Abstract: Objective To observe the morphological feature and the expression of connexin(Cx)43 and Cx40 in the common arrhythmogenesis and radiofrequency ablation(RFA) region, the atrioventricular junction and its surrounding area, in order to provide the morphological basis for understanding the mechanism of arrhythmogenesis and possible sites for therapy. Methods Ten fresh normal adult hearts were used in this study. The specimens containing the atrioventricular node(AVN) was harvested, embedded in paraffin and sectioned. The sections were stained with HE, Masson staining. Expression of Cx43 and Cx40 was examined by immunohistochemistry and image analysis. Results There were the bundles of transition cells at cavotricuspid isthmus, around the opening and distal end of coronary sinus, at the annulus fibrosus of bicuspid valve, and at the central fibrous body(CFB) near the annulus of right atrium upper-valve muscles. The bundles connected between the AVN and posterior node extension(PNE) in different directions. The transition cells were different from ordinary cardial muscle cells in the cell component, assignment, course and association of fibers. The distribution of Cx43 and Cx40 in bundles14 of transition cells and annulus of right atrium uppervavle muscles was limited in a small area. Expression of Cx43 and 40 was widely present in the transition cells of left atrium close to the membranous part of interventricular septum and extensively localized at cavo-tricuspid isthmus. Conclusion Dual atrioventricular(AV) nodal pathways(DAVNP) and multiple AV nodal pathways(TAVNP) are at and nearby the atrioventricular junction. And they are coherent with the target spots using in radiofrequency ablation to treat arrhythmia, which may indicate a new potential radiofrequency ablation site wher

Key words: Atrioventricular junction, Transition cell, Morphology, Connexin, Masson staining, Immunohistochemistry, Adult