular bundle and its branches in relation to small induced ventricular septal defects in the heart of chick embryos.

The form and position of the developing atrioventricular bundle and its branches has been studied in the hearts of domestic fowl embryos operated upon to induce delayed closure of the interventricular foramen. The operation was performed at 3 days of incubation when the outflow tract of the tubular heart was temporarily distorted by placing a nylon rod beneath the tract for 48 hours. The hearts from operated embryos were sectioned serially and examined with the light microscope at 11 days of incubation; normal hearts of the same incubation age were available for comparison. Small ventricular septal defects were found in a proportion of the operated hearts, and the form and position of developing conducting tissue with relation to the defects was of particular significance. The atrioventricular bundle and the septal component of the ring branch were represented by discrete fasciculi, but the right and left limbs of the bundle were disposed as a drape of tissue overlying the crest of the developing muscular septum, and situated beneath both the ventricular septal defect and cushion tissue of the septal truncus ridge. These observations of the developing conducting tissue in operated hearts are compatible with the concept that the bundle and branches develop normally from part of a precursor tissue distributed as a complete subendocardial sleeve in the tubular heart. They also imply that in the normal adult avian heart the positions of the atrioventricular bundle and its branches denote the junctional zones of the components forming the ventricular septum.

The development of the atrioventricular bundle and its branches in the avian heart.

A light microscopic study of serial sections of the hearts of 64 avian embryos (chicken, turkey, duck and goose) was undertaken in order to elucidate the development of the atrioventricular bundle and its branches. The developing atrioventricular bundle and its branches were shown to appear simultaneously (at about a third of the way through incubation) as relatively large, discrete fasciculi, which were dark-staining when compared with developing cardiac muscle. Prior to the appearance of the bundle and branches, the developing conducting tissue was found to be present as a widespread precursor tissue, which was distributed as a complete, dark-staining subendocardial sleeve, lining the whole of the ventricular canal, ventriculobulbar loop, and extending into the common atrial cavity. From these observations, it is concluded that, with septation and chamber formation, the tissue of the sleeve is partly incorporated within the chamber walls to give origin to diffuse Purkinje tissue, and is partly retained as the bundle and branches. The manner of derivation of the bundle and branches is discussed, and it is stressed that the definitive positions of the bundle and branches denote the junctional zones of the components of septation.