Histology of the fetal AV node.   A. Histologic section of the atrio-ventricular-septal junction in a 25 week old fetus. The right and left ventricular cavities are indicated. At the junction of the atrial and ventricular septum there is a pale structure that represents the distal portion of the AV node as it becomes the bundle of His and gives origin to one of the left bundle branch fascicles (H&E 5X). B. The central fibrous body and the mitral valve are seen at top and the AV node at the bottom of this higher magnification field. The immature, pale appearance of these two structures can interfere with the identification of the AV which is only slightly paler (H&E 20X). C. The central fibrous body and the ventricular myocardium are seen. The spindle-shaped connective tissue cells of the central fibrous body are distinct from the pale more rounded to polygonal cells of the conduction system. The working myocytes show conspicuous eosinophilia (H&E 50X). D. The histologic features of the distal AV node, the bundle of His and the Purkinje cells are rather similar in the fetuses and premature infants. These cells are shorter in length, larger in diameter and paler than the working myocytes (300X). E. In the subendocardium the large, pale Purkinje myocytes are easily distinguished from the eosinophilic more striated working myocytes (H&E 400X).