Clarifying the morphology of the ostium primum defect

Abstract

The 'ostium primum' defect is still frequently considered to be the consequence of deficient atrial septation, although the key feature is a common atrioventricular junction. The bridging leaflets of the common atrioventricular valve, which are joined to each other, are depressed distal to the atrioventricular junction, and fused to the crest of the muscular ventricular septum, which is bowed in the concave direction towards the ventricular apex. As a result, shunting across the defect occurs between the atrial chambers. These observations suggest that the basic deficiency in the 'ostium primum' defect is best understood as a product of defective atrioventricular septation, rather than an atrial septal defect. We have now encountered four examples of 'ostium primum' defects in mouse embryos that support this view. These were identified from a large number of mouse embryo hearts collected from a normal, outbred mouse colony and analysed by episcopic microscopy as part of an ongoing study of normal mouse cardiac development. The abnormal hearts were identified from embryos collected at embryonic days 15.5, 16.5 and 18.5 (two cases). We have analysed the features of the abnormal hearts, and compared the findings with those obtained in the large number of normally developed embryos. Our data show that the key feature of normal atrioventricular septation is the ventral growth through the right pulmonary ridge of a protrusion from the dorsal pharyngeal mesenchyme, confirming previous findings. This protrusion, known as the vestibular spine, or the dorsal mesenchymal protrusion, reinforces the closure of the primary atrial foramen, and muscularises along with the mesenchymal cap of the primary atrial septum to form the ventro-caudal buttress of the oval foramen, identified by some as the 'canal septum'. Detailed analysis of the four abnormal hearts suggests that in each case there has been failure of growth of the vestibular spine, with the result that the common atrioventricular junction found earlier during normal development now persists during cardiac development. Failure of separation of the common junction also accounts for the trifoliate arrangement of the left atrioventricular valve in the abnormal hearts. Analysis of the episcopic datasets also permits recognition of the location of the atrioventricular conduction axis. Comparison of the location of this tract in the normal and abnormal hearts shows that there is no separate formation of a ventricular component of the 'canal septum' as part of normal development. We conclude that it is abnormal formation of the primary atrial septum that is the cause of so-called 'secundum' atrial septal defects, whereas it is the failure to produce a second contribution to atrial septation (via growth of the vestibular spine) that results in the 'ostium primum' defect.

Keywords: atrioventricular septal defect; endocardial cushions; heart; primary atrial septum; vestibular spine.

Fig 1

The images, from episcopic datasets from developing mice at embryonic (E) day 10.5, show the initial site of formation of the primary atrial septum and the primary atrial foramen. (A) The dorsal mesocardial connection, (B) the site of formation of the primary atrial septum in the atrial roof. Note the location of the primordium of the apical muscular ventricular septum (star). The interventricular communication (dashed double-headed white arrow) at this stage has the inner heart curvature as its cranial margin, as the developing primary atrial septum is malaligned relative to the developing apical ventricular septum. The white brackets show the atrioventricular (AV) canal musculature, which surrounds the opening of the atrioventricular canal. The right side of the orifice of the canal, at this stage, opens to the apical component of the developing left ventricle (white arrow).

Fig 2

(A) Prepared in the sagittal plane and viewed from the left, from a dataset at E10.5. At this stage, the atrioventricular (AV) canal opens to the cavity of the developing left ventricle. The view from the left shows how the primary atrial foramen (double-headed white arrow with black borders) is bounded cranially by the primary atrial septum, and caudally by the atrial surfaces of the atrioventricular cushions, which are located superiorly and inferiorly within the canal. (B) A frontal section at E11.5, by which time the atrioventricular canal has expanded so that the cavity of the right atrium is opening into the inlet of the developing right ventricle (double-headed white arrow). The primary septum has broken away from the atrial roof to form the secondary inter-atrial communication. The primary septum itself, however, is now aligned to the developing muscular ventricular septum, although the inferior atrioventricular cushion has yet to fuse to the ventricular septal crest. The primary atrial foramen is now bounded by the mesenchymal cap cranially, and the atrial surface of the atrioventricular cushions caudally. The white star with black borders in this, and subsequent images, shows the gap between the left venous valve and the developing atrial septum, known as the interseptovalvar space.

