Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC135
Right atrioventricular ring tissue in human embryonic hearts - what can be learnt from histological and immunohistochemical investigation?
H. Azhar1, A. Atkinson1, H. Dobrzynski1
1. Cardiovascular Medicine, University of Manchester, Manchester, United Kingdom.
The cardiac conduction system (CCS) comprises specialized myocytes which have the ability to elicit an action potential and then conduct it from the atria to the ventricles. This system includes the sinus node, atrioventricular node, His bundle and Purkinje fibres. In the early 20th century principles were set regarding histological characterisation of conducting myocytes. Three criteria had to be met for cells to be accepted as part of the CCS; they had to be histologically distinct, serially detectable from section to section and encapsulated from surrounding myocytes by fibrous tissue (1, 2). The generation of improved immunohistochemical methods allow the morphology of the CCS to be highlighted more accurately. These methods have enabled the discovery of other nodal-like tissue, such as the atrioventricular ring tissue (AVRT). Cells of the AVRT encircle the tricuspid, mitral and aortic valves and form the right, left and aortic rings. These three rings combine to produce the retroaortic node. The function of the AVRT remains to be defined although it has been demonstrated that catheter ablation of this region attenuates atrial tachycardias. The aim of this pilot study was to determine if atrioventricular ring tissue is present in human embryos at the stage when the heart is fully developed, using markers, such as, HCN4, Cx40 and Cx43. 4 embryonic hearts at the fully developed stage (youngest heart at Carnegie stage 23) were frozen and cryosectioned in the longitudinal axis from the dorsal to the ventral surface to give tissue sections 25 µm in thickness. Histology was carried out on sections (every 300 µm) in order to find the approximate location of the ring tissues within the heart. Adjacent sections to those used for histology were immunolabelled for HCN4 (marker of the CCS and AVRT), Cx40 (marker of the atrial myocardium) and Cx43 (marker of the working myocardium). Since, HCN4 was found to be a non-specific marker in human embryos, attention was drawn to Cx40 and Cx43. The antibody concentration was optimised (Cx40, 1:50, Cx43, 1:50) so that atrial and ventricular myocardium could be easily identified. The results showed that Cx40 is present in the atrial myocardium and Cx43 is present in the working myocardium. Myocytes of the right atrioventricular ring tissue did not show either Cx40 or Cx43 to be present in this region. Our findings indicate that specialized atrioventricular ring tissues may be present in the human embryonic hearts, which can explain atrial tachycardias arising in the adult heart, in this region. We will now carry out further investigations in order to determine the expression of different proteins in the right ring tissue. We will also establish the relationship of the human embryonic AVRT to the CCS, and determine whether it is similar to that described in the rat model.
Where applicable, experiments conform with Society ethical requirements