[Study of atrial conduction tissue in the 17 mm V-C human embryo. Morphological contribution to the pathogenesis of...

Is there a specialised conduction pathway connecting the sinoatrial and atrioventricular nodes within the crista terminalis? As lesions of this zone can cause sinus node dysfunction and the sick-sinus syndrome, we decided to study this structure at the beginning of its embryonic development. Serial 10 micronthichness transverse sections of a 17 mm vertex-coccyx (Stage 19) human embryo were studied by light microscopy, anatomical reconstruction (Born) and morphometry. The region of the sinus venosus and venous valves, of the superior vena cava and the atrioventricular region were drawn and reconstructed with 150 X magnification by Born's method using wax plates of the following dimensions: 175 X 95X 1.5 mm. The volume of the region of the sino-atrial node was determined by multiplying the thickness of the sections by the surface of the region (measured by planimetry using the point counting method); its value was 15.4 X 10(6) microns3 with an antero-posterior diameter of 490,0 micron. These results show that this region is already proportionally larger in Stage 19 embryos. Reconstruction showed the sinoatrial node region to be a round bulge with a groove on either side: that of the sulcus terminalis and the superior vena cava on the left and the same vein with the mesocardium on the right. The right venous valve connects this zone with the interatrial septum and comprises two distinct cellular components, on derived from the atrial wall and the other from the muscle of the sinus venous. The artery fo the sinoatrial node was identified.(ABSTRACT TRUNCATED AT 250 WORDS)