Fig 3

The images, in the frontal plane, are from developing mice at the beginning of E11.5. (A) This image confirms that, as shown in Fig.2B, expansion of the atrioventricular canal (AV) has brought the cavity of the right atrium into direct continuity with that of the right ventricle. The section shows how the expansion of the mesenchymal tissue within the right pulmonary ridge, forming the vestibular spine, commits the orifice of the pulmonary vein to the developing left atrium. (B) This image, taken more ventrally, shows how growth of the primary septum towards the atrioventricular cushions has reduced the size of the primary atrial foramen. The reconstruction shows how the spine protrudes ventrally into the cavity of the developing right atrium, with its point overlapping the mesenchymal cap carried on the leading edge of the primary atrial septum.

Fig 4

The episcopic images are from datasets prepared from developing mice at E12.5. (A) This image, in the frontal plane, shows how the atrioventricular (AV) cushions have fused with each other, and how the vestibular spine has fused caudally with their atrial margins, completing the commitment of the pulmonary vein to the left atrium (compare with Fig.3A). (B) This image, in short axis, shows how the developing mitral valve, at this stage, has an obviously trifoliate arrangement formed by the appositions of the lateral margins of the superior and inferior atrioventricular cushions with the lateral cushion. The ventricular surfaces of the cushions are continuous apically with the trabecular layer of the ventricular wall. Note the locations of the developing primordiums of the papillary muscles of the mitral valve (stars). LV, left ventricle.

Fig 5

(A) This image, prepared from an episcopic dataset from a developing mouse at E13.5, and taken in four-chamber orientation, shows how it remains possible to recognise the contributions made by the atrioventricular (AV) cushions, the vestibular spine and the mesenchymal cap in closing the primary atrial foramen. The vestibular spine, together with the cap, now form the ventro-caudal margin of the developing oval foramen. The right atrial margin of the foramen (double-headed white arrow) is bounded cranially by the small muscular ridge now formed at the site of breakdown of the cranial margin of the primary septum. (B) This image, a short axis section from a mouse at E14.5, shows how the fusion of the two major atrioventricular cushions (white dotted line), together with expansion of the lateral cushion, is beginning to provide a bifoliate configuration for the developing mitral valve (double-headed white arrow). There is a small cleft (white arrow with black margins) at this stage within what will become the aortic leaflet of the mitral valveLV, left ventricle.

Fig 6

The images are frontal sections taken across the oval fossa, with (A) taken dorsally and (C) ventrally, from an episcopic dataset prepared from a mouse embryo at E15.5. The images show how it is possible to trace the atrioventricular (AV) conduction axis from its origin in the atrioventricular node (A), through the penetrating atrioventricular bundle (B), to the branching bundle, which continues as the left bundle branch down the endocardial aspect of the ventricular septum (dotted lines in C). The sections also show the margins of the oval fossa (double-headed white arrow in C). The dorsal rim, seen in (A), is a fold between the wall of the right atrium and the pulmonary vein. The cranial margin, seen in (C), is the muscular ridge formed at the site of breakdown of the primary septum from the atrial roof. The ventro-caudal margin has now been formed by muscularisation of the vestibular spine and the mesenchymal cap carried on the leading edge of the atrial septum. Note that, in (A), the atrioventricular node is in continuity with the atrial cardiomyocytes developing within the atrial septum (white arrows with white borders). The dotted lines have been placed to show the boundaries between the axis and the adjacent myocardial tissues evident in the serial sections reconstructed to produce the episcopic dataset.

