and the mound of tissue, with its own mesenchymal covering, that grows into the heart from the posterior mediastinum (see Fig 31.13B) This latter structure, the vestibular spine, was initially discovered and described by His the elder in the latter part of the 19th century.35 It disappeared from consideration for some considerable time before its recent resurrection.36–39 We now know that in several mouse models the vestibular spine is lacking, such as the Tbx1 knockout; it is also absent in some genetically normal mice (Fig 31.14, left) The genetically normal mice have typical “ostium primum” defects, with the findings confirming that the common atrioventricular junction is present despite the fusion of the atrioventricular cushions (Fig 31.14, right) FIG 31.14 Features of an ostium primum discovered in a genetically normal mouse sacrificed at embryonic day 15.5, by which time the primary atrial foramen is normally closed Left, Frontal section revealing absence of the septal component normally derived from the vestibular spine (compare with Fig 31.13A) Right, Short axis of the atrioventricular junctions viewed from the atrial aspect showing fusion between the leaflets derived from the superior and inferior atrioventricular (AV) cushions (compare with Fig 31.7, left) Yet other genetically perturbed mice have now been discovered with common atrioventricular junctions and with shunting through the atrioventricular septal defect exclusively at the ventricular level because the atrial septum is fused to the atrioventricular cushions The vestibular spine has been shown to be normally formed in mice with this arrangement.40 Ongoing studies of these mice, and other mouse models, are likely to identify the specific genes involved in normal and abnormal development It will be necessary to take careful note of the precise structure of the cardiac abnormalities produced in these experimental animals to make appropriate inferences for the situation pertaining in human maldevelopment Pathophysiology Atrial Shunting There is almost always an atrial component of the defect, the so-called ostium primum defect, across which there is a left-to-right shunt In about one-third of patients, even those with separate right and left atrioventricular valvar orifices, there is also a small right-to-left shunt When the atrial septum is malaligned leftward in relation to the common atrioventricular junction, a proportion of the systemic venous blood can be directed into the left ventricle and thus into the aorta In individuals in whom the atrial septum is virtually absent, often described as a common atrium, there is usually admixture of the systemic and pulmonary venous returns This is largely because of the absence of a septum, but it is also in part because of the high incidence of anomalous venous connections, particularly in hearts of patients having isomerism of the atrial appendages In the latter setting, with isomeric right appendages, the blood from a left-sided superior caval vein, if present, empties directly into the left upper corner of the atrium instead of flowing into the coronary sinus and thence into the right-sided atrium The coronary sinus, of course, is absent in the setting of right isomerism, permitting coronary venous blood to drain directly into the atrial cavities As a result, a mild degree of desaturation is not uncommon in patients with right isomerism also having an atrioventricular septal defect Ventricular Shunting If there is an unobstructed ventricular component to the defect, the direction of flow through it will depend on resistance to outflow from the two ventricles Subaortic stenosis, or aortic coarctation, will favor left-to-right shunting, whereas obstruction of the right ventricular outflow tract or pulmonary vascular obstructive disease will favor right-to-left shunting When none of these complicating factors is present, the shunt is left to right, as in a simple ventricular septal defect Ventriculoatrial Shunting Ventriculoatrial shunting is a phenomenon found most typically in patients with