Fig 7

The images are from an episcopic dataset prepared from a mouse embryo at E16.5. (A) This image, in four-chamber projection, shows the structure of the oval foramen (double-headed white arrow). The primary septum now forms its floor, with the ventro-caudal buttress derived by muscularisation of the vestibular spine along with the mesenchymal cap. The fused atrioventricular (AV) cushions now form the insulating plane between the atrial and ventricular septal structures. The cranial margin of the oval foramen is the muscular ridge developed at the initial site of breakdown of the primary septum away from the atrial roof. (B) This image is a short axis section across the left ventricle (LV). It shows how the mitral valve, by E15.5, has achieved its bifoliate pattern of closure along a solitary zone of apposition between its aortic and mural leaflets (double-headed white arrow). The subaortic outflow tract now interposes almost completely between the plane of mitral valvar opening and the muscular ventricular septum.

Fig 8

The images are from the episcopic datasets prepared from the developing mice found to have ostium primum defects. (A) This image is from an embryo killed at E15.5; (B) an embryo killed at E16.5; and (C) and (D) from embryos killed at E18.5. All sections are prepared to replicate the echocardiographic four-chamber frontal plane. They all show the commonality of the atrioventricular (AV) junction (double-headed white arrows in A and B). In each dataset, it is also possible to recognise that, while the mesenchymal cap has muscularised to form a knob on the leading edge of the primary atrial septum, there has been no formation of the vestibular spine. The venous valves guarding the systemic venous sinus are located dorsally within the right atrial chamber. In each dataset, it is also possible to recognise the atrioventricular conduction axis sandwiched between the inferior bridging leaflet of the common atrioventricular valve and the crest of the muscular ventricular septum. The fusion of the inferior bridging leaflet to the crest of the muscular ventricular septum produces separate right and left atrioventricular valvar orifices within the common atrioventricular junction (see also Fig.9).

Fig 9

The images show the common atrioventricular junction in one of the abnormal mouse embryos found, at E18.5, to have persistence of the ostium primum defect. (A) This image shows the view of the junction from the atrial aspect. The bridging leaflets of the common atrioventricular valve are fused to each other (double-headed white arrow), thus dividing the common junction into separate orifices for the right and left ventricles. The left atrioventricular orifice is guarded by leaflets that close in trifoliate fashion (white circle), as confirmed by the view obtained from the ventricular apex (B). Both images show that the aortic root, unlike the situation in the normally developed heart (Fig.7B), is no longer wedged between the leaflets of the left atrioventricular valve and the ventricular septum. The dotted line in (B) shows the line of fusion between the bridging leaflets of the common atrioventricular valve.

Fig 10

(A) This image shows the right-sided view of the ostium primum defect in one of the mouse embryos killed at E18.5. In this mouse, the primary septum has failed to grow down to the level of the atrioventricular (AV) junction (double-headed dotted black arrow), and there has been no ventral growth of the vestibular spine, the systemic venous sinus forming the dorsal wall of the right atrial chamber. The muscular ventricular septum has also failed to grow cranially to the level of the atrioventricular junction. Because the bridging leaflets of the common valve are fused to each other (white arrow with black borders), but also to the crest of the bowed ventricular septum, part of the shunting across the ostium primum defect is distal to the level of the atrioventricular junction (white star with black borders). (B) This image, prepared by staining one of the original episcopic sections with haematoxylin and eosin, shows how the atrioventricular conduction axis is sandwiched between the bridging leaflets and the crest of the muscular ventricular septum. Note the trifoliate configuration of the left atrioventricular valvar orifice.

Fig 11

The images show the varying size of the ostium primum defects in the embryonic mice with deficient atrioventricular septation. The defect is of good size in the image shown in (A), from an embryonic mouse killed at E18.5, but small in the one shown in (B), from the other embryo killed at E18.5. In both mice, shunting across the defect is distal to the plane of the atrioventricular junction (black dotted double-headed arrow) by virtue of the bridging leaflets of the common valve being depressed into the ventricular cavity and fused to the scooped-out ventricular septal crest. Shunting, therefore, is at atrial level even though it is below the level of the atrioventricular junction (white stars with black borders). Note also that the systemic venous sinus forms the dorsal wall of the right atrium, as there has been no ventral growth of the vestibular spine.

Fig 12

The images compare the morphology of the normal septal components in the setting of separate atrioventricular (AV) junctions as seen in the three orthogonal planes (D–F) with the arrangement in the abnormal mice with ostium primum defects (A–C).